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Showing content from https://www.w3.org/TR/2019/WD-css-color-4-20191105/ below:

CSS Color Module Level 4

This section is not normative.

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.

This property describes the foreground fill color of an element’s text content. In addition, it provides the value that currentcolor resolves to.

There are several different ways to syntactically specify a given color.

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% */

<color>

The <color> type is defined in a later section.

Note: In general, this property, including its alpha component, has no effect on "color glyphs", such as emoji in some fonts, which are colored by a built-in palette. Some colored fonts are able to refer to the foreground color, such as palette entry 0xFFFF in COLR table of OpenType, and context-fill value in SVG-in-OpenType. In that case, the foreground color is set by this property, identical to how currentcolor value works.

CSS has several syntaxes for specifying color values. Some directly specify an sRGB color by its channels, such as the hex color notation or rgb() function. Others are more human-friendly to write and understand, such as the hsl() and lch() functions, or the long list of named colors.

Colors are represented as a list of components, also sometimes called “channels”, representing axises in the color space. Each channel 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 behind the color.

Colors in CSS are represented by the <color> type:

<color> = <hex-color> | <named-color> | currentcolor | transparent
          <rgb()> | <rgba()> | <hsl()> | <hsla()> | <hwb()> |
          <lab()> | <lch()> | <gray()> |
          <color()> | <device-cmyk()> |
          <system-color>

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%).

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 [WCAG20] when using color in their documents.

1.4.1 Use of Color: Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element

Colors specified in HTML and untagged images are in the sRGB color space ([SRGB]) unless otherwise specified.

An untagged image is an image that is not explicitly assigned a color profile, as defined by the image format.

Note that this rule does not apply to videos, since untagged video should be presumed to be in ITU.

• 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]

Unless otherwise specified for a particular property, specified colors are resolved to computed colors and then further to used colors as follows:

transparent

The computed and used value is transparent black.

hex colors

rgb() and rgba() values

hsl() and hsla() values

hwb() values

named colors

The computed and used value is the corresponding sRGB color paired with the specified alpha channel (defaulting to opaque if unspecified).

lab(), lch(), and gray() values

The computed and used value is the corresponding CEI Lab color paired with the specified alpha channel (defaulting to opaque if unspecified).

Does working color space affect these values? <https://github.com/w3c/csswg-drafts/issues/3844>

color() values

The computed and used value is the color in the specified colorspace, paired with the specified alpha channel (defaulting to opaque if unspecified).

device-cmyk() values

The computed and used value is the specified device-specific CMYK color, paired with the specified alpha channel (defaulting to opaque if unspecified).

The actual value can vary based on the operation; for rendering to a CMYK-capable device, it may be rendered as a CMYK color; for blending with non-CMYK colors or rendering to a non-CMYK device, it must be converted as specified in §12 Device-dependent CMYK Colors: the device-cmyk() function.

system colors

Each <system-color> keyword computes to itself. Its used value is the corresponding color in its color space.

Define something for <deprecated-system-color>? <https://github.com/w3c/csswg-drafts/issues/3873>

currentcolor

The currentcolor keyword computes to itself.

In the color property, its used value is the 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.

The resolved value of a <color> is its used value.

When should channel-clamping occur? <https://github.com/w3c/csswg-drafts/issues/3845>

CSS 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 colorspaces, as described in §10.2 Predefined colorspaces: srgb, display-p3, a98-rgb, prophoto-rgb and rec-2020..

CSS provides several methods of directly specifying an sRGB color: hex colors, rgb()/rgba() notation, hsl()/hsla() notation, hwb() notation, named colors, and the transparent keyword.

The rgb() function defines an sRGB color by specifying the red, green, and blue channels directly. Its syntax is:

rgb() = rgb( <percentage>{3} [ / <alpha-value> ]? ) |
        rgb( <number>{3} [ / <alpha-value> ]? )
<alpha-value> = <number> | <percentage>

The first three arguments specify the red, green, and blue channels of the color, respectively. 0% represents the minimum value for that color channel in the sRGB gamut, and 100% represents the maximum value. A <number> is equivalent to a <percentage>, but with a different range: 0 again represents the minimum value for the color channel, but 255 represents the maximum. These values come from the fact that many graphics engines store the color channels internally as a single byte, which can hold integers between 0 and 255. Implementations should honor the precision of the channel as authored or calculated wherever possible. If this is not possible, the channel should be rounded to the closest value at the highest precision used, rounding up if two values are equally close.

The final argument, the <alpha-value>, specifies the alpha of the color. If given as a <number>, the useful range of the value is 0 (representing a fully transparent color) to 1 (representing a fully opaque color). If given as a <percentage>, 0% represents a fully transparent color, while 100% represents a fully opaque color. If omitted, it defaults to 100%.

Values outside these ranges are not invalid, but are clamped to the ranges defined here at computed-value time.

For legacy reasons, rgb() also supports an alternate syntax that separates all of its arguments with commas:

rgb() = rgb( <percentage>#{3} , <alpha-value>? ) |
        rgb( <number>#{3} , <alpha-value>? )

Also for legacy reasons, an rgba() function also exists, with an identical grammar and behavior to rgb().

The CSS hex color notation allows an sRGB color to be specified by giving the channels 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:

6 digits

The first pair of digits, interpreted as a hexadecimal number, specifies the red channel of the color, where 00 represents the minimum value and ff (255 in decimal) represents the maximum. The next pair of digits, interpreted in the same way, specifies the green channel, and the last pair specifies the blue. The alpha channel of the color is fully opaque.

In other words,

#00ff00

represents the same color as

rgb(0 255 0)

(a lime green).

8 digits

The first 6 digits are interpreted identically to the 6-digit notation. The last pair of digits, interpreted as a hexadecimal number, specifies the alpha channel of the color, where 00 represents a fully transparent color and ff represent a fully opaque color.

In other words,

#0000ffcc

represents the same color as

rgb(0 0 100% / 80%)

(a slightly-transparent blue).

