CSS defines a finite set of parameters, called properties, that direct the rendering of a document. Each property has a name (e.g., color, font-size, or border-style), a value space (e.g., <color>, <length-percentage>, [ solid | dashed | dotted | … ]), and a defined behavior on the rendering of the document. Properties values are assigned to various parts of the document via property declarations, which assign the property a value (e.g. red, 12pt, dotted) for the associated element or box.
One of the fundamental design principles of CSS is cascading, which allows several style sheets to influence the presentation of a document. When different declarations try to set a value for the same element/property combination, the conflicts must somehow be resolved.
The opposite problem arises when no declarations try to set a value for an element/property combination. In this case, a value is be found by way of inheritance or by looking at the property’s initial value.
The cascading and defaulting process takes a set of declarations as input, and outputs a specified value for each property on each element.
The rules for finding the specified value for all properties on all elements in the document are described in this specification. The rules for finding the specified values in the page context and its margin boxes are described in [css-page-3].
1.1. Module InteractionsThis section is normative.
This module replaces and extends the rules for assigning property values, cascading, and inheritance defined in [CSS2] chapter 6.
Other CSS modules may expand the definitions of some of the syntax and features defined here. For example, the Media Queries Level 4 specification, when combined with this module, expands the definition of the <media-query> value type as used in this specification.
For the purpose of this specification, text nodes are treated as element children of their associated element, and possess the full set of properties; since they cannot be targeted by selectors all of their computed values are assigned by defaulting.
2. Importing Style Sheets: the @import ruleThe @import rule allows users to import style rules from other style sheets. If an @import rule refers to a valid stylesheet, user agents must treat the contents of the stylesheet as if they were written in place of the @import rule, with two exceptions:
If a feature (such as the @namespace rule) explicitly defines that it only applies to a particular stylesheet, and not any imported ones, then it doesn’t apply to the imported stylesheet.
If a feature relies on the relative ordering of two or more constructs in a stylesheet (such as the requirement that @namespace rules must not have any other rules other than @import preceding it), it only applies between constructs in the same stylesheet.
For example, declarations in style rules from imported stylesheets interact with the cascade as if they were written literally into the stylesheet at the point of the @import.
Any @import rules must precede all other valid at-rules and style rules in a style sheet (ignoring @charset), or else the @import rule is invalid. The syntax of @import is:
@import [ <url> | <string> ]
<import-conditions> ;
<import-conditions> = [ supports( [ <supports-condition> | <declaration> ] ) ]?
<media-query-list>?
where the <url> or <string> gives the URL of the style sheet to be imported, and the optional <import-conditions> state the import conditions under which it applies.
If a <string> is provided, it must be interpreted as a <url> with the same value.
The following lines are equivalent in meaning and illustrate both
@importsyntaxes (one with
url()and one with a bare string):
@import "mystyle.css";
@import url("mystyle.css");
2.1. Conditional @import Rules
Import conditions allow the import to be media– or feature-support–dependent. In the absence of any import conditions, the import is unconditional. (Specifying all for the <media-query-list> has the same effect.) If the import conditions do not match, the rules in the imported stylesheet do not apply, exactly as if the imported stylesheet were wrapped in @media and/or @supports blocks with the given conditions.
The following rules illustrate how
@importrules can be made media-dependent:
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
@import url("narrow.css") handheld and (max-width: 400px);
User agents may therefore avoid fetching a conditional import as long as the import conditions do not match. Additionally, if a <supports-condition> blocks the application of the imported style sheet, the UA must not fetch the style sheet (unless it is loaded through some other link) and must return null for the import rule’s CSSImportRule.styleSheet value (even if it is loaded through some other link).
The following rule illustrates how an author can provide rules for modern user agents that support newer web features without impacting network performance on older user agents. In this example, a CSS file which loads COLRv1 fonts is only fetched when COLRv1 is supported:
@import url("COLRv1_fonts.css") supports(font-tech(color-COLRv1));
The import conditions are given by <media-query-list>, which is parsed and interpreted as a media query list, and <supports-condition>, is parsed and interpreted as a [[supports query]]. If a <declaration> is given in place of a <supports-condition>, it must be interpreted as a <supports-decl> (i.e. the extra set of parentheses is implied) and treated as a <supports-condition>.
For example, the following two lines are equivalent:
@import "mystyle.css" supports(display: flex); @import "mystyle.css" supports((display: flex));
The evaluation and full syntax of the import conditions are defined by the Media Queries [MEDIAQ] and CSS Conditional Rules [CSS-CONDITIONAL-3] specifications.
