This section is not normative.
TestsThis section is not normative, it does not need tests.
This module describes CSS properties which allow authors to specify the foreground color and opacity of the text content of an element. This module also describes in detail the CSS <color> value type.
It not only defines the color-related properties and values that already exist in CSS1, CSS2, and CSS Color 3, but also defines new properties and values.
In particular, it allows specifying colors in other color spaces than sRGB; previously, the more saturated colors outside the sRGB gamut could not be used in CSS even if the display device supported them.
A draft implementation report is available.
1.1. Value DefinitionsThis specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.
2. Color Terminology TestsThis section provides definitions used later, it does not need tests.
A color is a definition (numeric or textual) of the human visual perception of a light or a physical object illuminated with light. The objective study of human color perception is termed colorimetry.
The color of a physical object depends on how much light it reflects at each visible wavelength, plus the actual color of the light illuminating it (again, the amount of light at each wavelength). It is measured by a spectrophotometer .
The color of something that emits light (including colors on a computer screen) depends on how much light it emits at each visible wavelength. It is measured by a spectroradiometer.
If two objects have different spectra, but still produce the same physical sensation, we say they have the same color. We can calculate whether two colors are the same by converting the spectra to CIE XYZ (three numbers).
For example a green leaf, a photograph of that leaf displayed on a computer screen, and a print of that photograph, are all producing a green sensation by different means. If the screen and the printer are
calibrated, the green in the leaf, and the photo, and the print will look the same.
A color space is an organization of colors with respect to an underlying colorimetric model, such that there is a clear, objectively-measurable meaning for any color in that color space. This also means that the same color can be expressed in multiple color spaces, or transformed from one color space to another, while still looking the same.
A leaf is measured with a spectrophotometer and found to have the color lch(51.2345% 21.2 130) which is lab(51.2345% -13.6271 16.2401).
This same color could be expressed in various color spaces:
color(sRGB 0.41587 0.503670 0.36664); color(display-p3 0.43313 0.50108 0.37950); color(a98-rgb 0.44091 0.49971 0.37408); color(prophoto-rgb 0.36589 0.41717 0.31333); color(rec2020 0.42210 0.47580 0.35605);
An additive color space means that the coordinate system is linear in light intensity. The CIE XYZ color space is an additive color space. The Y component of XYZ is the luminance, the light intensity per unit area, or 'how bright it is'. Luminance is measured in candelas per square meter. cd/m², also called nits.
In an additive color space, calculations can be done to accurately predict color mixing. Most RGB spaces are not additive, because the components are gamma encoded. Undoing this gamma encoding produces linear-light values.
For example, if a light fixture contains two identical colored lights, and only one is switched on, and the color is measured to be
color(xyz 0.13 0.12 0.04), then the color when both are switched on will be exactly twice that,
color(xyz 0.26 0.24 0.08).
If we have two differently colored spotlights shining on a stage, and one has the measured value color(xyz 0.15 0.24 0.17) while the other is color(xyz 0.11 0.06 0.06) then we can accurately predict that if the colored beams are made to overlap, the color of the mixture will be the sum of the XYZ component values, or color(xyz 0.26 0.30 0.23).
A chromaticity is a color measurement where the lightness component has been factored out. From the identical lights example above, the u',v' chromaticity with one light is (0.2537, 0.5268) and the chromaticity is the same with both lights (they are the same color, it is just brighter).
Chromaticities are additive, so they accurately predict the chromaticity (but not the resulting lightness) of a mixture. Being two-dimensional, chromaticity is easily represented on a chromaticity diagram to predict the chromaticity of a color mixture. Any two colors can be mixed, and the resulting colors will lie on the line joining them on the diagram. Three colors form a plane, and the resulting colors will lie in the triangle they form on the diagram.
A chromaticity diagram showing (in solid colors) the display-p3 color space and for comparison (faded) the sRGB color space. The white point (D65) is also shown.Thus, once linearized, RGB color spaces are additive, and their gamut is defined by the chromaticities of the red, green and blue primaries, plus the chromaticity of the white point (the color formed by all three primaries at full intensity).
Most color spaces use one of a few daylight-simulating white points, which are named by the correlated color temperature (CCT) [Understanding_CCT] of the corresponding black-body radiator. For example, D65 is a daylight whitepoint corresponding to a correlated color temperature of 6500 Kelvin (actually 6504, because the value of Plank’s constant has changed since the color was originally defined).
To avoid cumulative round-trip errors, it is important that the identical chromaticity values are used consistently, at all places in a calculation. Thus, for maximum compatibility, for this specification, the following two standard daylight-simulating white points are defined:
Name x y CCT D50 0.345700 0.358500 5003K D65 0.312700 0.329000 6504KWhen the measured physical characteristics (such as the chromaticities of the primary colors it uses, or the colors produced in response to a given set of inputs) of a color space or a color-producing device are known, it is said to be characterized.
If in addition adjustments have been made so that a device meets calibration targets such as white point, neutrality of greys, predictability and consistency of tone response, then it is said to be calibrated.
Real physical devices cannot yet produce every possible color that the human eye can see. The range of colors that a given device can produce is termed the gamut (not to be confused with gamma). Devices with a limited gamut cannot produce very saturated colors, like those found in a rainbow.
A top-down view of three gamuts, plotted in Oklab with the positive a-axis towards the right and the positive b-axis towards the top; looking down the l-axis so white and neutrals are in the center. The largest of the three gamuts is ITU Rec BT.2020; the medium-sized one is Display P3, and the smallest is sRGB. Rendering by Alexey Ardov.The gamuts of different color spaces may be compared by looking at the volume (in cubic Lab units) of colors that can be expressed. The following table examines the predefined color spaces available in CSS.
color space Volume (million Lab units) sRGB 0.820 display-p3 1.233 a98-rgb 1.310 prophoto-rgb 2.896 rec2020 2.042A color in CSS is either an invalid color, as described below for each syntactic form, or a valid color.
Any color which is not an invalid color is a valid color.
A color may be a valid color but still be outside the range of colors that can be produced by an output device (a screen, projector, or printer)
It is said to be out of gamut.
Each valid color is either in-gamut for a particular output device (screen, or printer) or it is out of gamut.
For example, given a screen which covers 100% of the display-p3 color space, but no more, the following color is out of gamut:
color(prophoto-rgb 0.88 0.45 0.10)
because, expressed in display-p3, one or more coordinates are either greater that 1.0 or less than 0.0:
color(display-p3 1.0844 0.43 0.1)
This color is valid, and could, for example, be used as a gradient stop, but would need to be CSS gamut mapped for display, producing a similar-looking but lower chroma (less saturated) color.
3. Applying Color in CSS 3.1. Accessibility and Conveying Information By Color TestsThis section provides authoring guidance, it does not need tests.
Although colors can add significant information to documents and make them more readable, color by itself should not be the sole means to convey important information. Authors should consider the W3C Web Content Accessibility Guidelines [WCAG21] when using color in their documents.
3.2. Foreground Color: the color property Tests
This property specifies the primary foreground color of the element. This is used as the fill color of its text content, and in addition specifies the used value that currentcolor resolves to, which allows indirect references to this foreground color and affects the initial values of various other color properties such as border-color and text-emphasis-color.
The
<color>type provides multiple ways to syntactically specify a given color. For example, the following declarations all specify the sRGB color “lime”:
em { color: lime; } /* color keyword */
em { color: rgb(0 255 0); } /* RGB range 0-255 */
em { color: rgb(0% 100% 0%); } /* RGB range 0%-100% */
em { color: color(sRGB 0 1 0); } /* sRGB range 0.0-1.0 */
When applied to text, this property, including its alpha component, has no effect on “color glyphs” (such as the emoji in some fonts), which are colored by a built-in palette. However, some colored fonts are able to refer to a contextual “foreground color”, such as by palette entry 0xFFFF in the COLR table of OpenType, or by the context-fill value in SVG-in-OpenType. In such cases, the foreground color is set by this property, identical to how it sets the currentcolor value.
Opacity can be thought of as a postprocessing operation. Conceptually, after the element (including its descendants) is rendered into an RGBA offscreen image, the opacity setting specifies how to blend the offscreen rendering into the current composite rendering. See simple alpha compositing for details.
TestsOpacity values outside the range [0,1] are not invalid, and are preserved in specified values, but are clamped to the range [0, 1] in computed values.
Opacity in CSS is represented using the <opacity-value> syntax, for example in the opacity property.
<opacity-value> = <number> | <percentage>
Represented as a <number>, the useful range of the value is 0 (representing full transparency) to 1 (representing full opacity). It can also be written as a <percentage>, which computes to the equivalent <number> (0% to 0, 100% to 1).
The opacity property applies the specified opacity to the element as a whole, including its contents, rather than applying it to each descendant individually. This means that, for example, an opaque child occluding part of the element’s background will continue to do so even when opacity is less than 1, but the element and child as a whole will show the underlying page through themselves.
It also means that the glyphs corresponding to all characters in the element are treated as a whole; any overlapping portions do not increase the opacity.
Tests Correct and incorrect rendering of text with an opacity value of less than one, whose glyphs overlap.If separate opacity for each glyph is desired, it can be achieved by using a color value which includes alpha, rather than setting the opacity property.
If a box has opacity less than 1, it forms a stacking context for its children. (This prevents its contents from interleaving in the z-axis with content outside it.)
TestsFurthermore, if the z-index property applies to the box, the auto value is treated as 0 for the element; it is otherwise painted on the same layer within its parent stacking context as positioned elements with stack level 0 (as if it were a positioned element with z-index:0).
See section 9.9 and Appendix E of [CSS2] for more information on stacking contexts.
These rules about z-order do not apply to SVG elements, since SVG has its own rendering model ([SVG11], Chapter 3).
The value of the opacity property does not affect hit testing.
3.4. Color Space of Tagged ImagesAn tagged image is an image that is explicitly assigned a color profile, as defined by the image format. This is usually done by including an International Color Consortium (ICC) profile [ICC].
For example JPEG [JPEG], PNG [PNG] and TIFF [TIFF] all specify a means to embed an ICC profile.
Image formats may also use other, equivalent methods, often for brevity.
For example, PNG specifies a means (the sRGB chunk) to explicitly tag an image as being in the sRGB color space, without including the sRGB ICC profile.
Similarly, PNG specifies a compact means (the cICP chunk) to explicitly tag an image as being one of various SDR or HDR color spaces, such as Display P3 or BT.2100 HLG, without including an ICC profile.
Tagged RGB images, and tagged images using a transformation of RGB such as YCbCr, if the color profile or other identifying information is valid, must be treated as being in the specified color space.
TestsFor example, when a browser running on a system with a Display P3 monitor displays an JPEG image tagged as being in the ITU Rec BT.2020 [Rec.2020] color space, it must convert the colors from ITU Rec BT.2020 to Display P3 so that they display correctly. It must not treat the ITU Rec BT.2020 values as if they were Display P3 values, which would produce incorrect colors.
If the color profile or other identifying information is invalid, the image is treated as described for untagged images.
3.5. Color Spaces of Untagged ColorsFor compatibility, colors specified in HTML, and untagged images must be treated as being in the sRGB color space ([SRGB]) unless otherwise specified.
TestsAn untagged image is an image that is not explicitly assigned a color profile, as defined by the image format.
This rule does not apply to untagged videos, since untagged video should be presumed to be in an ITU-defined color space.
At below 720p, it is Recommendation ITU-R BT.601 [ITU-R-BT.601]
At 720p, it is SMPTE ST 296 (same colorimetry as 709) [SMPTE296]
At 1080p, it is Recommendation ITU-R BT.709 [ITU-R-BT.709]
At 4k (UHDTV) and above, it is ITU-R BT.2020 [Rec.2020] for SDR video
This section describes a type, it is primarily tested where that type is used.
Colors in CSS are represented as a list of color components, also sometimes called “channels”, representing axises in the color space. Each component has a minimum and maximum value, and can take any value between those two. Additionally, every color is accompanied by an alpha component, indicating how transparent it is, and thus how much of the backdrop one can see through the color.
CSS has several syntaxes for specifying color values:
the sRGB hex color notation which represents the RGB and alpha components in hexadecimal notation
the various color functions which can represent colors using a variety of color spaces and coordinate systems
the constant named color keywords
the variable <system-color> keywords and currentColor keyword.
The color functions use CSS functional notation to represent colors in a variety of color spaces by specifying their component coordinates. Some of these use a cylindrical polar color model, specifying color by a <hue> angle, a central axis representing lightness (black-to-white), and a radius representing saturation or chroma (how far the color is from a neutral grey). The others use a rectangular orthogonal color model, specifying color using three orthogonal component axes.
The color functions available in Level 4 are
rgb() and its rgba() alias, which (like the hex color notation) specify sRGB colors directly by their red/green/blue/alpha components.
hsl() and its hsla() alias, which specify sRGB colors by hue, saturation, and lightness using the HSL cylindrical coordinate model.
hwb(), which specifies an sRGB color by hue, whiteness, and blackness using the HWB cylindrical coordinate model.
lab(), which specifies a CIELAB color by CIE Lightness and its a- and b-axis hue coordinates (red/green-ness, and yellow/blue-ness) using the CIE LAB rectangular coordinate model.
lch() , which specifies a CIELAB color by CIE Lightness, Chroma, and hue using the CIE LCH cylindrical coordinate model
oklab(), which specifies an Oklab color by Oklab Lightness and its a- and b-axis hue coordinates (red/green-ness, and yellow/blue-ness) using the Oklab rectangular coordinate model.
oklch() , which specifies an Oklab color by Oklab Lightness, Chroma, and hue using the OKLCh cylindrical coordinate model.
color(), which allows specifying colors in a variety of color spaces including sRGB, Linear-light sRGB, Display P3, A98 RGB, ProPhoto RGB, ITU-R BT.2020-2, and CIE XYZ.
For easy reference in other specifications, opaque black is defined as the color rgb(0 0 0 / 100%); transparent black is the same color, but fully transparent—i.e. rgb(0 0 0 / 0%).
TestsThis section provides definitions used later, it does not need tests.
Colors in CSS are represented by the <color> type:
<color> = <color-base> | currentColor | <system-color>
<color-base> = <hex-color> | <color-function> | <named-color> | transparent
<color-function> = <rgb()> | <rgba()> |
<hsl()> | <hsla()> | <hwb()> |
<lab()> | <lch()> | <oklab()> | <oklch()> |
<color()>
An absolute color is a <color> whose computed value has an absolute, colorimetric interpretation. This means that the value is not:
currentColor (which depends on the value of the color property)
a <system-color> (which depends on the color mode)
The colors that resolve to sRGB are:
The functions that support legacy color syntax are:
The <hsl()>, <hsla()>, <hwb()>, <lch()>, and <oklch()> color functions are cylindrical polar color representations using a <hue> angle; the other color functions use rectangular orthogonal color representations.
4.1.1. Modern (Space-separated) Color Function SyntaxAll of the absolute color functional forms first defined in this specification use the modern color syntax, meaning:
color components are separated by whitespace
the optional alpha term is separated by a solidus ("/")
minimum required precision when serializing is defined, and may be greater than 8 bits per component
the none value is allowed, to represent missing components
components using <percentage> and <number> may be freely mixed
The following represents a saturated sRGB red that is 50% opaque:
rgb(100% 0% 0% / 50%)4.1.2. Legacy (Comma-separated) Color Function Syntax
For Web compatibility, the syntactic forms of rgb(), rgba(), hsl(), and hsla(), (those defined in earlier specifications) also support a legacy color syntax which has the following differences:
color components are separated by commas (optionally preceded and/or followed by whitespace)
non-opaque forms use a separate notation (for example hsla() rather than hsl()) and the alpha term is separated by commas (optionally preceded and/or followed by whitespace)
minimum required precision is lower, 8 bits per component
the none value is not allowed
color components must be specified using either all-<percentage> or all-<number>, they can not be mixed.
The following represents a saturated sRGB red that is 50% opaque:
rgba(100%, 0%, 0%, 0.5)
For the color functions introduced in this or subsequent levels, where there is no Web compatibility issue, the legacy color syntax is invalid.
4.2. Representing Transparency in Colors: the <alpha-value> syntax TestsThis section provides definitions used later, it does not need tests.
<alpha-value> = <number> | <percentage>
Unless otherwise specified, an <alpha-value> component of a color defaults to 100% when omitted. Values outside the range [0,1] are not invalid, but are clamped to that range at parsed-value time.
4.3. Representing Cylindrical-coordinate Hues: the <hue> syntax TestsThis section provides definitions used later, it does not need tests.
Hue is represented as an angle of the color circle (the rainbow, twisted around into a circle, and with purple added between violet and red).
<hue> = <number> | <angle>
Because this value is so often given in degrees, the argument can also be given as a number, which is interpreted as a number of degrees and is the canonical unit.
This number is normalized to the range [0,360).
For example, in
hsl(-540 0 0)or
hsl(540 0 0), the
<hue>component is normalized to 180 degrees.
In hsl(360 0 0) the <hue> component is normalized to 0 degrees.
In hsl(calc(-infinity) 0 0) or hsl(calc(infinity) 0 0), the <hue> component is again normalized to 0 degrees.
Note: The angles and spacing corresponding to particular hues depend on the color space. For example, in HSL and HWB, which use the sRGB color space, sRGB green is 120 degrees. In LCH, sRGB green is 134.39 degrees, display-p3 green is 136.01 degrees, a98-rgb green is 145.97 degrees and prophoto-rgb green is 141.04 degrees (because these are all different shades of green).
<hue> components are the most common components to become powerless; any color sufficiently close to the central achromatic axis will have a powerless hue component.
4.4. “Missing” Color Components and the none KeywordIn certain cases, a color can have one or more missing color components.