3 digits

This is a shorter variant of the 6-digit notation. The first digit, interpreted as a hexadecimal number, specifies the red channel of the color, where 0 represents the minimum value and f represents the maximum. The next two digits represent the green and blue channels, respectively, in the same way. The alpha channel of the color is fully opaque.

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

#123

specifies 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.

4 digits

This is a shorter variant of the 8-digit notation, "expanded" in the same way as the 3-digit notation is. The first digit, interpreted as a hexadecimal number, specifies the red channel of the color, where 0 represents the minimum value and f represents the maximum. The next three digits represent the green, blue, and alpha channels, respectively.

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.

CSS 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 case-insensitive.

The names resolve to colors in sRGB.

16 of CSS’s named colors come from HTML originally: 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. (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.

Note: this list of colors and their definitions is a superset of the list of named colors defined by SVG 1.1.

Note: The history of the X11 color system is interesting, and was excellently summarized by Alex Sexton in his talk “Peachpuffs and Lemonchiffons”.

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 Adjust §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.

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:

Canvas

Background of application content or documents.

CanvasText

Text in application content or documents.

LinkText

Text in non-active, non-visited links.

VisitedText

Text in visited links.

ActiveText

Text in active links.

ButtonFace

The face background color for push buttons.

ButtonText

Text on push buttons.

Field

Background of input fields.

FieldText

Text in input fields.

Highlight

Background of item(s) selected in a control.

HighlightText

Text of item(s) selected in a control.

GrayText

Disabled text. (Often, but not necessarily, gray.)

Note: As with all other keywords, these names are 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 pairings are expected to form legible background-foreground colors:

• Canvas background with CanvasText, LinkText, VisitedText, ActiveText foreground.

• ButtonFace background with ButtonText foreground.

• Field background with FieldText foreground.

• Highlight background with HighlightText foreground.

Additionally, GrayText is expected to be readable, though possibly at a lower contrast rating, over any of the backgrounds.

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 §16 Security and Privacy Considerations.

The keyword transparent specifies a transparent black. It is a type of <named-color>.

The keyword currentcolor represents value of the color property on the same element. Its used values is determined by resolving color values.

Here’s a simple example showing how to use the

keyword:

.foo {
  color: red;
  background-color: currentcolor;
}

This is equivalent to writing:

.foo {
  color: red;
  background-color: red;
}

For example, the

property

, whose initial value is

, by default matches the text color even as the

property 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 would be 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 it was originally introduced in SVG as a special attribute value spelled "currentColor", rather than a CSS value. Only later did CSS pick it up, at which point the capitalization stopped mattering, as CSS keywords are case-insensitive.

The 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 color scheme, which is much 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() function is:

hsl() = hsl( <hue> <percentage> <percentage> [ / <alpha-value> ]? )
<hue> = <number> | <angle>

The first argument specifies the hue. Hue is represented as an angle of the color circle (the rainbow, twisted around into a circle). The angle 0deg represents red (as does 360deg, 720deg, etc.), and the rest of the hues are spread around the circle, so 120deg represents green, 240deg represents blue, etc. 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.

The next two arguments are the saturation and lightness, respectively. For saturation, 100% is a fully-saturated, bright color, and 0% is a fully-unsaturated gray. For lightness, 50% represents the "normal" color, while 100% is white and 0% is black. If the saturation or lightness are less than 0% or greater than 100%, they are clamped to those values before being converted to an RGB color.

The final argument specifies the alpha channel of the color. It’s interpreted identically to the fourth argument of the rgb() function. If omitted, it defaults to 100%.

For example, an ordinary red, the same color you would see from the keyword

or the hex notation

, is represented in HSL as

.

The advantage of HSL over RGB is that it is far more intuitive: one can guess at the colors they want, and then tweak. It is also easier to create sets of matching colors (by keeping the hue the same and varying the saturation and lightness).

HSL colors resolve to sRGB.

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

For legacy reasons, hsl() also supports an alternate syntax that separates all of its arguments with commas:

hsl() = hsl( <hue>, <percentage>, <percentage>, <alpha-value>? )

Also for legacy reasons, an hsla() function also exists, with an identical grammar and behavior to hsl().

Converting an HSL color to sRGB is straightforward mathematically. Here’s a simple implementation of the conversion algorithm in JavaScript. For simplicity, this algorithm assumes that the hue has been normalized to a number in the half-open range [0, 6), and the saturation and lightness have been normalized to the range [0, 1]. It returns an array of three numbers representing the red, green, and blue channels of the colors, normalized to the range [0, 1].

function hslToRgb(hue, sat, light) {
  if( light <= .5 ) {
    var t2 = light * (sat + 1);
  } else {
    var t2 = light + sat - (light * sat);
  }
  var t1 = light * 2 - t2;
  var r = hueToRgb(t1, t2, hue + 2);
  var g = hueToRgb(t1, t2, hue);
  var b = hueToRgb(t1, t2, hue - 2);
  return [r,g,b];
}

function hueToRgb(t1, t2, hue) {
  if(hue < 0) hue += 6;
  if(hue >= 6) hue -= 6;

  if(hue < 1) return (t2 - t1) * hue + t1;
  else if(hue < 3) return t2;
  else if(hue < 4) return (t2 - t1) * (4 - hue) + t1;
  else return t1;
}

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.

<https://github.com/w3c/csswg-drafts/issues/3088>

The conversions in the table below are known to contain errors. They are copied from CSS Color 3, which also had the same errors. Those colors were supposedly computed by a program in ABC. A future spec will correctly compute those colors. Meanwhile, please note that these conversions are non-normative examples.

HWB (short for Hue-Whiteness-Blackness) is another method of specifying 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.

The syntax of the hwb() function is:

hwb() = hwb( <hue> <percentage> <percentage> [ / <alpha-value> ]? )

The first argument specifies the hue, and is interpreted identically to hsl().