2.2. Processing Stylesheet ImportsWhen the same style sheet is imported or linked to a document in multiple places, user agents must process (or act as though they do) each link as though the link were to an independent style sheet.
Note: This does not place any requirements on resource fetching, only how the style sheet is reflected in the CSSOM and used in specs such as this one. Assuming appropriate caching, it is perfectly appropriate for a UA to fetch a style sheet only once, even though it’s linked or imported multiple times.
The cascade origin of an imported style sheet is the cascade origin of the style sheet that imported it.
The environment encoding of an imported style sheet is the encoding of the style sheet that imported it. [css-syntax-3]
To
fetch an @import, given an
@importrule
rule:
Let parentStylesheet be rule’s parent CSS style sheet. [CSSOM]
If rule has a <supports-condition>, and that condition is not true, return.
Fetch a style resource from rule’s URL, with ruleOrDeclaration rule, destination "style", CORS mode "no-cors", and processResponse being the following steps given response response and byte stream, null or failure byteStream:
If byteStream is not a byte stream, return.
If parentStylesheet is in quirks mode and response is CORS-same-origin, let content type be "text/css". Otherwise, let content type be the Content Type metadata of response.
If content type is not "text/css", return.
Let importedStylesheet be the result of parsing byteStream given parsedUrl.
Set importedStylesheet’s origin-clean flag to parentStylesheet’s origin-clean flag.
If response is not CORS-same-origin, unset importedStylesheet’s origin-clean flag.
Set rule’s styleSheet to importedStylesheet.
The processing of imported style sheets depends on the actual type of the linked resource:
If the resource does not have Content-Type metadata, the type is treated as text/css.
If the host document is in quirks mode, and the host document’s origin is same origin with the linked resource response’s URL’s origin, the type is treated as text/css.
Otherwise, the type is determined from its Content-Type metadata.
If the linked resource’s type is text/css, it must be interpreted as a CSS style sheet. Otherwise, it must be interpreted as a network error.
Some properties are shorthand properties, meaning that they allow authors to specify the values of several properties with a single property. A shorthand property sets all of its longhand sub-properties, exactly as if expanded in place.
When values are omitted from a shorthand form, unless otherwise defined, each “missing” sub-property is assigned its initial value.
This means that a
shorthand property declarationalways sets
allof its
sub-properties, even those that are not explicitly set. Carelessly used, this might result in inadvertently resetting some
sub-properties. Carefully used, a
shorthandcan guarantee a “blank slate” by resetting
sub-propertiesinadvertently cascaded from other sources.
For example, writing background: green rather than background-color: green ensures that the background color overrides any earlier declarations that might have set the background to an image with background-image.
For example, the CSS Level 1
fontproperty is a
shorthandproperty for setting
font-style,
font-variant,
font-weight,
font-size,
line-height, and
font-familyall at once. The multiple declarations of this example:
h1 {
font-weight: bold;
font-size: 12pt;
line-height: 14pt;
font-family: Helvetica;
font-variant: normal;
font-style: normal;
}
can therefore be rewritten as
h1 { font: bold 12pt/14pt Helvetica }
As more font sub-properties are introduced into CSS, the shorthand declaration resets those to their initial values as well.
In some cases, a shorthand might have different syntax or special keywords that don’t directly correspond to values of its sub-properties. (In such cases, the shorthand will explicitly define the expansion of its values.)
In other cases, a property might be a reset-only sub-property of the shorthand: Like other sub-properties, it is reset to its initial value by the shorthand when unspecified, but the shorthand might not include syntax to set the sub-property to any of its other values. For example, the border shorthand resets border-image to its initial value of none, but has no syntax to set it to anything else. [css-backgrounds-3]
If a shorthand is specified as one of the CSS-wide keywords [css-values-3], it sets all of its sub-properties to that keyword, including any that are reset-only sub-properties. (Note that these keywords cannot be combined with other values in a single declaration, not even in a shorthand.)
Declaring a shorthand property to be !important is equivalent to declaring all of its sub-properties to be !important.
3.1. Property AliasingProperties sometimes change names after being supported for a while, such as vendor-prefixed properties being standardized. The original name still needs to be supported for compatibility reasons, but the new name is preferred. To accomplish this, CSS defines two different ways of “aliasing” old syntax to new syntax.