In this specification, this happens automatically due to hue-based interpolation for some colors (such as white); other specifications can define additional situations in which components are automatically missing.
It can also be specified explicitly, by providing the keyword none for a component in a color function. All color functions (with the exception of those using the legacy color syntax) allow any of their components to be specified as none.
This should be done with care, and only when the particular effect of doing so is desired.
TestsFor handling of missing components in situations which combine two colors, such as color interpolation, see § 12.2 Interpolating with Missing Components.
For all other purposes, a missing component behaves as a zero value, in the appropriate unit for that component: 0, 0%, or 0deg. This includes rendering the color directly, converting it to another color space, performing computations on the color component values, etc.
If a color with a missing component is serialized or otherwise presented directly to an author, then for legacy color syntax it represents that component as a zero value; otherwise, it represents that component as being the none keyword.
A missing hue is common when interpolating in cylindrical color spaces. For example, using the
color-mix()function specified in
[CSS-COLOR-5]one could write
color-mix(in hsl, white 30%, green 70%). Since
whiteis an achromatic color, it has a
missinghue when expressed in
hsl()(effectively
hsl(none 0% 100%)), since
anyhue will produce the same color) which means that the color-mix function will treat it as having the same hue as
green(effectively
hsl(120deg 0% 100%)), and then interpolate based on those components.
The result will be a color that truly looks like a blend of green and white, rather than perhaps looking reddish (if whites hue was defaulted to 0deg).
Explicitly specifying missing components can be useful to achieve an effect where you only
wantto interpolate certain components of a color.
For example, to animate a color to "grayscale", no matter what the color is, one can interpolate it with oklch(none 0 none). This will take the hue and lightness from the starting color, but animate its chroma down to 0, rendering it into an equal-lightness gray with a steady hue across the whole animation.
Doing this manually would require matching the hue and lightness of the starting color explicitly.
4.4.1. “Powerless” Color ComponentsIndividual color syntaxes can specify that, in some cases, a given component of their syntax becomes a powerless color component. This indicates that the value of the component doesn’t affect the rendered color; any value you give it will result in the same color displayed in the screen.
For example, in hsl(), the hue component is powerless when the saturation component is 0%; a 0% saturation indicates a grayscale color, which has no hue at all, so 0deg and 180deg, or any other angle, will give the exact same result.
If a powerless component is manually specified, it acts as normal; the fact that it’s powerless has no effect.
However, if a color is automatically produced by color space conversion, then any powerless components in the result must instead be set to missing, instead of whatever value was produced by the conversion process.
When performing color space conversion to a cylindrical polar color space, user agents shall treat a hue component as powerless if the chroma (or other measure of colorfulness, such as saturation in hsl) is less than the epsilon (ε) specified for that color space. For example, a gray color converted into oklch() may, due to numerical errors, have an extremely small chroma rather than precisely 0%; as a result, the hue component is powerless.
4.5. Parsing a <color> Value TestsThis section provides a definition referenced elsewhere, it does not need tests.
To
parse a CSS <color> value, given a
string input, and an optional context
element element:
Parse input as a <color>. If the result is failure, return failure; otherwise, let color be the result.
Let used color be the result of resolving color to a used color. If the value of other properties on the element a <color> is on is required to do the resolution (such as resolving a currentcolor or system color), use element if it was passed, or the initial values of the properties if not.
Return used color.
Note: This algorithm is not intented to parse a CSS <color> value specified in a CSS stylesheet or with a CSSOM interface, but in other places like HTML attributes or Canvas interfaces.
5. sRGB ColorsCSS colors in the sRGB color space are represented by a triplet of values—red, green, and blue—identifying a point in the sRGB color space [SRGB]. This is an internationally-recognized, device-independent color space, and so is useful for specifying colors that will be displayed on a computer screen, but is also useful for specifying colors on other types of devices, like printers.
CSS also allows the use of non-sRGB color spaces, as described in § 10 Predefined Color Spaces.
CSS provides several methods of directly specifying an sRGB color: hex colors, rgb()/rgba() color functions, hsl()/hsla() color functions, hwb() color function, named colors, and the transparent keyword.
5.1. The RGB functions: rgb() and rgba()The rgb() and rgba() functions define an sRGB color by specifying the r, g and b (red, green, and blue) components directly. Their syntax is:
rgb() = [ <legacy-rgb-syntax> | <modern-rgb-syntax> ]
rgba() = [ <legacy-rgba-syntax> | <modern-rgba-syntax> ]
<legacy-rgb-syntax> = rgb( <percentage>#{3} , <alpha-value>? ) |
rgb( <number>#{3} , <alpha-value>? )
<legacy-rgba-syntax> = rgba( <percentage>#{3} , <alpha-value>? ) |
rgba( <number>#{3} , <alpha-value>? )
<modern-rgb-syntax> = rgb(
[ <number> | <percentage> | none]{3}
[ / [<alpha-value> | none] ]? )
<modern-rgba-syntax> = rgba(
[ <number> | <percentage> | none]{3}
[ / [<alpha-value> | none] ]? )
Percentages Allowed for r, g and b Percent reference range For r, g and b: 0% = 0.0, 100% = 255.0 For alpha: 0% = 0.0, 100% = 1.0 Tests
The first three arguments specify the r, g and b (red, green, and blue) components of the color, respectively. 0% represents the minimum value for that color component in the sRGB gamut, and 100% represents the maximum value.
The percentage reference range of the color components comes from the historical fact that many graphics engines stored the color components internally as a single byte, which can hold integers between 0 and 255. Implementations should honor the precision of the component as authored or calculated wherever possible. If this is not possible, the component should be rounded towards +∞.
The final argument, the <alpha-value>, specifies the alpha of the color. If omitted, it defaults to 100%.
TestsValues outside these ranges are not invalid, but are clamped to the ranges defined here at parsed-value time.
For historical reasons, rgb() and rgba() also support a legacy color syntax.
TestsThe CSS hex color notation allows an sRGB color to be specified by giving the components as hexadecimal numbers, which is similar to how colors are often written directly in computer code. It’s also shorter than writing the same color out in rgb() notation.
The syntax of a <hex-color> is a <hash-token> token whose value consists of 3, 4, 6, or 8 hexadecimal digits. In other words, a hex color is written as a hash character, "#", followed by some number of digits 0-9 or letters a-f (the case of the letters doesn’t matter - #00ff00 is identical to #00FF00).
The number of hex digits given determines how to decode the hex notation into an RGB color:
In other words,
#00ff00represents the same color as
rgb(0 255 0)(a lime green).
In other words,
#0000ffccrepresents the same color as
rgb(0 0 100% / 80%)(a slightly-transparent blue).
This syntax is often explained by saying that it’s identical to a 6-digit notation obtained by "duplicating" all of the digits. For example, the notation
#123specifies the same color as the notation
#112233. This method of specifying a color has lower "resolution" than the 6-digit notation; there are only 4096 possible colors expressible in the 3-digit hex syntax, as opposed to approximately 17 million in 6-digit hex syntax.
In addition to the various numeric syntaxes for <color>s, CSS defines several sets of color keywords that can be used instead—each with their own advantages or use cases.
6.1. Named ColorsCSS defines a large set of named colors, so that common colors can be written and read more easily. A <named-color> is written as an <ident>, accepted anywhere a <color> is. As usual for CSS-defined <ident>s, all of these keywords are ASCII case-insensitive.
The names resolve to colors in sRGB.
16 of CSS’s named colors come from the VGA palette originally, and were then adopted into HTML: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow. Most of the rest come from one version of the X11 color system, used in Unix-derived systems to specify colors for the console, and were then adopted into SVG.
Note: these color names are standardized here, not because they are good, but because their use and implementation has been widespread for decades and the standard needs to reflect reality. Indeed, it is often hard to imagine what each name will look like (hence the list below); the names are not evenly distributed throughout the sRGB color volume, the names are not even internally consistent ( darkgray is lighter than gray, while lightpink is darker than pink), and some names (such as indianred, which was originally named after a red pigment from India), have been found to be offensive. Thus, their use is not encouraged.
(Two special color values, transparent and currentcolor, are specially defined in their own sections.)
The following table defines all of the opaque named colors, by giving equivalent numeric specifications in the other color syntaxes.
Named Numeric Color name Hex rgb Decimal aliceblue #f0f8ff 240 248 255 antiquewhite #faebd7 250 235 215 aqua #00ffff 0 255 255 aquamarine #7fffd4 127 255 212 azure #f0ffff 240 255 255 beige #f5f5dc 245 245 220 bisque #ffe4c4 255 228 196 black #000000 0 0 0 blanchedalmond #ffebcd 255 235 205 blue #0000ff 0 0 255 blueviolet #8a2be2 138 43 226 brown #a52a2a 165 42 42 burlywood #deb887 222 184 135 cadetblue #5f9ea0 95 158 160 chartreuse #7fff00 127 255 0 chocolate #d2691e 210 105 30 coral #ff7f50 255 127 80 cornflowerblue #6495ed 100 149 237 cornsilk #fff8dc 255 248 220 crimson #dc143c 220 20 60 cyan #00ffff 0 255 255 darkblue #00008b 0 0 139 darkcyan #008b8b 0 139 139 darkgoldenrod #b8860b 184 134 11 darkgray #a9a9a9 169 169 169 darkgreen #006400 0 100 0 darkgrey #a9a9a9 169 169 169 darkkhaki #bdb76b 189 183 107 darkmagenta #8b008b 139 0 139 darkolivegreen #556b2f 85 107 47 darkorange #ff8c00 255 140 0 darkorchid #9932cc 153 50 204 darkred #8b0000 139 0 0 darksalmon #e9967a 233 150 122 darkseagreen #8fbc8f 143 188 143 darkslateblue #483d8b 72 61 139 darkslategray #2f4f4f 47 79 79 darkslategrey #2f4f4f 47 79 79 darkturquoise #00ced1 0 206 209 darkviolet #9400d3 148 0 211 deeppink #ff1493 255 20 147 deepskyblue #00bfff 0 191 255 dimgray #696969 105 105 105 dimgrey #696969 105 105 105 dodgerblue #1e90ff 30 144 255 firebrick #b22222 178 34 34 floralwhite #fffaf0 255 250 240 forestgreen #228b22 34 139 34 fuchsia #ff00ff 255 0 255 gainsboro #dcdcdc 220 220 220 ghostwhite #f8f8ff 248 248 255 gold #ffd700 255 215 0 goldenrod #daa520 218 165 32 gray #808080 128 128 128 green #008000 0 128 0 greenyellow #adff2f 173 255 47 grey #808080 128 128 128 honeydew #f0fff0 240 255 240 hotpink #ff69b4 255 105 180 indianred #cd5c5c 205 92 92 indigo #4b0082 75 0 130 ivory #fffff0 255 255 240 khaki #f0e68c 240 230 140 lavender #e6e6fa 230 230 250 lavenderblush #fff0f5 255 240 245 lawngreen #7cfc00 124 252 0 lemonchiffon #fffacd 255 250 205 lightblue #add8e6 173 216 230 lightcoral #f08080 240 128 128 lightcyan #e0ffff 224 255 255 lightgoldenrodyellow #fafad2 250 250 210 lightgray #d3d3d3 211 211 211 lightgreen #90ee90 144 238 144 lightgrey #d3d3d3 211 211 211 lightpink #ffb6c1 255 182 193 lightsalmon #ffa07a 255 160 122 lightseagreen #20b2aa 32 178 170 lightskyblue #87cefa 135 206 250 lightslategray #778899 119 136 153 lightslategrey #778899 119 136 153 lightsteelblue #b0c4de 176 196 222 lightyellow #ffffe0 255 255 224 lime #00ff00 0 255 0 limegreen #32cd32 50 205 50 linen #faf0e6 250 240 230 magenta #ff00ff 255 0 255 maroon #800000 128 0 0 mediumaquamarine #66cdaa 102 205 170 mediumblue #0000cd 0 0 205 mediumorchid #ba55d3 186 85 211 mediumpurple #9370db 147 112 219 mediumseagreen #3cb371 60 179 113 mediumslateblue #7b68ee 123 104 238 mediumspringgreen #00fa9a 0 250 154 mediumturquoise #48d1cc 72 209 204 mediumvioletred #c71585 199 21 133 midnightblue #191970 25 25 112 mintcream #f5fffa 245 255 250 mistyrose #ffe4e1 255 228 225 moccasin #ffe4b5 255 228 181 navajowhite #ffdead 255 222 173 navy #000080 0 0 128 oldlace #fdf5e6 253 245 230 olive #808000 128 128 0 olivedrab #6b8e23 107 142 35 orange #ffa500 255 165 0 orangered #ff4500 255 69 0 orchid #da70d6 218 112 214 palegoldenrod #eee8aa 238 232 170 palegreen #98fb98 152 251 152 paleturquoise #afeeee 175 238 238 palevioletred #db7093 219 112 147 papayawhip #ffefd5 255 239 213 peachpuff #ffdab9 255 218 185 peru #cd853f 205 133 63 pink #ffc0cb 255 192 203 plum #dda0dd 221 160 221 powderblue #b0e0e6 176 224 230 purple #800080 128 0 128 rebeccapurple #663399 102 51 153 red #ff0000 255 0 0 rosybrown #bc8f8f 188 143 143 royalblue #4169e1 65 105 225 saddlebrown #8b4513 139 69 19 salmon #fa8072 250 128 114 sandybrown #f4a460 244 164 96 seagreen #2e8b57 46 139 87 seashell #fff5ee 255 245 238 sienna #a0522d 160 82 45 silver #c0c0c0 192 192 192 skyblue #87ceeb 135 206 235 slateblue #6a5acd 106 90 205 slategray #708090 112 128 144 slategrey #708090 112 128 144 snow #fffafa 255 250 250 springgreen #00ff7f 0 255 127 steelblue #4682b4 70 130 180 tan #d2b48c 210 180 140 teal #008080 0 128 128 thistle #d8bfd8 216 191 216 tomato #ff6347 255 99 71 turquoise #40e0d0 64 224 208 violet #ee82ee 238 130 238 wheat #f5deb3 245 222 179 white #ffffff 255 255 255 whitesmoke #f5f5f5 245 245 245 yellow #ffff00 255 255 0 yellowgreen #9acd32 154 205 50Note: this list of colors and their definitions is a superset of the list of named colors defined by SVG 1.1.
For historical reasons, this is also referred to as the X11 color set.Note: The history of the X11 color system is interesting, and was excellently summarized by Alex Sexton in their talk “Peachpuffs and Lemonchiffons”.
TestsIn general, the <system-color> keywords reflect default color choices made by the user, the browser, or the OS. They are typically used in the browser default stylesheet, for this reason.
To maintain legibility, the <system-color> keywords also respond to light mode or dark mode changes.
For example, traditional
blue link text is legible on a
white background (WCAG contrast 8.59:1, AAA pass) but would not be legible on a
black background (WCAG contrast 2.44:1, AA fail). Instead, a lighter blue such as
#81D9FE would be used in dark mode (WCAG contrast 13.28:1, AAA pass).
Legible link text
Illegible link text
Legible link text
However, in forced colors mode, most colors on the page are forced into a restricted, user-chosen palette. The <system-color> keywords expose these user-chosen colors so that the rest of the page can integrate with this restricted palette.
When the forced-colors media feature is active, authors should use the <system-color> keywords as color values in properties other than those listed in CSS Color Adjustment 1 § 3.1 Properties Affected by Forced Colors Mode, to ensure legibility and consistency across the page and avoid an uncoordinated mishmash of user-forced and page-chosen colors.
TestsWhen the values of <system-color> keywords come from the browser, (as opposed to being OS defaults or user choices) the browser should ensure that matching foreground/background pairs have a minimum of WCAG AA contrast. However, user preferences (for higher or lower contrast), whether set as a browser preference, a user stylesheet, or by altering the OS defaults, must take precedence over this requirement.
Authors may also use these keywords at any time, but should be careful to use the colors in matching background-foreground pairs to ensure appropriate contrast, as any particular contrast relationship across non-matching pairs (e.g. Canvas and ButtonText) is not guaranteed.
The <system-color> keywords are defined as follows:
mark element).
mark element).
Note: As with all other keywords, these names are ASCII case-insensitive. They are shown here with mixed capitalization for legibility.
For systems that do not have a particular system UI concept, the specified value should be mapped to the most closely related system color value that exists. The following system color pairings are expected to form legible background-foreground colors:
Canvas background with CanvasText, LinkText, VisitedText, ActiveText foreground.
Canvas background with a ButtonBorder border and adjacent color Canvas
ButtonFace background with ButtonText foreground.
ButtonFace or Field background with a ButtonBorder border and adjacent color Canvas'
Highlight background with HighlightText foreground.
SelectedItem background with SelectedItemText foreground.
AccentColor background with AccentColorText foreground.
Additionally, GrayText is expected to be readable, though possibly at a lower contrast rating, over any of the backgrounds.
For example, the system color combinations in the browser you are currently using:
Canvas with CanvasText: CanvasText
Canvas with LinkText: LinkText
Canvas with VisitedText: VisitedText
Canvas with ActiveText: ActiveText
Canvas with GrayText: GrayText
Canvas with ButtonBorder and adjacent Canvas: CanvasTextAdjacent
ButtonFace with ButtonText: ButtonText
ButtonFace with ButtonText and ButtonBorder: ButtonText
ButtonFace with GrayText: GrayText
Field with FieldText: FieldText
Field with GrayText: GrayText
Mark with MarkText: MarkText
Mark with GrayText: GrayText
Highlight with HighlightText: HighlightText
Highlight with GrayText: GrayText
SelectedItem with SelectedItemText: SelectedItemText
AccentColor with AccentColorText: AccentColorText
AccentColor with GrayText: GrayText
Earlier versions of CSS defined additional <system-color>s, which have since been deprecated. These are documented in Appendix A: Deprecated CSS System Colors.