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). Values outside of these ranges make the function invalid. If the sum of these two arguments is greater than 100%, then at computed-value time they are normalized to add up to 100%, with the same relative ratio.

The fourth argument specifies the alpha channel of the color. It’s interpreted identically to the fourth argument of the rgb() function. If omitted, it defaults to 100%.

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 white+black is equal to 100% (after normalization), it defines an achromatic color, or some shade of gray, without any hint of the chosen hue.

Converting an HWB color to sRGB is straightforward, and related to how one converts HSL to RGB. The following Javascript implementation of the algorithm assumes that the white and black components have already been normalized, so their sum is no larger than 100%, and have been converted into numbers in the range [0,1].

function hwbToRgb(hue, white, black) {
  var rgb = hslToRgb(hue, 1, .5);
  for(var i = 0; i < 3; i++) {
    rgb[i] *= (1 - white - black);
    rgb[i] += white;
  }
  return rgb;
}

Physical measurements of a color are typically expressed as the Lab color space, created in 1976 by the CIE. Color conversions from one device to another 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 axis. This value is usually written as a unitless number; for compatibility with the rest of CSS, it is written as a percentage. 100% means an L value of 100, not 1.0. L=0% is deep black (no light at all) while L=100% is a diffuse white (the illuminant is D50 white, a standardized daylight spectrum with a color temperature of 5000K, as reflected by a perfect diffuse reflector). Values greater than 100 would correspond to specular highlights, but their precise color is undefined in this specification. Usefully, L=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. 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.

D50 is also the whitepoint used for the profile connection space in ICC color interconversion, the values used in image editors which offer Lab editing, and the value used by physical measurement devices such as spectrometers, 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 Bradford algorithm [Bradford-CAT] is the industry standard chromatic adaptation transform, and is easy to calculate as it is a simple matrix multiplication.

LCH has the same L axis as Lab, but uses polar coordinates C (chroma) and H (hue). C is the geometric distance from the L axis and H is the angle from the positive a axis, with positive angles being more clockwise.

In Lab and LCH, if two colors have the same L value, they appear to have the same visual lightness—regardless of how different their hues are.

Note: The Lightness axis in Lab and LCH should not 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 even though visually, blue is much darker. In Lab, 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.

CSS allows colors to be directly expressed in Lab and LCH.

lab() = lab( <percentage> <number> <number> [ / <alpha-value> ]? )

The first argument specifies the CIE Lightness. This is typically a number between 0% (representing black) and 100% (representing white), However, CIE Lightness can exceed this range on some systems, with extra-bright whites using a lightness up to 400. Values less than 0% must be clamped to 0% at computed-value time; values greater than 100% are permitted (for forwards compatibility with High Dynamic Range (HDR), and must not be clamped.

The second and third arguments are the distances along the "a" and "b" axises in the Lab colorspace, 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, separated by a slash, and interpreted identically to the <alpha-value> in rgb().

lch() = lch( <percentage> <number> <hue> [ / <alpha-value> ]? )

The first argument specifies the CIE Lightness, interpreted identically to the Lightness argument of lab().

The second argument is the chroma (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 computed-value time.

The third argument is the hue angle. 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), 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). If the provided value is negative, or is greater than or equal to 360deg, it is set to the value modulo 360.

There is an optional fourth alpha value, separated by a slash, and interpreted identically to the <alpha-value> in rgb().

Conversion from sRGB to Lab requires several steps, although in practice all but the first step are linear calculations and can be combined.

1. Convert from sRGB to linear-light sRGB (undo gamma encoding). This has the same gamut as sRGB, but is now additive, so that arithmetic operations on the values give the correct results.
2. Convert from linear sRGB to CIE XYZ
3. Convert from a D65 whitepoint (used by sRGB) to the D50 whitepoint used in Lab, with the Bradford transform [Bradford-CAT]
4. Convert D50-adapted XYZ to Lab

There is sample JavaScript code for this conversion in §15 Sample code for color conversions.

Conversion from Lab to sRGB also requires multiple steps, and again in practice all but the last step are linear calculations and can be combined.

1. Convert Lab to (D50-adapted) XYZ
2. Convert from a D50 whitepoint (used by Lab) to the D65 whitepoint used in sRGB, with the Bradford transform
3. Convert from (D65-adapted) CIE XYZ to linear sRGB
4. Convert from linear-light sRGB to sRGB (do gamma encoding)

Conversion to LCH is trivial:

1. H = atan2(b, a)
2. C = sqrt(a^2 + b^2)
3. L is the same

Conversion to Lab is trivial:

1. a = C cos(H)
2. b = C sin(H)
3. L is the same

<https://github.com/w3c/csswg-drafts/issues/290>

Grays are fully desaturated colors. The gray() functional notation simplifies specifying this common set of colors, so that only a single numerical parameter is required, and so that gray(50) is a visual mid-gray (perceptually equidistant between black and white).

gray() = gray( <number>  [ / <alpha-value> ]? )

The first argument specifies the shade of gray, equal to the CIE Lightness, while the second optional argument specifies the alpha channel of the gray.

Note: In other words, gray(a / b) is equal to lab(a 0 0 / b).

Conversion from gray to sRGB requires multiple steps; in practice all but the last step are linear calculations and can be combined.

1. Convert to Lab by setting L to the gray value, a and b to 0
2. Convert Lab to XYZ
3. Adapt from D50 to D65 (Bradford transform)
4. Convert from (D65-adapted) CIE XYZ to linear sRGB
5. Convert from linear-light sRGB to sRGB (do gamma encoding)

When 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 characterised. This characterization information is stored in a profile. The most common type of color profile is defined by the International Color Consortium (ICC) [ICC].

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.