For example, if
old-nameis a
legacy name aliasfor
new-name,
getComputedStyle ( el ). oldName
will return the computed style of the
newName
property, and
el . style . setPropertyValue ( "old-name" , "value" )
will set the
new-nameproperty to
"value"
.
For example, the
page-break-*properties are
legacy shorthandsfor the
break-*properties (see
CSS Fragmentation 3 § 3.4 Page Break Aliases: the page-break-before, page-break-after, and page-break-inside properties).
Setting page-break-before: always expands to break-before: page at parse time, like other shorthands do. Similarly, if break-before: page is set, calling getComputedStyle ( el ). pageBreakBefore will return "always" . However, when serializing a style block (see CSSOM § 6.7.2 Serializing CSS Values), the page-break-before property will never be chosen as the shorthand to serialize to, regardless of whether it or break-before was specified; instead, break-before will always be chosen.
The all property is a shorthand that resets all CSS properties except direction and unicode-bidi. It only accepts the CSS-wide keywords. It does not reset custom properties [css-variables-1].
Note: The excepted CSS properties direction and unicode-bidi are actually markup-level features, and should not be set in the author’s style sheet. (They exist as CSS properties only to style document languages not supported by the UA.) Authors should use the appropriate markup, such as HTML’s dir attribute, instead. [css-writing-modes-3]
For example, if an author specifies
all: initialon an element, it will block all inheritance and reset all properties, as if no rules appeared in the author, user, or user-agent levels of the cascade.
This can be useful for the root element of a "widget" included in a page, which does not wish to inherit the styles of the outer page. Note, however, that any "default" style applied to that element (such as, e.g. display: block from the UA style sheet on block elements such as <div>) will also be blown away.
Once a user agent has parsed a document and constructed a document tree, it must assign, to every element in the flat tree, and correspondingly to every box in the formatting structure, a value to every property that applies to the target media type.
The final value of a CSS property for a given element or box is the result of a multi-step calculation:
Elements that are not connected or are not part of the document’s flattened element tree do not participate in CSS value processing, and do not have declared, cascaded, specified, computed, used, or actual values, even if they potentially have style declarations assigned to them (for example, by a style attribute).
Each property declaration applied to an element contributes a declared value for that property associated with the element. See Filtering Declarations for details.
These values are then processed by the cascade to choose a single “winning value”.
4.1.1. Value AliasingSome property values have legacy value aliases: at parse time, the legacy syntax is converted into the new syntax, resulting in a declared value different from the parsed input. These aliases are typically used for handling legacy compatibility requirements, such as converting vendor-prefixed values to their standard equivalents.
4.2. Cascaded ValuesThe cascaded value represents the result of the cascade: it is the declared value that wins the cascade (is sorted first in the output of the cascade). If the output of the cascade is an empty list, there is no cascaded value.
4.3. Specified ValuesThe specified value is the value of a given property that the style sheet authors intended for that element. It is the result of putting the cascaded value through the defaulting processes, guaranteeing that a specified value exists for every property on every element.
In many cases, the specified value is the cascaded value. However, if there is no cascaded value at all, the specified value is defaulted. The CSS-wide keywords are handled specially when they are the cascaded value of a property, setting the specified value as required by that keyword, see § 7.3 Explicit Defaulting.
4.4. Computed ValuesThe computed value is the result of resolving the specified value as defined in the “Computed Value” line of the property definition table, generally absolutizing it in preparation for inheritance.
Note: The computed value is the value that is transferred from parent to child during inheritance. For historical reasons, it is not necessarily the value returned by the getComputedStyle() function, which sometimes returns used values. [CSSOM] Furthermore, the computed value is an abstract data representation: their definitions reflect that data representation, not how that data is serialized. For example, serialization rules often allow omitting certain values which are implied during parsing; but those values are nonetheless part of the computed value.
A
specified valuecan be either absolute (i.e., not relative to another value, as in
redor
2mm) or relative (i.e., relative to another value, as in
auto,
2em). Computing a relative value generally absolutizes it:
See examples (f), (g) and (h) in the table below.
Note: In general, the computed value resolves the specified value as far as possible without laying out the document or performing other expensive or hard-to-parallelize operations, such as resolving network requests or retrieving values other than from the element and its parent.
The computed value exists even when the property does not apply. However, some properties may change how they determine the computed value based on whether the property applies to the element.
4.5. Used ValuesThe used value is the result of taking the computed value and completing any remaining calculations to make it the absolute theoretical value used in the formatting of the document.