Note: The <system-color>s incur some privacy and security risk, as detailed in § 21 Privacy Considerations and § 20 Security Considerations.
User Agents may, to mitigate privacy and security risks such as fingerprinting, elect to return fixed values for the used value of system colors which do not reflect customisation or theming choices made by the user.
6.3. The transparent keywordThe keyword transparent specifies a transparent black. It is a type of <named-color>.
TestsThe keyword currentcolor represents value of the color property on the same element. Unlike <named-color>s, it is not restricted to sRGB; the value can be any <color>. Its used values is determined by resolving color values.
TestsHere’s a simple example showing how to use the
currentcolorkeyword:
.foo {
color: red;
background-color: currentcolor;
}
This is equivalent to writing:
.foo {
color: red;
background-color: red;
}
For example, the
text-emphasis-colorproperty
[CSS3-TEXT-DECOR], whose initial value is
currentcolor, by default matches the text color even as the
colorproperty changes across elements.
<p><em>Some <strong>really</strong> emphasized text.</em>
<style>
p { color: black; }
em { text-emphasis: dot; }
strong { color: red; }
</style>
In the above example, the emphasis marks are black over the text "Some" and "emphasized text", but red over the text "really".
Note: Multi-word keywords in CSS usually separate their component words with hyphens. currentcolor doesn’t, because (deep breath) it was originally introduced in SVG as a property value, "current-color" with the usual CSS spelling. It (along with all other properties and their values) then became presentation attributes and attribute values, as well as properties, to make generation with XSLT easier. Then all of the presentation attributes were changed from hyphenated to camelCase, because the DOM had an issue with hyphen meaning "minus". But then, they didn’t follow CSS conventions anymore so all the properties and property values that were already part of CSS were changed back to hyphenated! currentcolor was not a part of CSS at that time, so remained camelCased. Only later did CSS pick it up, at which point the capitalization stopped mattering, as CSS keywords are ASCII case-insensitive.
7. HSL Colors: hsl() and hsla() functionsThe RGB system for specifying colors, while convenient for machines and graphic libraries, is often regarded as very difficult for humans to gain an intuitive grasp on. It’s not easy to tell, for example, how to alter an RGB color to produce a lighter variant of the same hue.
There are several other color schemes possible. One such is the HSL [HSL] color scheme, which is more intuitive to use, but still maps easily back to RGB colors.
HSL colors are specified as a triplet of hue, saturation, and lightness. The syntax of the hsl() and hsla() functions is:
hsl() = [ <legacy-hsl-syntax> | <modern-hsl-syntax> ]
hsla() = [ <legacy-hsla-syntax> | <modern-hsla-syntax> ]
<modern-hsl-syntax> = hsl(
[<hue> | none]
[<percentage> | <number> | none]
[<percentage> | <number> | none]
[ / [<alpha-value> | none] ]? )
<modern-hsla-syntax> = hsla(
[<hue> | none]
[<percentage> | <number> | none]
[<percentage> | <number> | none]
[ / [<alpha-value> | none] ]? )
<legacy-hsl-syntax> = hsl( <hue>, <percentage>, <percentage>, <alpha-value>? )
<legacy-hsla-syntax> = hsla( <hue>, <percentage>, <percentage>, <alpha-value>? )
Percentages Allowed for S and L Percent reference range for S and L: 0% = 0.0, 100% = 100.0 Powerless hue ε S <= 0.001 Tests
The first argument specifies the hue angle.
In HSL (and HWB) the angle 0deg represents sRGB primary red (as does 360deg, 720deg, etc.), and the rest of the hues are spread around the circle, so 120deg represents sRGB primary green, 240deg represents sRGB primary blue, etc.
The next two arguments are the saturation and lightness, respectively. For saturation, 100% or 100 is a fully-saturated, bright color, and 0% or 0 is a fully-unsaturated gray. For lightness, 50% or 50 represents the "normal" color, while 100% or 100 is white and 0% or 0 is black.
For historical reasons, if the saturation is less than 0% it is clamped to 0% at parsed-value time, before being converted to an sRGB color.
TestsThe final argument specifies the alpha component of the color. It’s interpreted identically to the fourth argument of the rgb() function. If omitted, it defaults to 100%.
HSL colors resolve to sRGB.
If the saturation of an HSL color is 0% or 0, then the hue component is powerless.
For example, an ordinary red, the same color you would see from the keyword
redor the hex notation
#f00, is represented in HSL as
hsl(0deg 100% 50%).
An advantage of HSL over RGB is that it is more intuitive: people can guess at the colors they want, and then tweak.
For example, the following colors can all be generated off of the basic "green" hue, just by varying the other two arguments:
hsl(120deg 100% 50%) lime green hsl(120deg 100% 25%) dark green hsl(120deg 100% 75%) light green hsl(120deg 75% 85%) pastel green
A disadvantage of HSL over OKLCh is that hue manipulation changes the visual lightness, and that hues are not evenly spaced apart.
It is thus easier in HSL to create sets of matching colors (by keeping the hue the same and varying the saturation and lightness), compared to manipulating the sRGB component values; however, because the lightness is simply the mean of the gamma-corrected red, green and blue components it does not correspond to the visual perception of lightness across hues.
For example,
blueis represented in HSL as
hsl(240deg 100% 50%)while
yellowis
hsl(60deg 100% 50%). Both have an HSL Lightness of 50%, but clearly the yellow looks much lighter than the blue.
In OKLCh, sRGB blue is oklch(0.452 0.313 264.1) while sRGB yellow is oklch(0.968 0.211 109.8). The OKLCh Lightnesses of 0.452 and 0.968 clearly reflect the visual lightnesses of the two colors.
The hue angle in HSL is not perceptually uniform; colors appear bunched up in some areas and widely spaced in others.
For example, the pair of hues
hsl(220deg 100% 50%)and
hsl(250deg 100% 50%)have an HSL hue difference of 250-220 =
30deg and look fairly similar, while another pair of colors
hsl(50deg 100% 50%)and
hsl(80deg 100% 50%), which
alsohave a hue difference of 80-50 =
30deg, look very different.
In OKLCh, the same pair of colors oklch(0.533 0.26 262.6) and oklch(0.462 0.306 268.9) have a hue difference of 268.9 - 262.6 = 6.3deg while the second pair oklch(0.882 0.181 94.24) and oklch(0.91 0.245 129.9) have a hue difference of 129.9 - 94.24 = 35.66deg, correctly reflecting the visual separation of hues.
For historical reasons, hsl() and hsla() also support a legacy color syntax.
TestsConverting an HSL color to sRGB is straightforward mathematically. Here’s a sample implementation of the conversion algorithm in JavaScript. It returns an array of three numbers representing the red, green, and blue components of the colors, which for colors in the sRGB gamut will be in the range [0, 1].
This code assumes that parse-time clamping of negative saturation has already been applied.
/**
* @param {number} hue - Hue as degrees 0..360
* @param {number} sat - Saturation in reference range [0,100]
* @param {number} light - Lightness in reference range [0,100]
* @return {number[]} Array of sRGB components; in-gamut colors in range [0..1]
*/
function hslToRgb(hue, sat, light) {
sat /= 100;
light /= 100;
function f(n) {
let k = (n + hue/30) % 12;
let a = sat * Math.min(light, 1 - light);
return light - a * Math.max(-1, Math.min(k - 3, 9 - k, 1));
}
return [f(0), f(8), f(4)];
}7.2. Converting sRGB Colors to HSL
Conversion in the reverse direction proceeds similarly.
Special care is taken to deal with intermediate negative values of saturation, which can be produced by colors far outside the sRGB gamut.
/**
* @param {number} red - Red component 0..1
* @param {number} green - Green component 0..1
* @param {number} blue - Blue component 0..1
* @return {number[]} Array of HSL values: Hue as degrees 0..360, Saturation and Lightness in reference range [0,100]
*/
function rgbToHsl (red, green, blue) {
let max = Math.max(red, green, blue);
let min = Math.min(red, green, blue);
let [hue, sat, light] = [NaN, 0, (min + max)/2];
let d = max - min;
let epsilon = 1 / 100000; // max Sat is 1, in this code
if (d !== 0) {
sat = (light === 0 || light === 1)
? 0
: (max - light) / Math.min(light, 1 - light);
switch (max) {
case red: hue = (green - blue) / d + (green < blue ? 6 : 0); break;
case green: hue = (blue - red) / d + 2; break;
case blue: hue = (red - green) / d + 4;
}
hue = hue * 60;
}
// Very out of gamut colors can produce negative saturation
// If so, just rotate the hue by 180 and use a positive saturation
// see https://github.com/w3c/csswg-drafts/issues/9222
if (sat < 0) {
hue += 180;
sat = Math.abs(sat);
}
if (hue >= 360) {
hue -= 360;
}
if (sat <= epsilon) {
hue = NaN;
}
return [hue, sat * 100, light * 100];
}7.3. Examples of HSL Colors
This section is not normative.
TestsThis section is not normative, it does not need tests.
The tables below illustrate a wide range of possible HSL colors. Each table represents one hue, selected at 30° intervals, to illustrate the common "core" hues: red, yellow, green, cyan, blue, magenta, and the six intermediary colors between these.
In each table, the X axis represents the saturation while the Y axis represents the lightness.
0° Reds 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 30° Reds-Yellows (=Oranges) 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 60° Yellows 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 90° Yellow-Greens 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 120° Greens 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 150° Green-Cyans 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 180° Cyans 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 210° Cyan-Blues 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 240° blues 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 270° Blue-Magentas 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 300° Magentas 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 330° Magenta-Reds 100% 80% 60% 40% 20% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 8. HWB Colors: hwb() functionHWB (short for Hue-Whiteness-Blackness) [HWB] is another method of specifying sRGB colors, similar to HSL', but often even easier for humans to work with. It describes colors with a starting hue, then a degree of whiteness and blackness to mix into that base hue.
Many color-pickers are based on the HWB color system, due to its intuitiveness.
HWB colors resolve to sRGB.
This is a screenshot of Chrome’s color picker, shown when a user activates an < input type = "color" > . The outer wheel is used to select the hue, then the relative amounts of white and black are selected by clicking on the inner triangle.
The syntax of the hwb() function is:
hwb() = hwb( [<hue> | none] [<percentage> | <number> | none] [<percentage> | <number> | none] [ / [<alpha-value> | none] ]? )Percentages Allowed for W and B Percent reference range for W and B: 0% = 0.0, 100% = 100.0 Powerless hue ε W + B >= 99.999
The first argument specifies the hue, and is defined identically to hsl(); this means it suffers the same disadvantages such as hue uniformity.
The second argument specifies the amount of white to mix in, as a percentage from 0% (no whiteness) to 100% (full whiteness). Similarly, the third argument specifies the amount of black to mix in, also from 0% (no blackness) to 100% (full blackness).
For example,
hwb(150 20% 10%) is the same color as
hsl(150 77.78% 55%) and
rgb(20% 90% 55%).
Values outside of these ranges are not invalid; hue angles outside the range [0,360) will be normalized to that range and values of white and black which sum to 100% or greater will produce achromatic colors as described below.
The resulting color can be thought of conceptually as a mixture of paint in the chosen hue, white paint, and black paint, with the relative amounts of each determined by the percentages.
If the sum white+black is greater than or equal to 100%, it defines an achromatic color, i.e. a shade of gray; when converted to sRGB the R, G and B values are identical and have the value white / (white + black).
For example, in the color
hwb(45 40% 80%) white and black adds to 120, so this is an achromatic color whose R, G and B components are 40 / 40 + 80 = 0.33
rgb(33.33% 33.33% 33.33%).
Achromatic HWB colors no longer contain any hint of the chosen hue. In this case, the hue component is powerless.
The fourth argument specifies the alpha component of the color. It’s interpreted identically to the fourth argument of the rgb() function. If omitted, it defaults to 100%.
There is no Web compatibility issue with hwb, which is new in this level of the specification, and so hwb() does not support a legacy color syntax that separates all of its arguments with commas. Using commas inside hwb() is an error.
TestsConverting an HWB color to sRGB is straightforward, and related to how one converts HSL to RGB. The following Javascript implementation of the algorithm first normalizes the white and black components, so their sum is no larger than 100%.
/**
* @param {number} hue - Hue as degrees 0..360
* @param {number} white - Whiteness in reference range [0,100]
* @param {number} black - Blackness in reference range [0,100]
* @return {number[]} Array of RGB components 0..1
*/
function hwbToRgb(hue, white, black) {
white /= 100;
black /= 100;
if (white + black >= 1) {
let gray = white / (white + black);
return [gray, gray, gray];
}
let rgb = hslToRgb(hue, 100, 50);
for (let i = 0; i < 3; i++) {
rgb[i] *= (1 - white - black);
rgb[i] += white;
}
return rgb;
}8.2. Converting sRGB Colors to HWB
Conversion in the reverse direction proceeds similarly.
/**
* @param {number} red - Red component 0..1
* @param {number} green - Green component 0..1
* @param {number} blue - Blue component 0..1
* @return {number} Hue as degrees 0..360
*/
function rgbToHue(red, green, blue) {
// Similar to rgbToHsl, except that saturation and lightness are not calculated, and
// potential negative saturation is ignored.
let max = Math.max(red, green, blue);
let min = Math.min(red, green, blue);
let hue = NaN;
let d = max - min;
if (d !== 0) {
switch (max) {
case red: hue = (green - blue) / d + (green < blue ? 6 : 0); break;
case green: hue = (blue - red) / d + 2; break;
case blue: hue = (red - green) / d + 4;
}
hue *= 60;
}
if (hue >= 360) {
hue -= 360;
}
return hue;
}
/**
* @param {number} red - Red component 0..1
* @param {number} green - Green component 0..1
* @param {number} blue - Blue component 0..1
* @return {number[]} Array of HWB values: Hue as degrees 0..360, Whiteness and Blackness in reference range [0,100]
*/
function rgbToHwb(red, green, blue) {
let epsilon = 1 / 100000; // account for multiply by 100
var hue = rgbToHue(red, green, blue);
var white = Math.min(red, green, blue);
var black = 1 - Math.max(red, green, blue);
if (white + black >= 1 - epsilon) {
hue = NaN;
}
return([hue, white*100, black*100]);
}
8.3. Examples of HWB Colors
This section is not normative.
TestsThis section is not normative, it does not need tests.
This section is not normative.
TestsThis section is not normative, it does not need tests.
Physical measurements of a color are typically expressed in the CIE L*a*b* [CIELAB] color space, created in 1976 by the CIE and commonly referred to simply as Lab. Color conversions from one device to another may also use Lab as an intermediate step. Derived from human vision experiments, Lab represents the entire range of color that humans can see.
Lab is a rectangular coordinate system with a central Lightness (L) axis. This value is usually written as a unitless number; for compatibility with the rest of CSS, it may also be written as a percentage. 100% means an L value of 100, not 1.0. L=0% or 0 is deep black (no light at all) while L=100% or 100 is a diffuse white.
Usefully, L=50% or 50 is mid gray, by design, and equal increments in L are evenly spaced visually: the Lab color space is intended to be perceptually uniform.
This figure shows, to the left, the Lightness axis of the CIE Lab color space. Twenty-one neutral swatches are shown (L=0%, L=5%, to L=100%). The steps are equally spaced, visually. To the right, the same number of steps in luminance are equally spaced in light energy but not equally spaced visually.The a and b axes convey hue; positive values along the a axis are a purplish red while negative values are the complementary color, a green. Similarly, positive values along the b axis are yellow and negative are the complementary blue/violet. Desaturated colors have small values of a and b and are close to the L axis; saturated colors lie far from the L axis.
The illuminant is D50 white, a standardized daylight spectrum with a color temperature of 5000K, as reflected by a perfect diffuse reflector; it approximates the color of sunlight on a sunny day. D50 is also the whitepoint used for the profile connection space in ICC color interconversion, the whitepoint used in image editors which offer Lab editing, and the value used by physical measurement devices such as spectrophotometers and spectroradiometers, when they report measured colors in Lab.
Conversion from colors specified using other white points is called a chromatic adaptation transform, which models the changes in the human visual system as we adapt to a new lighting condition. The linear Bradford algorithm [ICC] (a simplification of the original Bradford algorithm [Bradford-CAT]) is the industry standard chromatic adaptation transform, and is easy to calculate as it is a simple matrix multiplication.
CIE LCH has the same L axis as Lab, but uses polar coordinates C (chroma) and H (hue), making it a polar, cylindrical coordinate system. C is the geometric distance from the L axis and H is the angle from the positive a axis, towards the positive b axis.
This figure shows the L=50 plane of the CIE Lab color space. 20 degree increments in CIE LCH are displayed as circles at three levels of Chroma: 20, 40 and 60. All the 20 Chroma colors fit inside sRGB gamut, some of 40 and 60 Chroma are outside. These out of gamut colors are visualized as grey, with a red warning outer stroke.Note: The L axis in Lab and LCH is not to be confused with the L axis in HSL. For example, in HSL, the sRGB colors blue (#00F) and yellow (#FF0) have the same value of L (50%) even though visually, blue is much darker. This is much clearer in Lab: sRGB blue is lab(29.567% 68.298 -112.0294) while sRGB yellow is lab(97.607% -15.753 93.388). In Lab and LCH, if two colors have the same measured L value, they have identical visual lightness. HSL and related polar RGB models were developed in an attempt to give similar usability benefits for RGB that LCH gave to Lab, but are significantly less accurate.
Although the use of CIE Lab and LCH is widespread, it is known to have some problems. In particular:
These deficiencies affect, for example, creation of evenly spaced gradients, gamut mapping from one color space to a smaller one, and computation of the visual difference between two colors.