CSS allows colors to be specified by reference to a color profile. This could be for example a calibrated CMYK printer, or an RGB colorspace (such as ProPhoto , widely used by photographers), or any other color or monochrome output device which has been characterized. In addition, for convenience, CSS provides several -->predefined RGB color spaces: display-p3 [DCI-P3], which is a wide gamut space typical of current wide-gamut monitors, and Rec. 2020 [Rec.2020], which is a ultra-wide gamut space capable of representing almost all visible real-world colors. Both are broadcast industry standards.

This example specifies four profiled colors: for a standard SWOP-coated CMYK press, for a wide-gamut seven-ink printer, for ProPhoto RGB, and for the display-p3 standard RGB space. In each case, the numerical parameters are in the range 0.0 to 1.0 (rather than, for example, 0 to 255).

color: color(swopc 0.0134 0.8078 0.7451 0.3019);
color: color(indigo 0.0941 0.6274 0.3372 0.1647 0 0.0706 0.1216);
color: color(prophoto-rgb 0.9137 0.5882 0.4784);
color: color(display-p3 0.3804 0.9921 0.1412);

The colors not using a predefined colorspace also need a matching @color-profile at-rule somewhere in the stylesheet, to connect the name with the profile data.

@color-profile swopc {
  src: url('http://example.org/swop-coated.icc');}
@color-profile indigo {
  src: url('http://example.org/indigo-seven.icc');}

The color() function allows a color to be specified in a particular colorspace (rather than the implicit sRGB colorspace that the other color functions operate in). Its syntax is:

color() = color( [ <ident>? [ <number>+ | <string> ] [ / <alpha-value> ]? ]# , <color>? )

The color function takes one or more comma-separated arguments, with each argument specifying a color, and later colors acting as "fallback" if an earlier color can’t be displayed (for example, if the colorspace it specifies hasn’t been loaded yet).

Each argument has the following form:

• An optional <ident> denoting the colorspace. This can be one of the predefined colorspaces (such as display-p3), or one defined by a @color-profile rule. If omitted, it defaults to the predefined srgb color profile.

  If the <ident> names a non-existent colorspace, this argument represents an invalid color.

• Either one or more <number>s providing the parameter values that the colorspace takes, or a <string> giving the name of a color defined by the colorspace.

  If the colorspace takes numeric parameters

  If more <number>s are provided than parameters that the colorspace takes, the excess <number>s at the end are ignored.

  If less <number>s are provided than parameters that the colorspace takes, the missing parameters default to 0. (This is particularly convenient for multichannel printers where the additional inks are spot colors or varnishes that most colors on the page won’t use.)

  If a <string> is provided, this argument represents an invalid color.

  If the colorspace defines named colors

  If <number>s are provided, or a <string> is provided that doesn’t match any of the color names defined by the colorspace, this argument represents an invalid color.

• An optional slash-separated <alpha-value>. This is interpreted the same way as the <alpha-value> in rgb(), and if omitted it defaults to 100%.

After one or more arguments of the above form, a final <color> argument using any CSS color syntax can be provided.

The color() function represents the color specified by the first of its arguments that represent a valid color (that is, the first argument that isn’t an invalid color). If all of its arguments represent invalid colors, color() represents opaque black.

The following colorspaces are predefined for use in the color() function. They can be used without any @color-profile rule.

srgb

The srgb [SRGB] colorspace accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1]. The whitepoint is D65 (a daylight white, with a correlated color temperature of 6504°K).

[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 colorspace for CSS, identical to specifying a color with the rgb() function.

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 0.3127 0.3290 Transfer function see below White luminance 80.0 cd/m² Black luminance 0.80 cd/m²

var Cl;
if (C <= 0.04045)
  Cl = C / 12.92;
else
  Cl = Math.pow((C + 0.055) / 1.055, 2.4);

C is the red, green or blue component.

display-p3

The display-p3 colorspace accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1]. It uses the same primary chromaticities as [DCI-P3], but with a D65 whitepoint and the same transfer curve as sRGB.

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 0.3127 0.3290 Transfer function same as srgb White luminance 80.0 cd/m² Black luminance 0.80 cd/m²

a98-rgb

The a98-rgb colorspace accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1]. The transfer curve is a gamma function, close to but not exactly 1/2.2.

a98-rgb is compatible with Adobe® RGB (1998) [AdobeRGB].

Adobe® RGB (1998) uses primaries originally derived from the SMPTE 240M standard; errors in the original conversion turned out to produce a colorspace that was useful for digital photography, so Adobe® RGB (1998) is a common wider-gamut colorspace for photographic images. The a98-rgb colorspace allows CSS to specify colors that will match colors in such images having the same RGB values.

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 0.3127 0.3290 Transfer function 256/563 White luminance 160.0 cd/m² Black luminance 0.5557 cd/m²

prophoto-rgb

The prophoto-rgb colorspace accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1]. The transfer curve is a gamma function with a value of 1/1.8. The white point is D50, the same as is used by CIE Lab. Thus, conversion to Lab does not require the chromatic adaptation step.

The ProPhoto RGB space uses primaries chosen to allow a wide color gamut and to minimise hue shifts under tonal manipulation. It is often used in digital photography as a wide gamut colorspace for the master version of photographic images. The prophoto-rgb colorspace allows CSS to specify colors that will match colors in such images having the same RGB values.

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 0.7347 0.2653 Green chromaticity 0.1596 0.8404 Blue chromaticity 0.0366 0.0001 White chromaticity 0.3457 0.3585 Transfer function 1/1.800 White luminance 160.0 to 640.0 cd/m² Black luminance See text

rec-2020

The rec-2020 [Rec.2020] colorspace accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1]. ITU Reference 2020 is used for High Definition, 4k and 8k television.

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 0.3120 0.3290 Transfer function 1/2.4 (see note)

Note: rec-2020 references a different transfer curve for cameras. However this curve is never used in production cameras or 2020 displays.
"In typical production practice the encoding function of image sources is adjusted so that the final picture has the desired look, as viewed on a reference monitor having the reference decoding function of Recommendation ITU-R BT.1886, in the reference viewing environment defined in Recommendation ITU-R BT.2035."
The transfer function (1886) for reference Rec.2020 displays is gamma 2.4 [Rec.2020]

For both predefined color spaces, conversion to Lab requires several steps, although in practice all but the first step are linear calculations and can be combined.