For example, a declaration of width: auto can’t be resolved into a length without knowing the layout of the element’s ancestors, so the computed value is auto, while the used value is an absolute length, such as 100px. [CSS2]
As another example, a <div> might have a computed break-before value of auto, but acquire a used break-before value of page by propagation from its first child. [css-break-3]
If a property does not apply to this element or box type then it has no used value for that property.
For example, the flex property has no used value on elements that aren’t flex items.
4.5.1. Applicable PropertiesIf a property does not apply to an element or box type—as noted in its “Applies to” line—this means it does not directly take effect on that type of box or element.
Note: A property that does not apply can still have indirect formatting effects if its computed value affects the computation of other properties that do apply; and of course its computed value, which always exists, can still inherit to descendants and take effect on them.
Even though
writing-modeand
text-orientationdo not apply to table rows (they do not affect how the table row or its children are laid out), setting them on such boxes will still affect the calculation of font relative units such as
ch, and thus possibly any property that takes a
<length>.
Setting
text-transformon an HTML
p
element (which is
display: blockby default) will have an effect, even though
text-transformonly applies to
inline boxes, because the property inherits into the paragraph’s anonymous
root inline boxand applies to the text it contains.
Note: A property defined to apply to “all elements” applies to all elements and display types, but not necessarily to all pseudo-element types, since pseudo-elements often have their own specific rendering models or other restrictions. The ::before and ::after pseudo-elements, however, are defined to generate boxes almost exactly like normal elements and are therefore defined accept all properties that apply to “all elements”. See [CSS-PSEUDO-4] for more information about pseudo-elements.
4.6. Actual ValuesA used value is in principle ready to be used, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximate the used width. Also, the font size of an element may need adjustment based on the availability of fonts or the value of the font-size-adjust property. The actual value is the used value after any such adjustments have been made.
Note: By probing the actual values of elements, much can be learned about how the document is laid out. However, not all information is recorded in the actual values. For example, the actual value of the page-break-after property does not reflect whether there is a page break or not after the element. Similarly, the actual value of orphans does not reflect how many orphan lines there is in a certain element. See examples (j) and (k) in the table below.
4.7. Examples Examples of CSS Value Computation Property Winning declaration Cascaded value Specified value Computed value Used value Actual value (a) text-aligntext-align: left left left left left left (b) border-top-width, border-right-width, border-bottom-width, border-left-width border-width: inherit inherit 4.2px 4.2px 4.2px 4px (c) width (none) (none) auto (initial value) auto 120px 120px (d) list-style-position list-style-position: inherit inherit inside inside inside inside (e) list-style-position list-style-position: initial initial outside (initial value) outside outside outside (f) font-size font-size: 1.2em 1.2em 1.2em 14.1px 14.1px 14px (g) width width: 80% 80% 80% 80% 354.2px 354px (h) width width: auto auto auto auto 134px 134px (i) height height: auto auto auto auto 176px 176px (j) page-break-after (none) (none) auto (initial value) auto auto auto (k) orphans orphans: 3 3 3 3 3 3 5. Filtering
In order to find the declared values, implementations must first identify all declarations that apply to each element. A declaration applies to an element if:
The values of the declarations that apply form, for each property on each element, a list of declared values. The next section, the cascade, prioritizes these lists.
6. CascadingThe cascade takes an unordered list of declared values for a given property on a given element, sorts them by their declaration’s precedence as determined below, and outputs a single cascaded value.
6.1. Cascade Sorting OrderThe cascade sorts declarations according to the following criteria, in descending order of precedence:
Declarations from origins earlier in this list win over declarations from later origins.
When comparing two declarations that are sourced from different encapsulation contexts, then for normal rules the declaration from the outer context wins, and for important rules the declaration from the inner context wins. For this purpose, [DOM] tree contexts are considered to be nested in shadow-including tree order.
Note: This effectively means that normal declarations belonging to an encapsulation context can set defaults that are easily overridden by the outer context, while important declarations belonging to an encapsulation context can enforce requirements that cannot be overridden by the outer context.
The output of the cascade is a (potentially empty) sorted list of declared values for each property on each element.
6.2. Cascading OriginsEach style rule has a cascade origin, which determines where it enters the cascade. CSS defines three core origins:
Extensions to CSS define the following additional origins:
CSS attempts to create a balance of power between author and user style sheets. By default, rules in an author’s style sheet override those in a user’s style sheet, which override those in the user-agent’s default style sheet. To balance this, a declaration can be marked important, which increases its weight in the cascade and inverts the order of precedence.