To compensate for this, formulae to predict the visual difference between two colors (delta E) have been made more accurate over time (but also, much more complex to compute). The current industry standard formula, delta E 2000, works well to mitigate some of the Lab and LCH problems. A sample implementation is given in § 19.1 ΔE2000.
This does not help with hue curvature, however.
9.2. Oklab and OKLChThis section is not normative.
TestsThis section is not normative, it does not need tests.
Recently, Oklab, an improved Lab-like space has been developed [Oklab]. The corresponding polar form is called OKLCh. It was produced by numerical optimization of a large dataset of visually similar colors, and has improved hue linearity, hue uniformity, and chroma uniformity compared to CIE LCH.
Like CIE Lab, there is a central lightness L axis which is usually written as a unitless number in the range [0,1]; for compatibility with the rest of CSS, it may be written as a percentage. 100% means an L value of 1.0. L=0% or 0.0 is deep black (no light at all) while L=100% or 1.0 is a diffuse white.
Note: Unlike CIE Lab, which assumes adaptation to the diffuse white, Oklab assumes adaptation to the color being defined, which is intended to make it scale invariant.
As with CIE Lab, the a and b axes convey hue; positive values along the a axis are a purplish red while negative values are the complementary color, a green. Similarly, positive values along the b axis are yellow and negative are the complementary blue/violet.
The illuminant is D65, the same white point as most RGB color spaces.
OKLCh has the same L axis as Oklab, but uses polar coordinates C (chroma) and H (hue).
Note: Unlike CIE LCH, where Chroma can reach values of 200 or more, OKLCh Chroma ranges to 0.5 or so. The hue angles between CIE LCH and OKLCh are broadly similar, but not identical.
A constant CIE LCH hue slice, showing the sRGB gamut around primary blue. A noticeable purpling is immediately evident. A constant OKLCh hue slice, showing the sRGB gamut around primary blue. The visual hue remains constant.Because Oklab is more perceptually uniform than CIE Lab, the color difference is a straightforward distance in 3D space (root sum of squares). Although trivial, a sample implementation is give in § 19.2 ΔEOK.
9.3. Specifying Lab and LCH: the lab() and lch() functional notationsCSS allows colors to be directly expressed in Lab and LCH.
lab() = lab( [<percentage> | <number> | none]
[ <percentage> | <number> | none]
[ <percentage> | <number> | none]
[ / [<alpha-value> | none] ]? )
Percentages Allowed for L, a and b Percent reference range for L: 0% = 0.0, 100% = 100.0
In Lab, the first argument specifies the CIE Lightness, L. This is a number between 0% or 0 and 100% or 100 Values less than 0% or 0 must be clamped to 0% at parsed-value time; values greater than 100% or 100 are clamped to 100% at parsed-value time.
The second and third arguments are the distances along the "a" and "b" axes in the Lab color space, as described in the previous section. These values are signed (allow both positive and negative values) and theoretically unbounded (but in practice do not exceed ±160).
There is an optional fourth <alpha-value> component, separated by a slash, representing the alpha component.
If the lightness of a Lab color (after clamping) is 0%, or 100% the color will be displayed as black, or white, respectively due to gamut mapping to the display.
lab(29.2345% 39.3825 20.0664); lab(52.2345 40.1645 59.9971); lab(60.2345 -5.3654 58.956); lab(62.2345% -34.9638 47.7721); lab(67.5345 -8.6911 -41.6019); lab(29.69% 44.888% -29.04%)
lch() = lch( [<percentage> | <number> | none]
[ <percentage> | <number> | none]
[ <hue> | none]
[ / [<alpha-value> | none] ]? )
Percentages Allowed for L and C Percent reference range for L: 0% = 0.0, 100% = 100.0
In CIE LCH the first argument specifies the CIE Lightness L, interpreted identically to the Lightness argument of lab().
The second argument is the chroma C, (roughly representing the "amount of color"). Its minimum useful value is 0, while its maximum is theoretically unbounded (but in practice does not exceed 230). If the provided value is negative, it is clamped to 0 at parsed-value time.
The third argument is the hue angle H. It’s interpreted similarly to the <hue> argument of hsl(), but doesn’t map hues to angles in the same way because they are evenly spaced perceptually. Instead, 0deg points along the positive "a" axis (toward purplish red), (as does 360deg, 720deg, etc.); 90deg points along the positive "b" axis (toward mustard yellow), 180deg points along the negative "a" axis (toward greenish cyan), and 270deg points along the negative "b" axis (toward sky blue).
There is an optional fourth <alpha-value> component, separated by a slash, representing the alpha component.
If the chroma of an LCH color is 0%, the hue component is powerless. If the lightness of an LCH color (after clamping) is 0%, or 100%, the color will be displayed as black, or white, respectively due to gamut mapping to the display.
lch(29.2345% 44.2 27); lch(52.2345% 72.2 56.2); lch(60.2345 59.2 95.2); lch(62.2345% 59.2 126.2); lch(67.5345% 42.5 258.2); lch(29.69% 45.553% 327.1)
There is no Web compatibility issue with lab or lch', which are new in this level of the specification, and so lab() and lch() do not support a legacy color syntax that separates all of their arguments with commas. Using commas inside these functions is an error.
9.4. Specifying Oklab and OKLCh: the oklab() and oklch() functional notationsCSS allows colors to be directly expressed in Oklab and OKLCh.
oklab() = oklab( [ <percentage> | <number> | none]
[ <percentage> | <number> | none]
[ <percentage> | <number> | none]
[ / [<alpha-value> | none] ]? )
Percentages Allowed for L, a and b Percent reference range for L: 0% = 0.0, 100% = 1.0
In Oklab the first argument specifies the Oklab Lightness. This is a number between 0% or 0 and 100% or 1.0.
Values less than 0% or 0.0 must be clamped to 0% at parsed-value time; values greater than 100% or 1.0 are clamped to 100% at parsed-value time.
The second and third arguments are the distances along the "a" and "b" axes in the Oklab color space, as described in the previous section. These values are signed (allow both positive and negative values) and theoretically unbounded (but in practice do not exceed ±0.5).
There is an optional fourth <alpha-value> component, separated by a slash, representing the alpha component.
If the lightness of an Oklab color is 0% or 0, or 100% or 1.0, the color will be displayed as black, or white, respectively due to gamut mapping to the display.
oklab(40.101% 0.1147 0.0453); oklab(59.686% 0.1009 0.1192); oklab(0.65125 -0.0320 0.1274); oklab(66.016% -0.1084 0.1114); oklab(72.322% -0.0465 -0.1150); oklab(42.1% 41% -25%)
oklch() = oklch( [ <percentage> | <number> | none]
[ <percentage> | <number> | none]
[ <hue> | none]
[ / [<alpha-value> | none] ]? )
Percentages Allowed for L and C Percent reference range for L: 0% = 0.0, 100% = 1.0
In OKLCh the first argument specifies the OKLCh Lightness L, interpreted identically to the Lightness argument of oklab().
The second argument is the chroma C. Its minimum useful value is 0, while its maximum is theoretically unbounded (but in practice does not exceed 0.5). If the provided value is negative, it is clamped to 0 at parsed-value time.
The third argument is the hue angle H. It’s interpreted similarly to the <hue> arguments of hsl() and lch(), but doesn’t map hues to angles in the same way. 0deg points along the positive "a" axis (toward purplish red), (as does 360deg, 720deg, etc.); 90deg points along the positive "b" axis (toward mustard yellow), 180deg points along the negative "a" axis (toward greenish cyan), and 270deg points along the negative "b" axis (toward sky blue).
There is an optional fourth <alpha-value> component, separated by a slash, representing the alpha component.
If the chroma of an OKLCh color is 0% or 0, the hue component is powerless. If the lightness of an OKLCh color is 0% or 0, or 100% or 1.0, the color will be displayed as black, or white, respectively due to gamut mapping to the display.
oklch(40.101% 0.12332 21.555); oklch(59.686% 0.15619 49.7694); oklch(0.65125 0.13138 104.097); oklch(0.66016 0.15546 134.231); oklch(72.322% 0.12403 247.996); oklch(42.1% 48.25% 328.4)
There is no Web compatibility issue with oklab or oklch', which are new in this level of the specification, and so oklab() and oklch() do not support a legacy color syntax that separates all of their arguments with commas. Using commas inside these functions is an error.
9.5. Converting Lab or Oklab colors to LCH or OKLCh colorsConversion to the polar form is trivial:
For extremely small values of a and b (near-zero Chroma), although the visual color does not change from being on the neutral axis, small changes to the values can result in the reported hue angle swinging about wildly and being essentially random. In CSS, this means the hue is powerless, and treated as missing when converted into LCH or OKLCh; in non-CSS contexts this might be reflected as a missing value, such as NaN.
9.6. Converting LCH or OKLCh colors to Lab or Oklab colorsConversion to the rectangular form is trivial:
CSS provides several predefined color spaces including display-p3 [Display-P3], which is a wide gamut space typical of current wide-gamut monitors, prophoto-rgb, widely used by photographers and rec2020 [Rec.2020], which is a broadcast industry standard, ultra-wide gamut space capable of representing almost all visible real-world colors.
10.1. Specifying Predefined Colors: the color() functionThe color() function allows a color to be specified in a particular, specified color space (rather than the implicit sRGB color space that most of the other color functions operate in). Its syntax is:
color() = color( <colorspace-params> [ / [ <alpha-value> | none ] ]? )
<colorspace-params> = [ <predefined-rgb-params> | <xyz-params>]
<predefined-rgb-params> = <predefined-rgb> [ <number> | <percentage> | none ]{3}
<predefined-rgb> = srgb | srgb-linear | display-p3 | a98-rgb | prophoto-rgb | rec2020
<xyz-params> = <xyz-space> [ <number> | <percentage> | none ]{3}
<xyz-space> = xyz | xyz-d50 | xyz-d65
Tests
The color function takes parameters specifying a color, in an explicitly listed color space.
It represents either an invalid color, as described below, or a valid color.
The parameters have the following form:
An <ident> denoting one of the predefined color spaces (such as display-p3) Individual predefined color spaces may further restrict whether <number>s or <percentage>s or both, may be used.
If the <ident> names a non-existent color space (a name that does not match one of the predefined color spaces), this argument represents an invalid color.
The three parameter values that the color space takes (RGB or XYZ values).
An out of gamut color has component values less than 0 or 0%, or greater than 1 or 100%. These are not invalid, and are retained for intermediate computations; instead, for display, they are css gamut mapped using a relative colorimetric intent which brings the values (in the display color space) within the range 0/0% to 1/100% at actual-value time.
An optional slash-separated <alpha-value>.
There is no Web compatibility issue with color(), which is new in this level of the specification, and so color() does not support a legacy color syntax that separates all of its arguments with commas. Using commas inside this function is an error.
A color which is either an invalid color or an out of gamut color can’t be displayed.
If the specified color can be displayed, (that is, it isn’t an invalid color and isn’t out of gamut) then this is the actual value of the color() function.
If the specified color is a valid color but can’t be displayed, the actual value is derived from the specified color, css gamut mapped for display.
If the color is an invalid color, the used value is opaque black.
This very intense lime color is in-gamut for rec.2020:
color(rec2020 0.42053 0.979780 0.00579);
in LCH, that color is
lch(86.6146% 160.0000 136.0088);
in display-p3, that color is
color(display-p3 -0.6112 1.0079 -0.2192);
and is out of gamut for display-p3 (red and blue are negative, green is greater than 1). If you have a display-p3 screen, that color is:
The color used for display will be a less intense color produced automatically by gamut mapping.
This example has a typo! An intense green is provided in profoto-rgb space (which doesn’t exist). This makes it invalid, so the used value is
opaque blackcolor(profoto-rgb 0.4835 0.9167 0.2188)10.2. The Predefined sRGB Color Space: the sRGB keyword
The sRGB predefined color space defined below is the same as is used for legacy sRGB colors, such as rgb().
[SRGB] specifies two viewing conditions, encoding and typical. The [ICC] recommends using the encoding conditions for color conversion and for optimal viewing, which are the values in the table below.
sRGB is the default color space for CSS, used for all the legacy color functions.
It has the following characteristics:
x y Red chromaticity 0.640 0.330 Green chromaticity 0.300 0.600 Blue chromaticity 0.150 0.060 White chromaticity D65 Transfer function see below White luminance 80.0 cd/m2 Black luminance 0.20 cd/m2 Image state display-referred Percentages Allowed for R, G and B Percent reference range for R,G,B: 0% = 0.0, 100% = 1.0let sign = c < 0? -1 : 1;
let abs = Math.abs(c);
if (abs <= 0.04045) {
cl = c / 12.92;
}
else {
cl = sign * (Math.pow((abs + 0.055) / 1.055, 2.4));
}
c is the gamma-encoded red, green or blue component. cl is the corresponding linear-light component.
Visualization of the sRGB color space in LCH. The primaries and secondaries are shown.The sRGB-linear predefined color space is the same as srgb except that the transfer function is linear-light (there is no gamma-encoding).
It has the following characteristics:
x y Red chromaticity 0.640 0.330 Green chromaticity 0.300 0.600 Blue chromaticity 0.150 0.060 White chromaticity D65 Transfer function unity, see below White luminance 80.0 cd/m2 Black luminance 0.20 cd/m2 Image state display-referred Percentages Allowed for R, G and B Percent reference range for R,G,B: 0% = 0.0, 100% = 1.0cl = c;
c is the red, green or blue component. cl is the corresponding linear-light component, which is identical.
To avoid banding artifacts, a higher precision is required for srgb-linear than for srgb.
For example, these are the same color
color(srgb 0.691 0.139 0.259) color(srgb-linear 0.435 0.017 0.055)
Modern displays, TVs, laptop screens and phone screens are able to display all, or nearly all, of the display-p3 gamut.
It has the following characteristics:
x y Red chromaticity 0.680 0.320 Green chromaticity 0.265 0.690 Blue chromaticity 0.150 0.060 White chromaticity D65 Transfer function same as srgb White luminance 80.0 cd/m2 Black luminance 0.80 cd/m2 Image state display-referred Percentages Allowed for R, G and B Percent reference range for R,G,B: 0% = 0.0, 100% = 1.0 Visualization of the P3 color space in LCH. The primaries and secondaries are shown (but in sRGB, not in the correct colors). For comparison, the sRGB primaries and secondaries are also shown, as dashed circles. P3 primaries have higher Chroma.It has the following characteristics:
x y Red chromaticity 0.6400 0.3300 Green chromaticity 0.2100 0.7100 Blue chromaticity 0.1500 0.0600 White chromaticity D65 Transfer function 256/563 White luminance 160.0 cd/m2 Black luminance 0.5557 cd/m2 Image state display-referred Percentages Allowed for R, G and B Percent reference range for R,G,B: 0% = 0.0, 100% = 1.0 Visualization of the A98 color space in LCH. The primaries and secondaries are shown (but in sRGB, not in the correct colors). For comparison, the sRGB primaries and secondaries are also shown, as dashed circles. a98 primaries have higher Chroma, especially the yellow, green and cyan.The ProPhoto RGB space uses hyper-saturated, non physically realizable primaries. These were chosen to allow a wide color gamut and in particular, to minimize hue shifts under tonal manipulation. It is often used in digital photography as a wide gamut color space for the archival version of photographic images. The prophoto-rgb color space allows CSS to specify colors that will match colors in such images having the same RGB values.
The ProPhoto RGB space was originally developed by Kodak and is described in [Wolfe]. It was standardized by ISO as [ROMM],[ROMM-RGB].
The white luminance is given as a range, and the viewing flare (and thus, the black luminance) is 0.5% to 1.0% of this.
It has the following characteristics:
x y Red chromaticity 0.734699 0.265301 Green chromaticity 0.159597 0.840403 Blue chromaticity 0.036598 0.000105 White chromaticity D50 Transfer function see below White luminance 160.0 to 640.0 cd/m2 Black luminance See text Image state display-referred Percentages Allowed for R, G and B Percent reference range for R,G,B: 0% = 0.0, 100% = 1.0
const E = 16/512;
let sign = c < 0? -1 : 1;
let abs = Math.abs(c);
if (abs <= E) {
cl = c / 16;
}
else {
cl = sign * Math.pow(c, 1.8);
}
c is the gamma-encoded red, green or blue component. cl is the corresponding linear-light component.
Visualization of the prophoto-rgb color space in LCH. The primaries and secondaries are shown (but in sRGB, not in the correct colors). For comparison, the sRGB primaries and secondaries are also shown, as dashed circles. prophoto-rgb primaries and secondaries have much higher Chroma, but much of this ultrawide gamut does not correspond to physically realizable colors.The primaries are physically realizable, but with difficulty as they lie very close to the spectral locus.
Current displays are unable to reproduce the full gamut of rec2020. Coverage is expected to increase over time as displays improve.
It has the following characteristics:
x y Red chromaticity 0.708 0.292 Green chromaticity 0.170 0.797 Blue chromaticity 0.131 0.046 White chromaticity D65 Transfer function see below, from [Rec.2020] table 4 Image state display-referred Percentages Allowed for R, G and B Percent reference range for R,G,B: 0% = 0.0, 100% = 1.0const α = 1.09929682680944 ;
const β = 0.018053968510807;
let sign = c < 0? -1 : 1;
let abs = Math.abs(c);
if (abs < β * 4.5 ) {
cl = c / 4.5;
}
else {
cl = sign * (Math.pow((abs + α -1 ) / α, 1/0.45));
}
c is the gamma-encoded red, green or blue component. cl is the corresponding linear-light component.
Visualization of the rec2020 color space in LCH. The primaries and secondaries are shown (but in sRGB, not in the correct colors). For comparison, the sRGB primaries and secondaries are also shown, as dashed circles. rec2020 primaries have much higher Chroma.The reference white for xyz-d50 is D50, while the reference white for xyz-d65 and xyz is D65.