1. Convert from gamma-corrected RGB to linear-light RGB (undo gamma encoding)
2. Convert from linear RGB to CIE XYZ
3. Convert from a D65 whitepoint (used by both display-p3 and rec-2020) to the D50 whitepoint used in Lab, with the Bradford transform
4. Convert D50-adapted XYZ to Lab

Conversion from Lab to display-p3 or rec-2020 also requires multiple steps, and again in practice all but the last step are linear calculations and can be combined.

1. Convert Lab to (D50-adapted) XYZ
2. Convert from a D50 whitepoint (used by Lab) to the D65 whitepoint used in sRGB, with the Bradford transform
3. Convert from (D65-adapted) CIE XYZ to linear RGB
4. Convert from linear-light RGB to RGB (do gamma encoding)

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 the source and destination gamuts.

The @color-profile rule defines and names a color profile which can later be used in the color() function to specify a color. It’s defined as:

@color-profile = @color-profile <custom-ident> { <declaration-list> }

The <custom-ident> gives the color profile’s name. All of the predefined colorspace keywords (srgb, display-p3, a98-rgb, prophoto-rgb, rec-2020) are excluded from this <custom-ident>, as they’re predefined by this specification and always available.

The @color-profile rule accepts the descriptors defined in this specification.

The src descriptor specifies the URL to retrieve the color-profile information from.

Color profiles contain “rendering intents”, which define how to map their color to smaller gamuts than they’re defined over. Often a profile will contain only a single intent, but when there are multiple, the rendering-intent descriptor chooses one of them to use.

The four possible rendering intents are [ICC]:

relative-colorimetric

Media-relative colorimetric is required to leave source colors that fall inside the destination medium gamut unchanged relative to the respective media white points. Source colors that are out of the destination medium gamut are mapped to colors on the gamut boundary using a variety of different methods.

Note: the media-relative colorimetric rendering intent is often used with black point compensation, where the source medium black point is mapped to the destination medium black point as well. This method must map the source white point to the destination white point. If black point compensation is in use, the source black point must also be mapped to the destination black point. Adaptation algorithms should be used to adjust for the change in white point. Relative relationships of colors inside both source and destination gamuts should be preserved. Relative relationships of colors outside the destination gamut may be changed.

absolute-colorimetric

ICC-absolute colorimetric is required to leave source colors that fall inside the destination medium gamut unchanged relative to the adopted white (a perfect reflecting diffuser). Source colors that are out of the destination medium gamut are mapped to colors on the gamut boundary using a variety of different methods. This method produces the most accurate color matching of in-gamut colors, but will result in highlight clipping if the destination medium white point is lower than the source medium white point. For this reason it is recommended for use only in applications that need exact color matching and where highlight clipping is not a concern.

This method MUST disable white point matching and black point matching when converting colors. In general, this option is not recommended except for testing purposes.

perceptual

This method is often the preferred choice for images, especially when there are substantial differences between the source and destination (such as a screen display image reproduced on a reflection print). It takes the colors of the source image and re-optimizes the appearance for the destination medium using proprietary methods. This re-optimization may result in colors within both the source and destination gamuts being changed, although perceptual transforms are supposed to maintain the basic artistic intent of the original in the reproduction. They will not attempt to correct errors in the source image.

Note: With v2 ICC profiles there is no specified perceptual reference medium, which can cause interoperability problems. When v2 ICC profiles are used it may be safer to use the media-relative colorimetric rendering intent with black point compensation, instead of the perceptual rendering intent, unless the specific source and destination profiles to be used have been checked to ensure the combination produces the desired result.

This method should maintain relative color values among the pixels as they are mapped to the target device gamut. This method may change pixel values that were originally within the target device gamut, in order to avoid hue shifts and discontinuities and to preserve as much as possible the overall appearance of the scene.

saturation

This option was created to preserve the relative saturation (chroma) of the original, and to keep solid colors pure. However, it experienced interoperability problems like the perceptual intent, and as solid color preservation is not amenable to a reference medium solution using v4 profiles does not solve the problem. Use of this rendering intent is not recommended unless the specific source and destination profiles to be used have been checked to ensure the combination produces the desired result. This option should preserve the relative saturation (chroma) values of the original pixels. Out of gamut colors should be converted to colors that have the same saturation but fall just inside the gamut.

<https://github.com/w3c/csswg-drafts/issues/300>

While screens typically display colors directly with RGB pixels, printers often represent colors in different ways. In particular, one of the most common print-based ways of representing colors is with CMYK: a combination of cyan, magenta, yellow, and black which yields a particular color on that device. The device-cmyk() function allows authors to specify a color in this way:

device-cmyk() = device-cmyk( <cmyk-component>{4} [ / <alpha-value> ]? , <color>? )
<cmyk-component> = <number> | <percentage>

The arguments of the device-cmyk() function specify the cyan, magenta, yellow, and black components, in order, as a number between 0 and 1 or a percentage between 0% and 100%. These two usages are equivalent, and map to each other linearly. Values less than 0 or 0%, or greater than 1 or 100%, are not invalid; instead, they are clamped to 0/0% or 1/100% at computed-value time.

The fifth argument specifies the alpha channel of the color. It’s interpreted identically to the fourth argument of the rgb() function. If omitted, it defaults to 100%.

The sixth argument specifies the fallback color, used when the user agent doesn’t know how to accurately transform the CMYK color to RGB. If omitted, it defaults to the CMYK color naively converted to RGBA.

Typically, print-based applications will actually store the used colors as CMYK, and send them to the printer in that form. Unfortunately, CSS cannot do that; various CSS features require an RGB color, so that compositing/blending/etc. can be done. As such, CMYK colors must be converted to an equivalent RGB color. This is not trivial, like the conversion from HSL or HWB to RGB; the precise conversion depends on the precise characteristics of the output device.

If the user agent has information about the output device such that it believes it can accurately convert the CMYK color to a correct RGB color, the computed value of the device-cmyk() function must be that RGBA color. Otherwise, the computed value must be the fallback color.

For example, the following colors are equivalent (under the default conversion listed above):

color: device-cmyk(0 81% 81% 30%);
color: rgb(178 34 34);
color: firebrick;

Note: these colors might not match precisely if the browser knows a more precise conversion between CMYK and RGB colors. It’s recommended that if authors use any CMYK colors in their document, that they use only CMYK colors in their document to avoid any color-matching difficulties.

While most colors defined in this specification are directly compatible with RGBA, and thus can be mechanically and consistently converted back and forth with it, device-CMYK colors are not directly compatible; a given device-CMYK color will map to various RGBA colors depending on the physical characteristics of the output device.

Ideally, the user agent will be aware of the output device’s color profiles for RGBA and device-CMYK. If this is true, then the user agent must convert between device-CMYK and RGBA colors (and vice versa) by first converting the color into an appropriate device-independent color space, such as CIELab, and then converting into the output color space, using the appropriate color profiles for each operation.

This is not always possible, however. In that case, the user agent must use the following naive conversion algorithms.

To naively convert from CMYK to RGBA:

If a fallback color was specified, return that color (converting it to RGB as well, if necessary). Otherwise:

• red = 1 - min ( 1 , cyan * ( 1 - black ) + black )
• green = 1 - min ( 1 , magenta * ( 1 - black ) + black )
• blue = 1 - min ( 1 , yellow * ( 1 - black ) + black )
• Alpha is same as for input color.

To naively convert from RGBA to CMYK:

• black = 1 - max ( red , green , blue )
• cyan = ( 1 - red - black ) / ( 1 - black ), or 0 if black is 1
• magenta = ( 1 - green - black ) / ( 1 - black ), or 0 if black is 1
• yellow = ( 1 - blue - black ) / ( 1 - black ), or 0 if black is 1
• alpha is the same as the input color
• fallback color must be set to the input color

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.

<alpha-value>

The opacity to be applied to the element. It is interpreted identically to its definition in rgb(), except that the resulting opacity is applied to the entire element, rather than a particular color.

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.

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.)

Furthermore, 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 [CSS21] 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).

When drawing, implementations must handle alpha according to the rules in Section 5.1 Simple alpha compositing of [Compositing].

The following stylesheet is informative, not normative. This style sheet could be used by an implementation as part of its default styling of HTML4, XHTML1, XHTML1.1, XHTML Basic, and other XHTML Family documents.

html {
  color: black;
}

/* traditional desktop user agent colors for hyperlinks */
:link    { color: blue; }
:visited { color: purple; }

The default background of the root element must be transparent. The default color of the canvas (the surface on which the document is painted) is UA-dependent, but is recommended to be white, especially if the above color rules are used.

This section is not normative.

// 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 manipulations

// sRGB-related functions

function lin_sRGB(RGB) {
  // convert an array of sRGB values in the range 0.0 - 1.0
  // to linear light (un-companded) form.
  // https://en.wikipedia.org/wiki/SRGB
  return RGB.map(function (val) {
    if (val < 0.04045) {
      return val / 12.92;
    }

    return Math.pow((val + 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
  return RGB.map(function (val) {
    if (val > 0.0031308) {
      return 1.055 * Math.pow(val, 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)
  // http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
  var M = math.matrix([
    [0.4124564,  0.3575761,  0.1804375],
    [0.2126729,  0.7151522,  0.0721750],
    [0.0193339,  0.1191920,  0.9503041]
  ]);

  return math.multiply(M, rgb).valueOf();
}

function XYZ_to_lin_sRGB(XYZ) {
  // convert XYZ to linear-light sRGB
  var M = math.matrix([
    [ 3.2404542, -1.5371385, -0.4985314],
    [-0.9692660,  1.8760108,  0.0415560],
    [ 0.0556434, -0.2040259,  1.0572252]
  ]);

  return math.multiply(M, XYZ).valueOf();
}

//  image-3-related functions


function lin_P3(RGB) {
  // convert an array of image-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 image-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 image-p3 values to CIE XYZ
  // using  D65 (no chromatic adaptation)
  // http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
  var M = math.matrix([
    [0.4865709486482162, 0.26566769316909306, 0.1982172852343625],
    [0.2289745640697488, 0.6917385218365064,  0.079286914093745],
    [0.0000000000000000, 0.04511338185890264, 1.043944368900976]
  ]);
  // 0 was computed as -3.972075516933488e-17

  return math.multiply(M, rgb).valueOf();
}

function XYZ_to_lin_P3(XYZ) {
  // convert XYZ to linear-light P3
  var M = math.matrix([
    [ 2.493496911941425,   -0.9313836179191239, -0.40271078445071684],
    [-0.8294889695615747,   1.7626640603183463,  0.023624685841943577],
    [ 0.03584583024378447, -0.07617238926804182, 0.9568845240076872]
  ]);

  return math.multiply(M, XYZ).valueOf();
}

// prophoto-rgb functions

function lin_ProPhoto(RGB) {
  // convert an array of prophoto-rgb values in the range 0.0 - 1.0
  // to linear light (un-companded) form.
  // Transfer curve is gamma 1.8 with a small linear portion
  return RGB.map(function (val) {
    if (val < 0.031248) {
      return val / 16;
    }

    return Math.pow(val, 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
  return RGB.map(function (val) {
    if (val > 0.001953) {
      return Math.pow(val, 1/1.8);
    }