Values greater than 1.0/100% are allowed and must not be clamped; colors where Y is greater than 1.0 represent colors brighter than diffuse white. Values less than 0/0% are uncommon, but can occur as a result of chromatic adaptation, and likewise must not be clamped.
It has the following characteristics:
Percentages Allowed for X,Y,Z Percent reference range for X,Y,Z: 0% = 0.0, 100% = 1.0These are exactly equivalent:
#7654CD rgb(46.27% 32.94% 80.39%) lab(44.36% 36.05 -58.99) color(xyz-d50 0.2005 0.14089 0.4472) color(xyz-d65 0.21661 0.14602 0.59452)
These colors are exactly equivalent, and represent white:
#FFFFFF color(xyz-d50 0.9643 1 0.8251) color(xyz-d65 0.9505 1 1.089)
For all predefined RGB color spaces, conversion to Lab requires several steps, although in practice all but the first step are linear calculations and can be combined.
Conversion to Oklab is similar, but the chromatic adaptation step is only needed for prophoto-rgb.
There is sample JavaScript code for these conversions in § 18 Sample code for Color Conversions.
10.10. Converting Lab or Oklab to Predefined RGB Color SpacesConversion from Lab to predefined spaces like display-p3 or rec2020 also requires multiple steps, and again in practice all but the last step are linear calculations and can be combined.
Conversion from Oklab is similar, but the chromatic adaptation step is only needed for prophoto-rgb.
There is sample JavaScript code for these conversions in § 18 Sample code for Color Conversions.
Implementations may choose to implement these steps in some other way (for example, using an ICC profile with relative colorimetric rendering intent) provided the results are the same for colors inside both the source and destination gamuts.
10.11. Converting Between Predefined RGB Color SpacesConversion from one predefined RGB color space to another requires multiple steps, one of which is only needed when the whitepoints differ. To convert from src to dest:
There is sample JavaScript code for this conversion for the predefined RGB color spaces, in § 18 Sample code for Color Conversions.
10.12. Simple Alpha CompositingWhen drawing, implementations must handle alpha according to the rules in Section 5.1 Simple alpha compositing of [Compositing].
11. Converting Colors TestsThis section provides an algorithm used later, it does not need tests.
Colors may be converted from one color space to another and, provided that there is no gamut mapping and that each color space can represent out of gamut colors, (for RGB spaces, this means that the transfer function is defined over the extended range) then (subject to numerical precision and round-off error) the two colors will look the same and represent the same color sensation.
To convert a color col1 in a source color space src with white point src-white to a color col2 in destination color space dest with white point dest-white:
Color interpolation happens with gradients, compositing, filters, transitions, animations, and color mixing and color modification functions.
Interpolation between two <color> values takes place by executing the following steps:
Interpolating to or from currentcolor is possible. The numerical value used for this purpose is the used value.
12.1. Color Space for InterpolationVarious features in CSS depend on interpolating colors.
Mixing or otherwise combining colors has different results depending on the interpolation color space used. Thus, different color spaces may be more appropriate for each interpolation use case.
In some cases, the result of physically mixing two colored lights is desired. In that case, the CIE XYZ or srgb-linear color space is appropriate, because they are linear in light intensity.
If colors need to be evenly spaced perceptually (such as in a gradient), the Oklab color space (and the older Lab), are designed to be perceptually uniform.
If avoiding graying out in color mixing is desired, i.e. maximizing chroma throughout the transition, OKLCh (and the older LCH) work well for that.
Lastly, compatibility with legacy Web content may be the most important consideration. The sRGB color space, which is neither linear-light nor perceptually uniform, is the choice here, even though it produces poorer results (overly dark or greyish mixes).
These features are collectively termed the host syntax.
To permit a host syntax to indicate the interpolation color space, this specification exports a color-interpolation-method production. It is not used by this specification itself, only exposed so that other specifications can use it; see e.g. use in CSS Images 4 § 3.1 Linear Gradients: the linear-gradient() notation.
The host syntax should define what the default interpolation color space should be for each case, and preferably provide syntax for authors to override this default. If such syntax is part of a property value, it should use the color-interpolation-method production, defined below for easy reference from other specifications. This ensures consistency across CSS, and that further customizations on how color interpolation is performed can automatically percolate across all of CSS.
<color-space> = <rectangular-color-space> | <polar-color-space> <rectangular-color-space> = srgb | srgb-linear | display-p3 | a98-rgb | prophoto-rgb | rec2020 | lab | oklab | xyz | xyz-d50 | xyz-d65 <polar-color-space> = hsl | hwb | lch | oklch <hue-interpolation-method> = [ shorter | longer | increasing | decreasing ] hue <color-interpolation-method> = in [ <rectangular-color-space> | <polar-color-space> <hue-interpolation-method>? ]
The keywords in the definitions of <rectangular-color-space> and <polar-color-space> each refer to their corresponding color space, represented in CSS either by the functional syntax with the same name, or (if no such function is present), by the corresponding <ident> in the color() function.
TestsIf the host syntax does not define what color space interpolation should take place in, it defaults to Oklab.
Authors that prefer interpolation in sRGB in a particular instance can opt-in to the old behavior by explicitly specifying sRGB as the interpolation color space, for example on a particular gradient where that result is desired.
If the colors to be interpolated are outside the gamut of the interpolation color space , then once converted to that space, they will contain out of range values.
These are not clipped, but the values are interpolated as-is.
12.2. Interpolating with Missing ComponentsIn the course of converting the two colors to the interpolation color space, any missing components would be replaced with the value 0.
Thus, the first stage in interpolating two colors is to classify any missing components in the input colors, and compare them to the components of the interpolation color space. If any analogous components which are missing components are found, they will be carried forward and re-inserted in the converted color before premultiplication, and before linear interpolation takes place.
The analogous components are as follows:
Category Components Reds r,x Greens g,y Blues b,z Lightness L Colorfulness C, S Hue H Opponent a a Opponent b b Alpha alphaNote: for the purposes of this classification, the XYZ spaces are considered super-saturated RGB spaces. Also, despite Saturation being Lightness-dependent, it falls in the same category as Chroma here. The Whiteness and Blackness components of HWB have no analogs in other color spaces.
For example, if these two colors are to be interpolated in OKLCh, the missing hue in the CIE LCH color is analogous to the hue component of OKLCh and will be carried forward while the missing blue component in the second color is not analogous to any OKLCh component and will not be carried forward:
lch(50% 0.02 none) color(display-p3 0.7 0.5 none)
which convert to
oklch(56.897% 0.0001 0) oklch(63.612% 0.1522 78.748)
and with carried forward missing component re-inserted, the two colors to be interpolated are:
oklch(56.897% 0.0001 none) oklch(63.612% 0.1522 78.748)
If a color with a carried forward missing component is interpolated with another color which is not missing that component, the missing component is treated as having the other color’s component value.
Therefore, the carrying-forward step must be performed before any powerless component handling.
For example, if these two colors are interpolated, the second of which has a missing hue:
oklch(78.3% 0.108 326.5) oklch(39.2% 0.4 none)
Then the actual colors to be interpolated are
oklch(78.3% 0.108 326.5) oklch(39.2% 0.4 326.5)
and not
oklch(78.3% 0.108 326.5) oklch(39.2% 0.4 0)
If the carried forward missing component is alpha, the color must be premultiplied with this carried forward value, not with the zero value that would have resulted from color conversion.
For example, if these two colors are interpolated, the second of which has a missing alpha:
oklch(0.783 0.108 326.5 / 0.5) oklch(0.392 0.4 0 / none)
Then the actual colors to be interpolated are
oklch(78.3% 0.108 326.5 / 0.5) oklch(39.2% 0.4 0 / 0.5)
giving the premultiplied OKLCh values [0.3915, 0.054, 326] and [0.196, 0.2, 0].
If both colors are missing a given component, the interpolated color will also be missing that component.
12.3. Interpolating with AlphaWhen the colors to be interpolated are not fully opaque, they are first premultiplied as follows:
If the alpha value is none, the premultiplied value is the un-premultiplied value. Otherwise,
If any component value is none, the premultiplied value is also none.
For rectangular orthogonal color coordinate systems, all component values are multiplied by the alpha value.
For cylindrical polar color coordinate systems, the hue angle is not premultiplied, but the other two axes are premultiplied.
To obtain a color value from a premultiplied color value,
If the interpolated alpha value is zero or none, the un-premultiplied value is the premultiplied value. Otherwise,
If any component value is none, the un-premultiplied value is also none.
otherwise, each component which had been premultiplied is divided by the interpolated alpha value.
Interpolating colors using the premultiplied representations tends to produce more attractive transitions than the non-premultiplied representations, particularly when transitioning from a fully opaque color to fully transparent.
Note that transitions where either the transparency or the color are held constant (for example, transitioning between rgba ( 255 , 0 , 0 , 100 % ) (opaque red) and rgba ( 0 , 0 , 255 , 100 % ) (opaque blue), or rgba ( 255 , 0 , 0 , 100 % ) (opaque red) and rgba ( 255 , 0 , 0 , 0 % ) (transparent red)) have identical results whether the color interpolation is done in premultiplied or non-premultiplied color-space. Differences only arise when both the color and transparency differ between the two endpoints.
The following example illustrates the difference between a gradient transitioning via pre-multiplied values (in this case sRGB, since all colors involved are legacy colors) and one transitioning (incorrectly) via non-premultiplied values. In both of these examples, the gradient is drawn over a white background. Both gradients could be written with the following value:
linear-gradient(90deg, red, transparent, blue)
With premultiplied colors, transitions to or from "transparent" always look nice:
On the other hand, if a gradient were to incorrectly transition in non-premultiplied space, the center of the gradient would be a noticeably grayish color, because "transparent" is actually a shorthand for rgba(0,0,0,0), or transparent black, meaning that the red transitions to a black as it loses opacity, and similarly with the blue’s transition:
For example, to interpolate, in the sRGB color space, the two sRGB colors
rgb(24% 12% 98% / 0.4) and
rgb(62% 26% 64% / 0.6) they would first be converted to premultiplied form [9.6% 4.8% 39.2% ] and [37.2% 15.6% 38.4%] before interpolation.
The midpoint of linearly interpolating these colors would be [23.4% 10.2% 38.8%] which, with an alpha value of 0.5, is rgb(46.8% 20.4% 77.6% / 0.5) when premultiplication is undone.
To interpolate, in the Lab color space, the two colors
rgb(76% 62% 03% / 0.4) and
color(display-p3 0.84 0.19 0.72 / 0.6) they are first converted to lab
lab(66.927% 4.873 68.622 / 0.4)
lab(53.503% 82.672 -33.901 / 0.6) then the L, a and b coordinates are premultiplied before interpolation [26.771% 1.949 27.449] and [32.102% 49.603 -20.341].
The midpoint of linearly interpolating these would be [29.4365% 25.776 3.554] which, with an alpha value of 0.5, is lab(58.873% 51.552 7.108) / 0.5) when premultiplication is undone.
To interpolate, in the chroma-preserving LCH color space, the same two colors
rgb(76% 62% 03% / 0.4) and
color(display-p3 0.84 0.19 0.72 / 0.6) they are first converted to LCH
lch(66.93% 68.79 85.94 / 0.4)
lch(53.5% 89.35 337.7 / 0.6) then the L and C coordinates (but not H) are premultiplied before interpolation [26.771% 27.516 85.94] and [32.102% 53.61 337.7].
The midpoint of linearly interpolating these, along the shorter hue arc (the default) would be [29.4365% 40.563 31.82] which, with an alpha value of 0.5, is lch(58.873% 81.126 31.82) / 0.5) when premultiplication is undone.
There is sample JavaScript code for alpha premultiplication and un-premultiplication, for both polar and rectangular color spaces, in § 18 Sample code for Color Conversions.
12.4. Hue InterpolationFor color functions with a hue angle (LCH, HSL, HWB etc), there are multiple ways to interpolate. As arcs greater than 360° are rarely desirable, hue angles are fixed up prior to interpolation so that per-component interpolation is done over less than 360°, often less than 180°.
Host syntax can specify any of the following algorithms for hue interpolation (angles in the following are in degrees, but the logic is the same regardless of how they are specified). Specifying a hue interpolation strategy is already part of the <color-interpolation-method> syntax via the <hue-interpolation-method> token.
Unless otherwise specified, if no specific hue interpolation algorithm is selected by the host syntax, the default is shorter.
TestsNote: As a reminder, if the interpolating colors were not already in the specified interpolation color space, then converting them will turn any powerless components into missing components.
12.4.1. shorterHue angles are interpolated to take the shorter of the two arcs between the starting and ending hues.
For example, the midpoint when interpolating in OKLCh from a red
oklch(0.6 0.24 30) to a yellow
oklch(0.8 0.15 90) would be at a hue angle of 30 + (90 - 30) * 0.5 = 60 degrees, along the shorter arc between the two colors, giving a deep orange
oklch(0.7 0.195 60)
Angles are adjusted so that θ₂ - θ₁ ∈ [-180, 180]. In pseudo-Javascript:
if (θ₂ - θ₁ > 180) {
θ₁ += 360;
}
else if (θ₂ - θ₁ < -180) {
θ₂ += 360;
}
12.4.2. longer
Hue angles are interpolated to take the longer of the two arcs between the starting and ending hues.
For example, the midpoint when interpolating in OKLCh from a red
oklch(0.6 0.24 30) to a yellow
oklch(0.8 0.15 90) would be at a hue angle of (30 + 360 + 90) * 0.5 = 240 degrees, along the longer arc between the two colors, giving a sky blue
oklch(0.7 0.195 240)
Angles are adjusted so that θ₂ - θ₁ ∈ {(-360, -180], [180, 360)}. In pseudo-Javascript:
if (0 < θ₂ - θ₁ < 180) {
θ₁ += 360;
}
else if (-180 < θ₂ - θ₁ <= 0) {
θ₂ += 360;
}
12.4.3. increasing
Hue angles are interpolated so that, as they progress from the first color to the second, the angle is always increasing. If the angle increases to 360 it is reset to zero, and then continues increasing.
Depending on the difference between the two angles, this will either look the same as shorter or as longer. However, if one of the hue angles is being animated, and the hue angle difference passes through 180 degrees, the interpolation will not flip to the other arc.
For example, the midpoint when interpolating in OKLCh from a deep brown
oklch(0.5 0.1 30) to a turquoise
oklch(0.7 0.1 190) would be at a hue angle of (30 + 190) * 0.5 = 110 degrees, giving a khaki
oklch(0.6 0.1 110).
However, if the hue of the second color is animated to oklch(0.7 0.1 230), the midpoint of the interpolation will be (30 + 230) * 0.5 = 130 degrees, continuing in the same increasing direction, giving another green oklch(0.6 0.1 130) rather than flipping to the opponent color part-way through the animation.
Angles are adjusted so that θ₂ - θ₁ ∈ [0, 360). In pseudo-Javascript:
if (θ₂ < θ₁) {
θ₂ += 360;
}
12.4.4. decreasing
Hue angles are interpolated so that, as they progress from the first color to the second, the angle is always decreasing. If the angle decreases to 0 it is reset to 360, and then continues decreasing.
Depending on the difference between the two angles, this will either look the same as shorter or as longer. However, if one of the hue angles is being animated, and the hue angle difference passes through 180 degrees, the interpolation will not flip to the other arc.
For example, the midpoint when interpolating in OKLCh from a deep brown
oklch(0.5 0.1 30) to a turquoise
oklch(0.7 0.1 190) would be at a hue angle of (30 + 360 + 190) * 0.5 = 290 degrees, giving a purple
oklch(0.6 0.1 290).
However, if the hue of the second color is animated to oklch(0.7 0.1 230), the midpoint of the interpolation will be (30 + 360 + 230) * 0.5 = 310 degrees, continuing in the same decreasing direction, giving another purple oklch(0.6 0.1 310) rather than flipping to the opponent color part-way through the animation.
Angles are adjusted so that θ₂ - θ₁ ∈ (-360, 0]. In pseudo-Javascript:
if (θ₁ < θ₂) {
θ₁ += 360;
}
13. Gamut Mapping 13.1. An Introduction to Gamut Mapping
Note: This section provides important context for the specific requirements described elsewhere in the document.
This section is non-normative
TestsThis section is not normative, it does not need tests.
When a color in an origin color space is converted to another, destination color space which has a smaller gamut, some colors will be outside the destination gamut.
For intermediate color calculations, these out of gamut values are preserved. However, if the destination is the display device (a screen, or a printer) then out of gamut values must be converted to an in-gamut color.
Gamut mapping is the process of finding an in-gamut color with the least objectionable change in visual appearance.
13.1.1. ClippingThe simplest and least acceptable method is simply to clip the component values to the displayable range. This changes the proportions of the three primary colors (for an RGB display), resulting in a hue shift.
For example, consider the color
color ( srgb-linear 0.5 1 3 )
. Because this is a linear-light color space, we can compare the intensities of the three components and see that the amount of blue light is three times the amount of green, while the amount of red light is half that of green. There is six times as much blue primary as red. In OKLCh, this color has a hue angle of 265.1°
If we now clip this color to bring it into gamut for sRGB, we get color ( srgb-linear 0.5 1 1 ) . The amount of blue light is the same as green. In OKLCh, this color has a hue angle of 196.1°, a substantial change of 69°.
A better method is to map colors, in a perceptually uniform color space, by finding the closest in-gamut color (so-called minimum ΔE or MINDE). Clearly, the success of this technique depends on the degree of uniformity of the gamut mapping color space and the predictive accuracy of the deltaE function used.
However, when doing gamut mapping changes in Hue are particularly objectionable; changes in Chroma are more tolerable, and small changes in Lightness can also be acceptable especially if the alternative is a larger Chroma reduction. MINDE weights changes in each dimension equally, and thus gives suboptimal results.