    return 16 * val;
  });
}

function lin_ProPhoto_to_XYZ(rgb) {
  // convert an array of linear-light prophoto-rgb values to CIE XYZ
  // using  D50 (so no chromatic adaptation needed afterwards)
  // http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
  var M = Math.matrix([
  [ 0.7977604896723027,  0.13518583717574031,  0.0313493495815248     ],
  [ 0.2880711282292934,  0.7118432178101014,   0.00008565396060525902 ],
  [ 0.0,                 0.0,                  0.8251046025104601     ]
  ]);

  return Math.multiply(M, rgb).valueOf();
}

function XYZ_to_lin_ProPhoto(XYZ) {
  // convert XYZ to linear-light prophoto-rgb
  var M = Math.matrix([
    [  1.3457989731028281,  -0.25558010007997534,  -0.05110628506753401 ],
    [ -0.5446224939028347,   1.5082327413132781,    0.02053603239147973 ],
    [  0.0,                  0.0,                   1.2119675456389454  ]
  ]);

  return Math.multiply(M, XYZ).valueOf();
}

// 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.
  return RGB.map(function (val) {
    return Math.pow(val, 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
  return RGB.map(function (val) {
    return Math.pow(val, 256/563);
  });
}

function lin_a98rgb_to_XYZ(rgb) {
  // convert an array of linear-light a98-rgb values to CIE XYZ
  // using  D50 (so no chromatic adaptation needed afterwards)
  // http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
  // which has greater numerical precsion than section 4.3.5.3 of
  // https://www.adobe.com/digitalimag/pdfs/AdobeRGB1998.pdf
  var M = Math.matrix([
  [ 0.5766690429101305,   0.1855582379065463,   0.1882286462349947  ],
  [ 0.29734497525053605,  0.6273635662554661,   0.07529145849399788 ],
  [ 0.02703136138641234,  0.07068885253582723,  0.9913375368376388  ]
  ]);

  return Math.multiply(M, rgb).valueOf();
}

function XYZ_to_lin_a98rgb(XYZ) {
  // convert XYZ to linear-light a98-rgb
  var M = Math.matrix([
  [  2.0415879038107465,    -0.5650069742788596,   -0.34473135077832956 ],
  [ -0.9692436362808795,     1.8759675015077202,    0.04155505740717557 ],
  [  0.013444280632031142,  -0.11836239223101838,   1.0151749943912054  ]
  ]);

  return Math.multiply(M, XYZ).valueOf();
}

//Rec. 2020-related functions

function lin_2020(RGB) {
  // convert an array of rec-2020 RGB values in the range 0.0 - 1.0
  // to linear light (un-companded) form.
  const α = 1.09929682680944 ;
  const β = 0.018053968510807;

  return RGB.map(function (val) {
    if (val < β * 4.5 ) {
      return val / 4.5;
    }

    return Math.pow((val + α -1 ) / α, 2.4);
  });
}
//check with standard this really is 2.4 and 1/2.4, not 0.45 was wikipedia claims

function gam_2020(RGB) {
  // convert an array of linear-light rec-2020 RGB  in the range 0.0-1.0
  // to gamma corrected form
  const α = 1.09929682680944 ;
  const β = 0.018053968510807;

  return RGB.map(function (val) {
    if (val > β ) {
      return α * Math.pow(val, 1/2.4) - (α - 1);
    }

    return 4.5 * val;
  });
}

function lin_2020_to_XYZ(rgb) {
  // convert an array of linear-light rec-2020 values to CIE XYZ
  // using  D65 (no chromatic adaptation)
  // http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html
  var M = math.matrix([
    [0.6369580483012914, 0.14461690358620832,  0.1688809751641721],
    [0.2627002120112671, 0.6779980715188708,   0.05930171646986196],
    [0.000000000000000,  0.028072693049087428, 1.060985057710791]
  ]);
  // 0 is actually calculated as  4.994106574466076e-17

  return math.multiply(M, rgb).valueOf();
}

function XYZ_to_lin_2020(XYZ) {
  // convert XYZ to linear-light rec-2020
  var M = math.matrix([
    [1.7166511879712674,   -0.35567078377639233, -0.25336628137365974],
    [-0.6666843518324892,   1.6164812366349395,   0.01576854581391113],
    [0.017639857445310783, -0.042770613257808524, 0.9421031212354738]
  ]);

  return math.multiply(M, XYZ).valueOf();
}

// 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
  // http://www.brucelindbloom.com/index.html?Eqn_ChromAdapt.html
  var M = math.matrix([
    [ 1.0478112,  0.0228866, -0.0501270],
    [ 0.0295424,  0.9904844, -0.0170491],
    [-0.0092345,  0.0150436,  0.7521316]
   ]);

  return math.multiply(M, XYZ).valueOf();
}

function D50_to_D65(XYZ) {
  // Bradford chromatic adaptation from D50 to D65
  var M = math.matrix([
    [ 0.9555766, -0.0230393,  0.0631636],
    [-0.0282895,  1.0099416,  0.0210077],
    [ 0.0122982, -0.0204830,  1.3299098]
   ]);

  return math.multiply(M, XYZ).valueOf();
}

// 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
  var white = [0.96422, 1.00000, 0.82521]; // D50 reference white

  // compute xyz, which is XYZ scaled relative to reference white
  var xyz = XYZ.map((value, i) => value / white[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
  ];
}

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 white = [0.96422, 1.00000, 0.82521]; // D50 reference white
  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 * white[i]);
}

function Lab_to_LCH(Lab) {
  // Convert to polar form
  var hue = Math.atan2(Lab[2], Lab[1]) * 180 / Math.PI;
  return [
    Lab[0], // L is still L
    Math.sqrt(Math.pow(Lab[1], 2) + Math.pow(Lab[2], 2)), // Chroma
    hue >= 0 ? hue : hue + 360 // 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
  ];
}

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, but are encouraged to map them to default values not based on the user’s OS settings or match them to appropriate (undeprecated) system colors. Authors must not use these keywords.

The deprecated system colors are represented as the <deprecated-colors> sub-type, and are defined as:

ActiveBorder

Active window border.

ActiveCaption

Active window caption.

AppWorkspace

Background color of multiple document interface.

Background

Desktop background.

ButtonHighlight

The color of the border facing the light source for 3-D elements that appear 3-D due to one layer of surrounding border.

ButtonShadow

The color of the border away from the light source for 3-D elements that appear 3-D due to one layer of surrounding border.

CaptionText

Text in caption, size box, and scrollbar arrow box.

InactiveBorder

Inactive window border.

InactiveCaption

Inactive window caption.

InactiveCaptionText

Color of text in an inactive caption.

InfoBackground

Background color for tooltip controls.

InfoText

Text color for tooltip controls.

Menu background.

Text in menus.

Scrollbar

Scroll bar gray area.

ThreeDDarkShadow

The color of the darker (generally outer) of the two borders away from the light source for 3-D elements that appear 3-D due to two concentric layers of surrounding border.

ThreeDFace

The face background color for 3-D elements that appear 3-D due to two concentric layers of surrounding border.

ThreeDHighlight

The color of the lighter (generally outer) of the two borders facing the light source for 3-D elements that appear 3-D due to two concentric layers of surrounding border.

ThreeDLightShadow

The color of the darker (generally inner) of the two borders facing the light source for 3-D elements that appear 3-D due to two concentric layers of surrounding border.

ThreeDShadow

The color of the lighter (generally inner) of the two borders away from the light source for 3-D elements that appear 3-D due to two concentric layers of surrounding border.

Window

Window background.

WindowFrame

Window frame.

WindowText

Text in windows.

Thanks to Brad Pettit both for writing up color-profiles, and for implementing it. Thanks to Steven Pemberton for his write-up on HSL colors. Thanks to Melanie Richards for updating the system colors write-up. Thanks especially to the feedback from Marc Attinasi, Bert Bos, Joe Clark, fantasai, Patrick Garies, Tony Graham, Ian Hickson, Susan Lesch, Alex LeDonne, Cameron McCormack, Krzysztof Maczyński, Chris Moschini, Chris Murphy, Christoph Päper, David Perrell, Jacob Refstrup, Dave Singer, Jonathan Stanley, Andrew Thompson, Russ Weakley, Etan Wexler, David Woolley, Boris Zbarsky, Steve Zilles, the XSL FO subgroup of the XSL working group, and all the rest of the www-style community.

And thanks to Chris Lilley for being the resident CSS Color expert.

• Changed Lightness in Lab and LCH to be a percentage, for CSS compatibility
• Clamping of color values clarified
• Percentage opacity is now allowed
• Define terms sRGB and linear-light sRGB, for use by other specs
• Add new list of CSS system colors; renaming Text to CanvasText
• Make system color keywords compute to themselves
• Add computed/used entry for system colors
• Rewrite intro to non-deprecated system colors to center their use around forced-colors mode rather than generic use
• Consistent hyphenation of predefined colorspaces
• Restore text about non-opaque elements painting at layeres even when not positioned
• Initial value of the "color" property is now black
• Clarify hue in LCH is modulo 360deg
• Clarify allowed range of L in LCH and Lab, and meaning of L=100
• Update references for colorspaces used in video
• Add prophoto-rgb predefined colorspace
• Correct black and white luminance levels for display-p3
• Clarify display-p3 transfer function
• Add a98-rgb colorspace, correct table of primary chromaticities
• Clarify that currentColor’s computed value is not the resolved color
• Update syntax is examples to conform to latest specification
• Remove the color-mod() function
• Drop the "media" from propdef tables
• Export, and consistently use, "transparent black" and "opaque black"
• Clarify calculated values such as percents
• Clarify required precision and rounding behavior for color channels
• Clarify "color" property has no effect on color font glyphs (unless specifically referenced, e.g. with currentColor)
• Clarify how color values are resolved
• larify that HSL, HWB and named colors resolve to sRGB
• Simplify conversion from device-cmyk to sRGB
• Describe previous, comma-using color syntaxes as "legacy"; change examples to commaless form
• Remove superfluous requirement that displayed colors be restricted to device gamut (like there was any other option!)
• Rename P3 to display-p3; avoid claiming this is DCI P3, as these are not the same
• Improved description of the parameters to the "color()" function
• Disallow predefined spaces from "@color-profile" identifier
• Add canonical order to "color", "color-adjust" and "opacity" property definitions
• Switch definition of alpha compositing from SVG11 to CSS Compositing
• Clarify sample conversion code is non-normative
• Add Security and Privacy Considerations
• Update several references to most current versions
• Convert inline issues to links to GitHub issues
• Minor editorial clarifications, formatting and markup improvements

1. rgb() and rgba() functions now accept <number> rather than <integer>.
2. hsl() and hsla() functions now accept <angle> as well as <number> for hues.
3. rgb() and rgba(), and hsl() and hsla() are now aliases of each other (all of them have an optional alpha).
4. rgb(), rgba(), hsl(), and hsla() have all gained a new syntax consisting of space-separated arguments and an optional slash-separated opacity. All the color functions use this syntax form now, in keeping with CSS’s functional-notation design principles.
5. All uses of <alpha-value> now accept <percentage> as well as <number>.
6. 4 and 8-digit hex colors have been added, to specify transparency.

Several brand new features have been added:

1. gray() function, for specifying grays compactly.
2. hwb() function, for specifying colors in the HWB notation.
3. predefined, wide color gamut RGB colorspaces
4. lab() and lch() functions, for device-independent color
5. color() function and @color-profile at-rule, for profiled device-dependent color, including calibrated CMYK.
6. device-cmyk() function, for specifying uncalibrated colors in an output-device-specific CMYK colorspace.
7. Addition of named color rebeccapurple.

This specification defines "system" colors, which theoretically can expose details of the user’s OS settings, which is a fingerprinting risk.

The system colors, if they actually correspond to the user’s system colors, also 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.

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.

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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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