13.1.3. Chroma ReductionTo improve on MINDE algorithms, colors are mapped in a perceptually uniform, polar color space by holding the hue constant, and reducing the chroma until the color falls in gamut.
In this example, Display P3 primary yellow (
color ( display-p3 1 1 0 )
) is being mapped to an sRGB display. The gamut mapping color space is OKLCh.
color(display-p3 1 1 0)
is
color(srgb 1 1 -0.3463)
which is
color(oklch 0.96476 0.24503 110.23)
By progressively reducing the chroma component until the resulting color falls inside the sRGB gamut (has no components negative, or greater than one) a gamut mapped color is obtained.
color(oklch 0.96476 0.21094 110.23)
which is
color(srgb 0.99116 0.99733 0.00001)A constant-hue slice of OKLCh color space. The vertical axis represents lightness, the horizontal axis is chroma. The color to be mapped, shown as a yellow circle, has the chroma reduced while keeping hue and lightness constant. The color therefore moves along the maroon line in the diagram, towards the neutral axis on the left. The gamut boundary of sRGB is shown in green.
A practical implementation will converge more quickly than a linear reduction; either by binary search, or by computing the geometric intersection of the line of constant hue and lightness with the gamut boundary.
13.1.4. Excessive Chroma ReductionAlso, this simple approach will give sub-optimal results for certain colors, principally very light colors like yellow and cyan, if the upper edge of the gamut boundary is shallow, or even slightly concave. The line of constant lightness can skim just above the gamut boundary, resulting in an excessively low chroma in those cases.
The choice of color space will affect the acceptability of the gamut mapped colors.
In this example, Display P3 primary yellow (
color ( display-p3 1 1 0
) has the chroma progressively reduced in CIE LCH color space.
In the upper part of this diagram, colors which are inside the gamut of sRGB are displayed as-is. Colors inside the gamut of Display P3 (but outside sRGB) are in salmon. Colors outside the gamut of Display P3 are in red. The lower part of the diagram shows the linear-light intensities of the Display P3 red, green and blue components.It can be seen that reduction in CIE LCH chroma makes the red intensity curve up, out of Display P3 gamut; by the time it falls again the chroma is very low. Simple gamut mapping in CIE LCH would give unsatisfactory results.
In this example, Display P3 primary yellow (
color ( display-p3 1 1 0
) has the chroma progressively reduced, but this time in OKLCh color space.
In the upper part of this diagram, colors which are inside the gamut of sRGB are displayed as-is. Colors inside the gamut of Display P3 (but outside sRGB) are in salmon. Colors outside the gamut of Display P3 are in red. The lower part of the diagram shows the linear-light intensities of the Display P3 red, green and blue components.It can be seen that reduction in OKLCh chroma is better behaved. Colors do not go outside the Display P3 gamut, and the resulting gamut-mapped yellow has good chroma. Simple gamut mapping in OK LCH would give acceptable results.
13.1.5. Chroma Reduction with Local ClippingThe simple chroma-reduction algorithm can be improved: at each step, the color difference is computed between the current mapped color and a clipped version of that color. If the current color is outside the gamut boundary, but the color difference between it and the clipped version is below the threshold for a just noticeable difference (JND), the clipped version of the color is returned as the mapped result. Effectively, this is doing a MINDE mapping at each stage, but constrained so the hue and lightness changes are very small, and thus are not noticeable.
In this example, Display P3 primary yellow (
color ( display-p3 1 1 0
) has the chroma progressively reduced in CIE LCH color space, with the local clip modification.
In the upper part of this diagram, colors which are inside the gamut of sRGB are displayed as-is. Colors inside the gamut of Display P3 (but outside sRGB) are in salmon. Colors outside the gamut of Display P3 are in red. The lower part of the diagram shows the linear-light intensities of the Display P3 red, green and blue components.It can be seen that reduction in CIE LCH chroma still makes the red intensity curve up, out of Display P3 gamut; but less than before and the sRGB boundary is found much more quickly. Gamut mapping in CIE LCH with local clip would give acceptable results.
In this example, Display P3 primary yellow (
color ( display-p3 1 1 0
) has the chroma progressively reduced, but this time in OKLCh color space and with the local clip modification.
In the upper part of this diagram, colors which are inside the gamut of sRGB are displayed as-is. Colors inside the gamut of Display P3 (but outside sRGB) are in salmon. Colors outside the gamut of Display P3 are in red. The lower part of the diagram shows the linear-light intensities of the Display P3 red, green and blue components.It can be seen that reduction in OKLCh chroma, which was already good, is further improved by the local clip modification. Simple gamut mapping in CIE LCH with local clip would give excellent results.
13.1.6. Deviations from Perceptual Uniformity: Hue CurvatureUsing the CIE LCH color space and deltaE2000 distance metric, is known to give suboptimal results with significant hue shifts, for colors in the hue range 270° to 330°.
A constant-hue slice of CIE LCH color space, at a hue angle of 301.37° corresponding to sRGB primary blue. The vertical axis is Lightness, the horizontal axis is Chroma. Between chroma of 25 and 75, the hue is visibly purple, becoming more blue between 100 and 131. The same phenomenon continues past 131, but cannot be shown on an sRGB display.Using OKLCh color space and deltaEOK distance metric avoids this issue at all hue angles.
A constant-hue slice of OKLCh color space, at a hue angle of 264.06° corresponding to sRGB primary blue. The vertical axis is Lightness, the horizontal axis is Chroma. The hue is visibly the same at all values of chroma, up to 0.315 (the sRGB limit at this hue). It continues to be constant beyond this point, although that cannot be shown on an sRGB diagram. 13.2. CSS Gamut Mapping to an RGB Destination TestsActual values of color are not exposed to script, making this hard to test in an automated manner.
The CSS gamut mapping algorithm applies to individual, Standard Dynamic Range (SDR) CSS colors which are out of gamut of an RGB display and thus require to be css gamut mapped.
It implements a relative colorimetric intent, and colors inside the destination gamut are unchanged.
Note: other situations, in particular mapping to printer gamuts where the maximum black level is significantly above zero, will require different algorithms which align the respective black and white points, which will result in lightness changes for very light and very dark colors as chroma is reduced..
Note: this algorithm is for individual, distinct colors; for color images, where relationships between neighboring pixels are important and the aim is to preserve detail and texture, a perceptual rendering intent is more appropriate and in that case, colors inside the destination gamut could be changed.
CSS gamut mapping occurs in the OKLCh color space, and the color difference formula used is deltaEOK. The local-MINDE improvement is used.
For colors which are out of range on the Lightness axis, white is returned in the destination color space if the Lightness is greater than or equal to 1.0, while black is returned in the destination color space if the Lightness is less than or equal to 0.0.
For the binary search implementation, at each step in the search, the deltaEOK is computed between the current mapped color and a clipped version of that color. If the current color is outside the gamut boundary, but the deltaEOK between it and the clipped version is below a threshold for a just noticeable difference (JND), the clipped version of the color is returned as the mapped result.
For the geometric implementation, having found the exact intersection, project outwards (towards higher chroma) along the line of constant lightness until either:
the deltaEOK between the projected point and a clipped version of that point exceeds one JND, or
the chroma of the projected point is equal to the chroma of the original color (i.e. do not project past the original color)
Then return the clipped version of the color as the mapped result.
For the OKLCh color space, one JND is is an OKLCh difference of 0.02.
Note: In CIE Lab color space, where the range of the Lightness component is 0 to 100, using deltaE2000, one JND is 2. Because the range of Lightness in Oklab and OKLCh is 0 to 1, using deltaEOK, one JND is 100 times smaller.
13.2.1. Sample Pseudocode for the Binary Search Gamut Mapping Algorithm with Local MINDETo
CSS gamut mapa color
originin color space
origin color spaceto be in gamut of a destination color space
destination:
Unless otherwise specified for a particular property, specified colors are resolved to computed colors and then further to used colors as described below.
The resolved value of a <color> is its used value.
TestsThis applies to:
hwb() values
It does not apply to:
color() values using the srgb or srgb-linear color spaces.
If the sRGB color was explicitly specified by the author as a named color, or as a system color, the declared value is that named or system color, converted to ASCII lowercase. The computed and used value is the corresponding sRGB color, paired with the specified alpha component (after clamping to [0, 1]) and defaulting to opaque if unspecified).
The author-provided mixed-case form below has a declared value in all lowercase.
pUrPlE purple
Otherwise, the declared, computed and used value is the corresponding sRGB color, paired with the specified alpha component (after clamping to [0, 1]) and defaulting to opaque if unspecified).
For historical reasons, when calc() in sRGB colors resolves to a single value, the declared value serialises without the "calc(" ")" wrapper.
For example, if a color is given as
rgb(calc(64 * 2) 127 255) the declared value will be rgb(128 127 255) and not rgb(calc(128) 127 255).
For example, if a color is given as
hsl(38.82 calc(2 * 50%) 50%) the declared value will be rgb(255 165.2 0) because the
calc()is lost during HSL to RGB conversion.
Also for historical reasons, when calc() is simplified down to a single value, the color values are clamped to [0.0, 255.0].
For example, if a color is given as
rgb(calc(100 * 4) 127 calc(20 - 35)) the declared value will be rgb(255 127 0) and not rgb(calc(400) 127 calc(-15)).
This clamping also takes care of values such as Infinity, -Infinity, and NaN which will clamp at 255, 0 and 0 respectively.
For example, the computed value of
hsl(38.824 100% 50%)
is
rgb(255, 165, 0)Tests
This applies to lab() and lch() values.
The declared, computed and used value is the corresponding CIE Lab or LCH color (after clamping of L, C and H) paired with the specified alpha component (as a <number>, not a <percentage>; and defaulting to opaque if unspecified).
For example, the computed value of
lch(52.2345% 72.2 56.2 / 1)
is
lch(52.2345% 72.2 56.2)Tests 14.3. Resolving Oklab and OKLCh values
This applies to oklab() and oklch() values.
The declared, computed and used value is the corresponding Oklab or OKLCh color (after clamping of L, C and H) paired with the specified alpha component (as a <number>, not a <percentage>; and defaulting to opaque if unspecified).
For example, the computed value of
oklch(42.1% 0.192 328.6 / 1)
is
oklch(42.1% 0.192 328.6)Tests 14.4. Resolving values of the color() function
The declared, computed and used value is the color in the specified color space, paired with the specified alpha component (as a <number>, not a <percentage>; and defaulting to opaque if unspecified).
For example, the computed value of
color(display-p3 0.823 0.6554 0.2537 /1)
is
color(display-p3 0.823 0.6554 0.2537)
For colors specified in the xyz color space, which is an alias of the xyz-d65 color space, the computed and used value is in the xyz-d65 color space.
For example, the computed value of
color(xyz 0.472 0.372 0.131)
is
color(xyz-d65 0.472 0.372 0.131)Tests 14.5. Resolving other colors
This applies to system colors (including the <deprecated-color>s), transparent, and currentcolor.
The declared value for each <system-color> keyword and <deprecated-color> keyword is itself. The computed value is the corresponding color in its color space. However, such colors must not be altered by 'forced colors mode'.
For example, in this html:
<button style="color: ButtonText; background: ButtonFace"></button>
The declared value of the color property is "ButtonText" while the computed value could be, for example, rgb(0, 0, 0).
The declared value of transparent is "transparent" while the computed and used value is transparent black.
The currentcolor keyword computes to itself.
In the color property, the used value of currentcolor is the resolved inherited value. In any other property, its used value is the used value of the color property on the same element.
Note: This means that if the currentcolor value is inherited, it’s inherited as a keyword, not as the value of the color property, so descendants will use their own color property to resolve it.
For example, given this html:
<div>
<p>Assume this example text is long enough
to wrap on multiple lines.
</p>
</div>
and this css:
div {
color: forestgreen;
text-shadow: currentColor;
}
p {
color: mediumseagreen;
}
p::firstline {
color: yellowgreen;
}
The used value of the inherited property text-shadow on the first line fragment would be yellowgreen.
TestsThis section updates and replaces that part of CSS Object Model, section Serializing CSS Values, which relates to serializing <color> values.
In this section, the strings used in the specification and the corresponding characters are as follows.
String Character(s) " " U+0020 SPACE "#" U+0023 NUMBER SIGN "," U+002C COMMA "-" U+002D HYPHEN-MINUS "." U+002E FULL STOP "/" U+002F SOLIDUS "none" U+006E LATIN SMALL LETTER NThe string "." shall be used as a decimal separator, regardless of locale, and there shall be no thousands separator.
For syntactic forms which support missing color components, the value none (equivalently NONE, nOnE, etc), shall be serialized in all-lowercase as the string "none".
15.1. Serializing alpha valuesThis applies to any <color> value which can take an optional alpha value. It does not apply to the opacity property.
If, after clamping to the range [0, 1] the alpha is 1, it is omitted from the serialization; an implicit value of 1 (fully opaque) is the default.
If the alpha is any other value than 1, it is explicitly included in the serialization as described below.
If the value is internally represented as an integer between 0 and 255 inclusive (i.e. 8-bit unsigned integer), follow these steps:
Otherwise, return the result of serializing the given value (as a <number>, not a <percentage>).
For example, if the alpha is stored as the 8-bit unsigned integer 237, the integer 93 satisfies the criterion because Math.round(93 * 2.55) is 237, and so the alpha is serialized as "0.93".
However, if the alpha is stored as the 8-bit unsigned integer 236, there is no such integer (92 maps to 235 while 94 maps to 240), and so since 236 ÷ 0.255 = 925.490196078 the alpha is serialized as "0.92549" (no more than 6 figures, trailing zeroes omitted).
The <number> value is expressed in base ten, with the "." character as decimal separator. The leading zero must not be omitted. Trailing zeroes must be omitted.
For example, an alpha value of 70% will be serialized as the string "0.7" which has a leading zero before the decimal separator, "." as decimal separator (even if the current locale would use some other character, such as ","), and all digits after the "7" would be "0" and are omitted.
The precision with which alpha values are retained, and thus the number of decimal places in the serialized value, is not defined in this specification, but must at least be sufficient to round-trip integer percentage values. Thus, the serialized value must contain at least two decimal places (unless trailing zeroes have been removed). Values must be rounded towards +∞, not truncated.
For example, an alpha value of 12.3456789% could be serialized as the strings "0.12" or "0.123" or "0.1234" or "0.12346" (rounding the value of 5 towards +∞ because the following digit is 6) or any longer, rounded serialization of the same form.
Because <alpha-value>s which were specified outside the valid range are clamped at parse time, the declared value will be clamped. However, per CSS Values 4 § 10.12 Range Checking, <alpha-value>s specified using calc() are not clamped when the specified form is serialized; but the computed values are clamped.
For example an alpha value which was specified directly as 120% would be serialized as the string "1". However, if it was specified as calc(2*60%) the declared value would be serialized as the string "calc(1.2)".
15.2. Serializing sRGB valuesThe serialized form of the following sRGB values:
hwb() values
is derived from the declared value.
When serializing the value of a property which was set by the author to a CSS named color, a system color, a deprecated-color, or transparent therefore, for the declared value, the ASCII lowercase keyword value is retained. For the computed and used value, the corresponding sRGB value is used.
TestsThus, the serialized declared value of transparent is the string "transparent", while the serialized computed value of transparent is the string "rgba(0, 0, 0, 0)".
For all other sRGB values, the declared, computed and used value is the corresponding sRGB value.
During serialization, any missing values are converted to 0.
15.2.1. HTML-compatible serialization of sRGB valuesIf the following conditions are all true:
Then corresponding sRGB values are serialized in 6-digit hex color notation as follows:
A seven-character string consisting of the character "#", followed immediately by the two-digit hexadecimal representations of the red component, the green component, and the blue component, in that order, using ASCII lower hex digits. No spaces are permitted.
For example, fill style is set to
magenta:
context.fillStyle = "rgb(255, 0, 255)" console.log(context.fillStyle); // "#ff00ff"
The color space is sRGB, the representation is 8 bits per component, the data format does not produce none values nor does it support extended range values, and the alpha is 1.
The HTML-compatible serialization is the string "#ff00ff" (not "#FF00FF").
Otherwise, for sRGB the CSS serialization of sRGB values is used and for other color spaces, the relevant serialization of the <color> value.
For example, fill style is set to
a dark brown, in CIE Lab:
context.fillStyle = "lab(29% 39 20)"; console.log(context.fillStyle); // "lab(29 39 20)"
The CSS serialization is the string "lab(29 39 20)".
For example, fill style is set to
semi-transparent magenta:
context.fillStyle = "#ff00ffed"; console.log(context.fillStyle); // "rgba(255, 0, 255, 0.93)"
The alpha is not 1, so the CSS serialization is the string "rgba(255, 0, 255, 0.93)".
15.2.2. CSS serialization of sRGB valuesCorresponding sRGB values use either the rgb() or rgba() form (depending on whether the (clamped) alpha is exactly 1, or not), with all ASCII lowercase letters for the function name.
For compatibility, the sRGB component values are serialized in <number> form, not <percentage>. Also for compatibility, the component values are serialized in base 10, with a range of [0-255], regardless of the bit depth with which they are stored.
As noted earlier, unitary alpha values are not explicitly serialized. Also, for compatibility, if the alpha is exactly 1, the rgb() form is used, with an implicit alpha; otherwise, the rgba() form is used, with an explicit alpha value.
For compatibility, the legacy form with comma separators is used; exactly one ASCII space follows each comma. This includes the comma (not slash) used to separate the blue component of rgba() from the alpha value.
For example, the serialized value of
rgb(29 164 192 / 95%)
is the string "rgba(29, 164, 192, 0.95)"
For example, an author-supplied value:
hwb(740deg 20% 30% / 50%)
Would be normalized first to
hwb(20 20% 30% / 50%)
and then converted to sRGB and serialized as
rgba(178.5, 93.5, 51, 0.5)
The precision of the returned result is described below.
Note: contrary to CSS Color 3, the parameters of the rgb() function are of type <number>, not <integer>. Thus, any higher precision than eight bits is indicated with a fractional part.
The precision with which sRGB component values are retained, and thus the number of significant figures in the serialized value, is not defined in this specification, but must at least be sufficient to round-trip eight bit values. Values must be rounded towards +∞, not truncated.
Note: authors of scripts which expect color values returned from getComputedStyle to have <integer> component values, are advised to update them to also cope with <number>.
For example,
rgb(146.064 107.457 131.223)
is now valid, and equal to
rgb(57.28% 42.14% 51.46%)
A conformant serialized form for both, is the string "rgb(146.06, 107.46, 131.2)".
Trailing fractional zeroes in any component values must be omitted; if the fractional part consists of all zeroes, the decimal point must also be omitted. This means that sRGB colors specified with integer component values will serialize with backwards-compatible integer values.
The serialized computed value of
''goldenrod''
is the string "rgb(218, 165, 32)" and not the string "rgb(218.000, 165.000, 32.000)"
15.3. Serializing Lab and LCH valuesThe serialized form of lch() and lab() values is derived from the computed value and uses the lab() or lch() forms, with ASCII lowercase letters for the function name.
The component values are serialized in base 10; the L, a, b and C component values are serialized as <number>, using the Lab percentage reference ranges or the LCH percentage reference ranges as appropriate to perform percentage to number conversion; thus 0% L maps to 0 and 100% L maps to 100. A single ASCII space character " " must be used as the separator between the component values.
TestsThe serialized value of
lab(56.200% 0.000 83.600)
is the string "lab(56.2 0 83.6)"
The serialized value of
lab(56.200% 0.000 66.88%)
is the string "lab(56.2 0 83.6)"
Trailing fractional zeroes in any component values must be omitted; if the fractional part consists of all zeroes, the decimal point must also be omitted.
The serialized value of
lch(37% 105.0 305.00)
is the string "lch(37 105 305)", not "lch(37 105.0 305.00)".
The precision with which lab() component values are retained, and thus the number of significant figures in the serialized value, is not defined in this specification, but due to the wide gamut must be sufficient to round-trip L values between 0 and 100, and a and b values between ±127, with at least sixteen bit precision; this will result in at least three decimal places unless trailing zeroes have been omitted. (half float or float, is recommended for internal storage). Values must be rounded towards +∞, not truncated.
Note: a and b values outside ±125 are possible with ultrawide gamut spaces. For example, all of the prophoto-rgb primaries and secondaries exceed this range, but are within ±200.
As noted earlier, unitary alpha values are not explicitly serialized. Non-unitary alpha values must be explicitly serialized, and the string " / " (an ASCII space, then forward slash, then another space) must be used to separate the b component value from the alpha value.
The serialized value of
lch(56.2% 83.6 357.4 /93%)
is the string "lch(56.2 83.6 357.4 / 0.93)" not "lch(56.2% 83.6 357.4 / 0.93)"
15.4. Serializing Oklab and OKLCh valuesThe serialized form of oklch() and oklab() values is derived from the computed value and uses the oklab() or oklch() forms, with ASCII lowercase letters for the function name.
The component values are serialized in base 10; the L, a, b and C component values are serialized as <number> using the Oklab percentage reference ranges or the OKLCh percentage reference ranges as appropriate to perform percentage to number conversion; thus 0% L maps to 0 and 100% L maps to 1.0. A single ASCII space character " " must be used as the separator between the component values.
TestsThe serialized value of
oklab(54.0% -0.10 -0.02)
is the string "oklab(0.54 -0.1 -0.02)" not "oklab(54 -0.1 -0.02)" or "oklab(54% -0.1 -0.02)"
The serialized value of
oklab(54.0 -25% -5%)
is the string "oklab(0.54 -0.1 -0.02)" not "oklab(54 -0.25 -0.05)"
Trailing fractional zeroes in any component values must be omitted; if the fractional part consists of all zeroes, the decimal point must also be omitted.
The serialized value of
oklch(56.43% 0.0900 123.40)
is the string "oklch(0.5643 0.09 123.4)", not "oklch(0.5643 0.0900 123.40)".
The precision with which oklab() component values are retained, and thus the number of significant figures in the serialized value, is not defined in this specification, but due to the wide gamut must be sufficient to round-trip L values between 0 and 1 (0% and 100%), and a, b and C values between ±0.5, with at least sixteen bit precision; this will result in at least five decimal places unless trailing zeroes have been omitted. (half float or float, is recommended for internal storage). Values must be rounded towards +∞, not truncated.
Note: a, b and C values outside ±0.5 are possible with ultrawide gamut spaces. For example, the prophoto-rgb green and blue primaries exceed this range, with C of 0.526 and 1.413 respectively.
As noted earlier, unitary alpha values are not explicitly serialized. Non-unitary alpha values must be explicitly serialized, and the string " / " (an ASCII space, then forward slash, then another space) must be used to separate the final color component (b, or C) value from the alpha value.
The serialized value of
oklch(53.85% 0.1725 320.67 / 70%)
is the string "oklch(0.5385 0.1725 320.67 / 0.7)"
15.5. Serializing values of the color() functionThe serialized form of color() values is derived from the computed value and uses the color() form, with ASCII lowercase letters for the function name and the color space name.
The component values are serialized in base 10, as <number>. A single ASCII space character " " must be used as the separator between the component values, and also between the color space name and the first color component.
TestsThe serialized value of
color(dIsPlAy-P3 0.964 0.763 0.787)
is the string "color(display-p3 0.96 0.76 0.79)", if two decimal places are retained. Notice that 0.787 has rounded up to 0.79, rather than being truncated to 0.78.
Trailing fractional zeroes in any component values must be omitted; if the fractional part consists of all zeroes, the decimal point must also be omitted.
The serialized value of
color(rec2020 0.400 0.660 0.340)
is the string "color(rec2020 0.4 0.66 0.34)", not "color(rec2020 0.400 0.660 0.340)".
If the color space is sRGB, the color space is still explicitly required in the serialized result.
For the predefined color spaces, the minimum precision for round-tripping is as follows:
(16bit, half-float, or float per component is recommended for internal storage). Values must be rounded towards +∞, not truncated.
Note: compared to the legacy forms such as rgb(), hsl() and so on, color(srgb) has a higher minimum precision requirement. Stylesheet authors who prefer higher precision are thus encouraged to use the color(srgb) form.
As noted earlier, unitary alpha values are not explicitly serialized. Non-unitary alpha values must be explicitly serialized, and the string " / " (an ASCII space, then forward slash, then another space) must be used to separate the final color component value from the alpha value.
The serialized value of
color(prophoto-rgb 0.2804 0.40283 0.42259/85%)
is the string "color(prophoto-rgb 0.28 0.403 0.423 / 0.85)", if three decimal places are retained.
15.6. Serializing other colorsThis applies to currentcolor.
The serialized form of this value is derived from the computed value and uses ASCII lowercase letters for the color name.
The serialized form of currentColor is the string "currentcolor".
16. Serializing <opacity-value>This applies to the opacity property.
The specified given opacity value is serialized as a <number>, not a <percentage>.
This <number> value is expressed in base ten, with the "." character as decimal separator. The leading zero must not be omitted. Trailing zeroes must be omitted.
Opacity values outside the range [0,1] are preserved, without clamping, in the serialized specified value.
The precision with which opacity values are retained, and thus the number of decimal places in the serialized value, is not defined in this specification, but must at least be sufficient to round-trip integer percentage values. Thus, the serialized value must contain at least two decimal places (unless trailing zeroes have been removed). Values must be rounded towards +∞, not truncated.
17. Default Style RulesThe following stylesheet is informative, not normative. This style sheet could be used by an implementation as part of its default styling of HTML documents.
/* traditional desktop user agent colors for hyperlinks */
:link { color: LinkText; }
:visited { color: VisitedText; }
:active { color: ActiveText; }
18. Sample code for Color Conversions
This section is not normative.
TestsThis section is not normative, it does not need tests.
For clarity, a library is used for matrix multiplication. (This is more readable than inlining all the multiplies and adds). The matrices are in column-major order.
// Sample code for color conversions
// Conversion can also be done using ICC profiles and a Color Management System
// For clarity, a library is used for matrix multiplication (multiply-matrices.js)
// standard white points, defined by 4-figure CIE x,y chromaticities
const D50 = [0.3457 / 0.3585, 1.00000, (1.0 - 0.3457 - 0.3585) / 0.3585];
const D65 = [0.3127 / 0.3290, 1.00000, (1.0 - 0.3127 - 0.3290) / 0.3290];
// sRGB-related functions
function lin_sRGB(RGB) {
// convert an array of sRGB values
// where in-gamut values are in the range [0 - 1]
// to linear light (un-companded) form.
// https://en.wikipedia.org/wiki/SRGB
// Extended transfer function:
// for negative values, linear portion is extended on reflection of axis,
// then reflected power function is used.
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
if (abs <= 0.04045) {
return val / 12.92;
}
return sign * (Math.pow((abs + 0.055) / 1.055, 2.4));
});
}
function gam_sRGB(RGB) {
// convert an array of linear-light sRGB values in the range 0.0-1.0
// to gamma corrected form
// https://en.wikipedia.org/wiki/SRGB
// Extended transfer function:
// For negative values, linear portion extends on reflection
// of axis, then uses reflected pow below that
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
if (abs > 0.0031308) {
return sign * (1.055 * Math.pow(abs, 1/2.4) - 0.055);
}
return 12.92 * val;
});
}
function lin_sRGB_to_XYZ(rgb) {
// convert an array of linear-light sRGB values to CIE XYZ
// using sRGB's own white, D65 (no chromatic adaptation)
var M = [
[ 506752 / 1228815, 87881 / 245763, 12673 / 70218 ],
[ 87098 / 409605, 175762 / 245763, 12673 / 175545 ],
[ 7918 / 409605, 87881 / 737289, 1001167 / 1053270 ],
];
return multiplyMatrices(M, rgb);
}
function XYZ_to_lin_sRGB(XYZ) {
// convert XYZ to linear-light sRGB
var M = [
[ 12831 / 3959, -329 / 214, -1974 / 3959 ],
[ -851781 / 878810, 1648619 / 878810, 36519 / 878810 ],
[ 705 / 12673, -2585 / 12673, 705 / 667 ],
];
return multiplyMatrices(M, XYZ);
}
// display-p3-related functions
function lin_P3(RGB) {
// convert an array of display-p3 RGB values in the range 0.0 - 1.0
// to linear light (un-companded) form.
return lin_sRGB(RGB); // same as sRGB
}
function gam_P3(RGB) {
// convert an array of linear-light display-p3 RGB in the range 0.0-1.0
// to gamma corrected form
return gam_sRGB(RGB); // same as sRGB
}
function lin_P3_to_XYZ(rgb) {
// convert an array of linear-light display-p3 values to CIE XYZ
// using D65 (no chromatic adaptation)
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
var M = [
[ 608311 / 1250200, 189793 / 714400, 198249 / 1000160 ],
[ 35783 / 156275, 247089 / 357200, 198249 / 2500400 ],
[ 0 / 1, 32229 / 714400, 5220557 / 5000800 ],
];
return multiplyMatrices(M, rgb);
}
function XYZ_to_lin_P3(XYZ) {
// convert XYZ to linear-light P3
var M = [
[ 446124 / 178915, -333277 / 357830, -72051 / 178915 ],
[ -14852 / 17905, 63121 / 35810, 423 / 17905 ],
[ 11844 / 330415, -50337 / 660830, 316169 / 330415 ],
];
return multiplyMatrices(M, XYZ);
}
// prophoto-rgb functions
function lin_ProPhoto(RGB) {
// convert an array of prophoto-rgb values
// where in-gamut colors are in the range [0.0 - 1.0]
// to linear light (un-companded) form.
// Transfer curve is gamma 1.8 with a small linear portion
// Extended transfer function
const Et2 = 16/512;
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
if (abs <= Et2) {
return val / 16;
}
return sign * Math.pow(abs, 1.8);
});
}
function gam_ProPhoto(RGB) {
// convert an array of linear-light prophoto-rgb in the range 0.0-1.0
// to gamma corrected form
// Transfer curve is gamma 1.8 with a small linear portion
// TODO for negative values, extend linear portion on reflection of axis, then add pow below that
const Et = 1/512;
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
if (abs >= Et) {
return sign * Math.pow(abs, 1/1.8);
}
return 16 * val;
});
}
function lin_ProPhoto_to_XYZ(rgb) {
// convert an array of linear-light prophoto-rgb values to CIE D50 XYZ
// matrix cannot be expressed in rational form, but is calculated to 64 bit accuracy
// see https://github.com/w3c/csswg-drafts/issues/7675
var M = [
[ 0.79776664490064230, 0.13518129740053308, 0.03134773412839220 ],
[ 0.28807482881940130, 0.71183523424187300, 0.00008993693872564 ],
[ 0.00000000000000000, 0.00000000000000000, 0.82510460251046020 ]
];
return multiplyMatrices(M, rgb);
}
function XYZ_to_lin_ProPhoto(XYZ) {
// convert D50 XYZ to linear-light prophoto-rgb
var M = [
[ 1.34578688164715830, -0.25557208737979464, -0.05110186497554526 ],
[ -0.54463070512490190, 1.50824774284514680, 0.02052744743642139 ],
[ 0.00000000000000000, 0.00000000000000000, 1.21196754563894520 ]
];
return multiplyMatrices(M, XYZ);
}
// a98-rgb functions
function lin_a98rgb(RGB) {
// convert an array of a98-rgb values in the range 0.0 - 1.0
// to linear light (un-companded) form.
// negative values are also now accepted
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
return sign * Math.pow(abs, 563/256);
});
}
function gam_a98rgb(RGB) {
// convert an array of linear-light a98-rgb in the range 0.0-1.0
// to gamma corrected form
// negative values are also now accepted
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
return sign * Math.pow(abs, 256/563);
});
}
function lin_a98rgb_to_XYZ(rgb) {
// convert an array of linear-light a98-rgb values to CIE XYZ
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
// has greater numerical precision than section 4.3.5.3 of
// https://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf
// but the values below were calculated from first principles
// from the chromaticity coordinates of R G B W
// see matrixmaker.html
var M = [
[ 573536 / 994567, 263643 / 1420810, 187206 / 994567 ],
[ 591459 / 1989134, 6239551 / 9945670, 374412 / 4972835 ],
[ 53769 / 1989134, 351524 / 4972835, 4929758 / 4972835 ],
];
return multiplyMatrices(M, rgb);
}
function XYZ_to_lin_a98rgb(XYZ) {
// convert XYZ to linear-light a98-rgb
var M = [
[ 1829569 / 896150, -506331 / 896150, -308931 / 896150 ],
[ -851781 / 878810, 1648619 / 878810, 36519 / 878810 ],
[ 16779 / 1248040, -147721 / 1248040, 1266979 / 1248040 ],
];
return multiplyMatrices(M, XYZ);
}
//Rec. 2020-related functions
function lin_2020(RGB) {
// convert an array of rec2020 RGB values in the range 0.0 - 1.0
// to linear light (un-companded) form.
// ITU-R BT.2020-2 p.4
const α = 1.09929682680944 ;
const β = 0.018053968510807;
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
if (abs < β * 4.5 ) {
return val / 4.5;
}
return sign * (Math.pow((abs + α -1 ) / α, 1/0.45));
});
}
function gam_2020(RGB) {
// convert an array of linear-light rec2020 RGB in the range 0.0-1.0
// to gamma corrected form
// ITU-R BT.2020-2 p.4
const α = 1.09929682680944 ;
const β = 0.018053968510807;
return RGB.map(function (val) {
let sign = val < 0? -1 : 1;
let abs = Math.abs(val);
if (abs > β ) {
return sign * (α * Math.pow(abs, 0.45) - (α - 1));
}
return 4.5 * val;
});
}
function lin_2020_to_XYZ(rgb) {
// convert an array of linear-light rec2020 values to CIE XYZ
// using D65 (no chromatic adaptation)
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
var M = [
[ 63426534 / 99577255, 20160776 / 139408157, 47086771 / 278816314 ],
[ 26158966 / 99577255, 472592308 / 697040785, 8267143 / 139408157 ],
[ 0 / 1, 19567812 / 697040785, 295819943 / 278816314 ],
];
// 0 is actually calculated as 4.994106574466076e-17
return multiplyMatrices(M, rgb);
}
function XYZ_to_lin_2020(XYZ) {
// convert XYZ to linear-light rec2020
var M = [
[ 30757411 / 17917100, -6372589 / 17917100, -4539589 / 17917100 ],
[ -19765991 / 29648200, 47925759 / 29648200, 467509 / 29648200 ],
[ 792561 / 44930125, -1921689 / 44930125, 42328811 / 44930125 ],
];
return multiplyMatrices(M, XYZ);
}
// Chromatic adaptation
function D65_to_D50(XYZ) {
// Bradford chromatic adaptation from D65 to D50
// The matrix below is the result of three operations:
// - convert from XYZ to retinal cone domain
// - scale components from one reference white to another
// - convert back to XYZ
// see https://github.com/LeaVerou/color.js/pull/354/files
var M = [
[ 1.0479297925449969, 0.022946870601609652, -0.05019226628920524 ],
[ 0.02962780877005599, 0.9904344267538799, -0.017073799063418826 ],
[ -0.009243040646204504, 0.015055191490298152, 0.7518742814281371 ]
];
return multiplyMatrices(M, XYZ);
}
function D50_to_D65(XYZ) {
// Bradford chromatic adaptation from D50 to D65
// See https://github.com/LeaVerou/color.js/pull/360/files
var M = [
[ 0.955473421488075, -0.02309845494876471, 0.06325924320057072 ],
[ -0.0283697093338637, 1.0099953980813041, 0.021041441191917323 ],
[ 0.012314014864481998, -0.020507649298898964, 1.330365926242124 ]
];
return multiplyMatrices(M, XYZ);
}
// CIE Lab and LCH
function XYZ_to_Lab(XYZ) {
// Assuming XYZ is relative to D50, convert to CIE Lab
// from CIE standard, which now defines these as a rational fraction
var ε = 216/24389; // 6^3/29^3
var κ = 24389/27; // 29^3/3^3
// compute xyz, which is XYZ scaled relative to reference white
var xyz = XYZ.map((value, i) => value / D50[i]);
// now compute f
var f = xyz.map(value => value > ε ? Math.cbrt(value) : (κ * value + 16)/116);
return [
(116 * f[1]) - 16, // L
500 * (f[0] - f[1]), // a
200 * (f[1] - f[2]) // b
];
// L in range [0,100]. For use in CSS, add a percent
}
function Lab_to_XYZ(Lab) {
// Convert Lab to D50-adapted XYZ
// http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
var κ = 24389/27; // 29^3/3^3
var ε = 216/24389; // 6^3/29^3
var f = [];
// compute f, starting with the luminance-related term
f[1] = (Lab[0] + 16)/116;
f[0] = Lab[1]/500 + f[1];
f[2] = f[1] - Lab[2]/200;
// compute xyz
var xyz = [
Math.pow(f[0],3) > ε ? Math.pow(f[0],3) : (116*f[0]-16)/κ,
Lab[0] > κ * ε ? Math.pow((Lab[0]+16)/116,3) : Lab[0]/κ,
Math.pow(f[2],3) > ε ? Math.pow(f[2],3) : (116*f[2]-16)/κ
];
// Compute XYZ by scaling xyz by reference white
return xyz.map((value, i) => value * D50[i]);
}
function Lab_to_LCH(Lab) {
var epsilon = 0.0015;
var chroma = Math.sqrt(Math.pow(Lab[1], 2) + Math.pow(Lab[2], 2)); // Chroma
var hue = Math.atan2(Lab[2], Lab[1]) * 180 / Math.PI;
if (hue < 0) {
hue = hue + 360;
}
if (chroma <= epsilon) {
hue = NaN;
}
return [
Lab[0], // L is still L
chroma, // Chroma
hue // Hue, in degrees [0 to 360)
];
}
function LCH_to_Lab(LCH) {
// Convert from polar form
return [
LCH[0], // L is still L
LCH[1] * Math.cos(LCH[2] * Math.PI / 180), // a
LCH[1] * Math.sin(LCH[2] * Math.PI / 180) // b
];
}
// OKLab and OKLCH
// https://bottosson.github.io/posts/oklab/
// XYZ <-> LMS matrices recalculated for consistent reference white
// see https://github.com/w3c/csswg-drafts/issues/6642#issuecomment-943521484
// recalculated for 64bit precision
// see https://github.com/color-js/color.js/pull/357
function XYZ_to_OKLab(XYZ) {
// Given XYZ relative to D65, convert to OKLab
var XYZtoLMS = [
[ 0.8190224379967030, 0.3619062600528904, -0.1288737815209879 ],
[ 0.0329836539323885, 0.9292868615863434, 0.0361446663506424 ],
[ 0.0481771893596242, 0.2642395317527308, 0.6335478284694309 ]
];
var LMStoOKLab = [
[ 0.2104542683093140, 0.7936177747023054, -0.0040720430116193 ],
[ 1.9779985324311684, -2.4285922420485799, 0.4505937096174110 ],
[ 0.0259040424655478, 0.7827717124575296, -0.8086757549230774 ]
];
var LMS = multiplyMatrices(XYZtoLMS, XYZ);
// JavaScript Math.cbrt returns a sign-matched cube root
// beware if porting to other languages
// especially if tempted to use a general power function
return multiplyMatrices(LMStoOKLab, LMS.map(c => Math.cbrt(c)));
// L in range [0,1]. For use in CSS, multiply by 100 and add a percent
}
function OKLab_to_XYZ(OKLab) {
// Given OKLab, convert to XYZ relative to D65
var LMStoXYZ = [
[ 1.2268798758459243, -0.5578149944602171, 0.2813910456659647 ],
[ -0.0405757452148008, 1.1122868032803170, -0.0717110580655164 ],
[ -0.0763729366746601, -0.4214933324022432, 1.5869240198367816 ]
];
var OKLabtoLMS = [
[ 1.0000000000000000, 0.3963377773761749, 0.2158037573099136 ],
[ 1.0000000000000000, -0.1055613458156586, -0.0638541728258133 ],
[ 1.0000000000000000, -0.0894841775298119, -1.2914855480194092 ]
];
var LMSnl = multiplyMatrices(OKLabtoLMS, OKLab);
return multiplyMatrices(LMStoXYZ, LMSnl.map(c => c ** 3));
}
function OKLab_to_OKLCH(OKLab) {
var epsilon = 0.000004;
var hue = Math.atan2(OKLab[2], OKLab[1]) * 180 / Math.PI;
var chroma = Math.sqrt(OKLab[1] ** 2 + OKLab[2] ** 2);
if (hue < 0) {
hue = hue + 360;
}
if (chroma <= epsilon) {
hue = NaN;
}
return [
OKLab[0], // L is still L
chroma,
hue
];
}
function OKLCH_to_OKLab(OKLCH) {
return [
OKLCH[0], // L is still L
OKLCH[1] * Math.cos(OKLCH[2] * Math.PI / 180), // a
OKLCH[1] * Math.sin(OKLCH[2] * Math.PI / 180) // b
];
}
// Premultiplied alpha conversions
function rectangular_premultiply(color, alpha) {
// given a color in a rectangular orthogonal colorspace
// and an alpha value
// return the premultiplied form
return color.map((c) => c * alpha)
}
function rectangular_un_premultiply(color, alpha) {
// given a premultiplied color in a rectangular orthogonal colorspace
// and an alpha value
// return the actual color
if (alpha === 0) {
return color; // avoid divide by zero
}
return color.map((c) => c / alpha)
}
function polar_premultiply(color, alpha, hueIndex) {
// given a color in a cylindicalpolar colorspace
// and an alpha value
// return the premultiplied form.
// the index says which entry in the color array corresponds to hue angle
// for example, in OKLCH it would be 2
// while in HSL it would be 0
return color.map((c, i) => c * (hueIndex === i? 1 : alpha))
}
function polar_un_premultiply(color, alpha, hueIndex) {
// given a color in a cylindicalpolar colorspace
// and an alpha value
// return the actual color.
// the hueIndex says which entry in the color array corresponds to hue angle
// for example, in OKLCH it would be 2
// while in HSL it would be 0
if (alpha === 0) {
return color; // avoid divide by zero
}
return color.map((c, i) => c / (hueIndex === i? 1 : alpha))
}
// Convenience functions can easily be defined, such as
function hsl_premultiply(color, alpha) {
return polar_premultiply(color, alpha, 0);
}
19. Sample Code for ΔE2000 and ΔEOK Color Differences
This section is not normative.
TestsThis section is not normative, it does not need tests.
The simplest color difference metric, ΔE76, is simply the Euclidean distance in Lab color space. While this is a good first approximation, color-critical industries such as printing and fabric dyeing soon developed improved formulae. Currently, the most widely used formula is ΔE2000. It corrects a number of known asymmetries and non-linearities compared to ΔE76. Because the formula is complex, and critically dependent on the sign of various intermediate calculations, implementations are often incorrect [Sharma].
The sample code below has been validated to five significant figures against the test suite of paired Lab values and expected ΔE2000 published by [Sharma] and is correct.
// deltaE2000 is a statistically significant improvement
// over deltaE76 and deltaE94,
// and is recommended by the CIE and Idealliance
// especially for color differences less than 10 deltaE76
// but is wicked complicated
// and many implementations have small errors!
/**
* @param {number[]} reference - Array of CIE Lab values: L as 0..100, a and b as around -150..150
* @param {number[]} sample - Array of CIE Lab values: L as 0..100, a and b as around -150..150
* @return {number} How different a color sample is from reference
*/
function deltaE2000 (reference, sample) {
// Given a reference and a sample color,
// both in CIE Lab,
// calculate deltaE 2000.
// This implementation assumes the parametric
// weighting factors kL, kC and kH
// (for the influence of viewing conditions)
// are all 1, as seems typical.
let [L1, a1, b1] = reference;
let [L2, a2, b2] = sample;
let C1 = Math.sqrt(a1 ** 2 + b1 ** 2);
let C2 = Math.sqrt(a2 ** 2 + b2 ** 2);
let Cbar = (C1 + C2)/2; // mean Chroma
// calculate a-axis asymmetry factor from mean Chroma
// this turns JND ellipses for near-neutral colors back into circles
let C7 = Math.pow(Cbar, 7);
const Gfactor = Math.pow(25, 7);
let G = 0.5 * (1 - Math.sqrt(C7/(C7+Gfactor)));
// scale a axes by asymmetry factor
// this by the way is why there is no Lab2000 color space
let adash1 = (1 + G) * a1;
let adash2 = (1 + G) * a2;
// calculate new Chroma from scaled a and original b axes
let Cdash1 = Math.sqrt(adash1 ** 2 + b1 ** 2);
let Cdash2 = Math.sqrt(adash2 ** 2 + b2 ** 2);
// calculate new hues, with zero hue for true neutrals
// and in degrees, not radians
const π = Math.PI;
const r2d = 180 / π;
const d2r = π / 180;
let h1 = (adash1 === 0 && b1 === 0)? 0: Math.atan2(b1, adash1);
let h2 = (adash2 === 0 && b2 === 0)? 0: Math.atan2(b2, adash2);
if (h1 < 0) {
h1 += 2 * π;
}
if (h2 < 0) {
h2 += 2 * π;
}
h1 *= r2d;
h2 *= r2d;
// Lightness and Chroma differences; sign matters
let ΔL = L2 - L1;
let ΔC = Cdash2 - Cdash1;
// Hue difference, taking care to get the sign correct
let hdiff = h2 - h1;
let hsum = h1 + h2;
let habs = Math.abs(hdiff);
let Δh;
if (Cdash1 * Cdash2 === 0) {
Δh = 0;
}
else if (habs <= 180) {
Δh = hdiff;
}
else if (hdiff > 180) {
Δh = hdiff - 360;
}
else if (hdiff < -180) {
Δh = hdiff + 360;
}
else {
console.log("the unthinkable has happened");
}
// weighted Hue difference, more for larger Chroma
let ΔH = 2 * Math.sqrt(Cdash2 * Cdash1) * Math.sin(Δh * d2r / 2);
// calculate mean Lightness and Chroma
let Ldash = (L1 + L2)/2;
let Cdash = (Cdash1 + Cdash2)/2;
let Cdash7 = Math.pow(Cdash, 7);
// Compensate for non-linearity in the blue region of Lab.
// Four possibilities for hue weighting factor,
// depending on the angles, to get the correct sign
let hdash;
if (Cdash1 == 0 && Cdash2 == 0) {
hdash = hsum; // which should be zero
}
else if (habs <= 180) {
hdash = hsum / 2;
}
else if (hsum < 360) {
hdash = (hsum + 360) / 2;
}
else {
hdash = (hsum - 360) / 2;
}
// positional corrections to the lack of uniformity of CIELAB
// These are all trying to make JND ellipsoids more like spheres
// SL Lightness crispening factor
// a background with L=50 is assumed
let lsq = (Ldash - 50) ** 2;
let SL = 1 + ((0.015 * lsq) / Math.sqrt(20 + lsq));
// SC Chroma factor, similar to those in CMC and deltaE 94 formulae
let SC = 1 + 0.045 * Cdash;
// Cross term T for blue non-linearity
let T = 1;
T -= (0.17 * Math.cos(( hdash - 30) * d2r));
T += (0.24 * Math.cos( 2 * hdash * d2r));
T += (0.32 * Math.cos(((3 * hdash) + 6) * d2r));
T -= (0.20 * Math.cos(((4 * hdash) - 63) * d2r));
// SH Hue factor depends on Chroma,
// as well as adjusted hue angle like deltaE94.
let SH = 1 + 0.015 * Cdash * T;
// RT Hue rotation term compensates for rotation of JND ellipses
// and Munsell constant hue lines
// in the medium-high Chroma blue region
// (Hue 225 to 315)
let Δθ = 30 * Math.exp(-1 * (((hdash - 275)/25) ** 2));
let RC = 2 * Math.sqrt(Cdash7/(Cdash7 + Gfactor));
let RT = -1 * Math.sin(2 * Δθ * d2r) * RC;
// Finally calculate the deltaE, term by term as root sum of squares
let dE = (ΔL / SL) ** 2;
dE += (ΔC / SC) ** 2;
dE += (ΔH / SH) ** 2;
dE += RT * (ΔC / SC) * (ΔH / SH);
return Math.sqrt(dE);
// Yay!!!
};19.2. ΔEOK
Because Oklab does not suffer from the hue linearity, hue uniformity, and chroma non-linearities of CIE Lab, the color difference metric does not need to correct for them and so is simply the Euclidean distance in Oklab color space.
// Calculate deltaE OK
// simple root sum of squares
/**
* @param {number[]} reference - Array of OKLab values: L as 0..1, a and b as -1..1
* @param {number[]} sample - Array of OKLab values: L as 0..1, a and b as -1..1
* @return {number} How different a color sample is from reference
*/
function deltaEOK (reference, sample) {
let [L1, a1, b1] = reference;
let [L2, a2, b2] = sample;
let ΔL = L1 - L2;
let Δa = a1 - a2;
let Δb = b1 - b2;
return Math.sqrt(ΔL ** 2 + Δa ** 2 + Δb ** 2);
} Appendix A: Deprecated CSS System Colors
Earlier versions of CSS defined several additional system colors. These color keywords have been deprecated, however, as they are insufficient for their original purpose (making website elements look like their native OS counterparts), represent a security risk by making it easier for a webpage to “spoof” a native OS dialog, and increase fingerprinting surface, compromising user privacy.
User agents must support these keywords, and to mitigate fingerprinting must map them to the (undeprecated) system colors as listed below. Authors must not use these keywords.
The deprecated system colors are represented as the <deprecated-color> sub-type, and are defined as:
When CSS is being parsed in quirks mode, <quirky-color> is a type of <color> that is only valid in certain properties:
It is not valid in properties that include or reference these properties, such as the background shorthand, or inside functional notations such as color-mix()
Additionally, while <quirky-color> must be valid as a <color> when parsing the affected properties in the @supports rule, it is not valid for those properties when used in the CSS.supports() method.
A <quirky-color> can be represented as a <number-token>, <dimension-token>, or <ident-token>, according to the following rules:
If it’s an <ident-token>, the token’s representation must contain exactly 3 or 6 characters, all hexadecimal digits. It represents a <hex-color> with the same value.
If it’s a <number-token>, it must have its integer flag set.
Serialize the integer’s value. If the serialization has less than 6 characters, prepend "0" characters to it until it is 6 characters long. It represents a <hex-color> with the same value.
If it’s a <dimension-token>, it must have its integer flag set.
Serialize the integer’s value, and append the representation of the token’s unit. If the result has less than 6 characters, prepend "0" characters to it until it is 6 characters long. It represents a <hex-color> with the same value.
(In other words, Quirks Mode allows hex colors to be written without the leading "#", but with weird parsing rules.)
Testsquirky hex colors
In addition to those who contributed to CSS Color 3, the editors would like to thank Emilio Cobos Álvarez, Alexey Ardov, Chris Bai, Amelia Bellamy-Royds, Lars Borg, Mike Bremford, Andreu Botella, Dan Burzo, Max Derhak, fantasai, Simon Fraser, Devon Govett, Phil Green, Dean Jackson, Andreas Kraushaar, Pierre-Anthony Lemieux, Tiaan Louw, Cameron McCormack, Romain Menke, Chris Murphy, Isaac Muse, Jonathan Neal, Chris Needham, Björn Ottosson, Christoph Päper, Brad Pettit, Xidorn Quan, Craig Revie, Melanie Richards, Florian Rivoal, Jacob Rus, Joseph Salowey, Simon Sapin, Igor Snitkin, Lea Verou, Mark Watson, James Stuckey Weber, Sam Weinig, and Natalie Weizenbaum.
Changes Changes since the Candidate Recommendation Draft of 24 April 2025The primary change, compared to CSS Color 3, is that CSS colors are no longer restricted to the narrow gamut of sRGB.
To support this, several brand new features have been added:
Other technical changes:
In addition, there have been some syntactic changes:
The system colors, if they actually correspond to the user’s system colors, pose a security risk, as they make it easier for a malware site to create user interfaces that appear to be from the system. However, as several system colors are now defined to be "generic", this risk is believed to be mitigated.
21. Privacy ConsiderationsThis specification defines "system" colors, which theoretically can expose details of the user’s OS settings, which is a fingerprinting risk.
22. Accessibility ConsiderationsThis specification encourages authors to not use color alone as a distinguishing feature.
This specification encourages browsers to ensure adequate contrast for specific system color foreground/background pairs. A harder requirement with specific AA or AAA contrast ratios was considered, but since browsers are often just passing along color choices made by the OS, or selected by users (who may have particular requirements, including lower contrast for people living with migraines or epileptic seizures), the CSSWG was unable to require a specific contrast level.
Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.
All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class= "example" , like this:
Informative notes begin with the word “Note” and are set apart from the normative text with class= "note" , like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class= "advisement" >, like this: UAs MUST provide an accessible alternative.
A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.
A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.
Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.
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