A declaration is important if it has a !important annotation as defined by [css-syntax-3], i.e. if the last two (non-whitespace, non-comment) tokens in its value are the delimiter token ! followed by the identifier token important. All other declarations are normal (non-important).
[hidden] { display: none !important; }
An important declaration takes precedence over a normal declaration. Author and user style sheets may contain important declarations, with user-origin important declarations overriding author-origin important declarations. This CSS feature improves accessibility of documents by giving users with special requirements (large fonts, color combinations, etc.) control over presentation.
Important declarations from all origins take precedence over animations. This allows authors to override animated values in important cases. (Animated values normally override all other rules.) [css-animations-1]
User-agent style sheets may also contain important declarations. These override all author and user declarations.
The first rule in the user’s style sheet in the following example contains an
!importantdeclaration, which overrides the corresponding declaration in the author’s style sheet. The declaration in the second rule will also win due to being marked
!important. However, the third declaration in the user’s style sheet is not
!importantand will therefore lose to the second rule in the author’s style sheet (which happens to set style on a
shorthandproperty). Also, the third author rule will lose to the second author rule since the second declaration is
!important. This shows that
!importantdeclarations have a function also within author style sheets.
/* From the user’s style sheet */
p { text-indent: 1em !important }
p { font-style: italic !important }
p { font-size: 18pt }
/* From the author’s style sheet */
p { text-indent: 1.5em !important }
p { font: normal 12pt sans-serif !important }
p { font-size: 24pt }
6.4. Precedence of Non-CSS Presentational Hints
The UA may choose to honor presentational hints in a source document’s markup, for example the bgcolor attribute or s element in [HTML]. All document language-based styling must be translated to corresponding CSS rules and enter the cascade as rules in either the UA-origin or a special-purpose author presentational hint origin between the regular user origin and the author origin. For the purpose of cascading this author presentational hint origin is treated as an independent origin, but for the purpose of the revert keyword it is considered part of the author origin.
A document language may define whether such a presentational hint enters the cascade as UA-origin or author-origin; if so, the UA must behave accordingly. For example, [SVG11] maps its presentation attributes into the author origin.
Note: Presentational hints entering the cascade as UA-origin rules can be overridden by author-origin or user-origin styles. Presentational hints entering the cascade as author presentational hint origin rules can be overridden by author-origin styles, but not by non-important user-origin styles. Host languages should choose the appropriate origin for presentational hints with these considerations in mind.
7. DefaultingWhen the cascade does not result in a value, the specified value must be found some other way. Inherited properties draw their defaults from their parent element through inheritance; all other properties take their initial value. Authors can explicitly request inheritance or initialization via the inherit and initial keywords.
7.1. Initial ValuesEach property has an initial value, defined in the property’s definition table. If the property is not an inherited property, and the cascade does not result in a value, then the specified value of the property is its initial value.
7.2. InheritanceInheritance propagates property values from parent elements to their children. The inherited value of a property on an element is the computed value of the property on the element’s parent element. For the root element, which has no parent element, the inherited value is the initial value of the property.
For a [DOM] tree with shadows, inheritance operates on the flattened element tree. This means that slotted elements inherit from the slot they’re assigned to, rather than directly from their light tree parent. Pseudo-elements inherit according to the fictional tag sequence described for each pseudo-element. [CSS-PSEUDO-4]
Some properties are inherited properties, as defined in their property definition table. This means that, unless the cascade results in a value, the value will be determined by inheritance.
A property can also be explicitly inherited. See the inherit keyword.
Note: Inheritance follows the document tree and is not intercepted by anonymous boxes, or otherwise affected by manipulations of the box tree.
7.3. Explicit DefaultingSeveral CSS-wide property values are defined below; declaring a property to have these values explicitly specifies a particular defaulting behavior. As specified in CSS Values and Units [css-values-3], all CSS properties can accept these values.
The keyword revert is a cascade-dependent keyword; some contexts may restrict its use while allowing the other CSS-wide keywords.
7.3.1. Resetting a Property: the initial keywordThe initial CSS-wide keyword represents the value defined as the property’s initial value.
If the cascaded value of a property is the initial keyword, the property’s specified value is its initial value.
7.3.2. Explicit Inheritance: the inherit keywordThe inherit CSS-wide keyword represents the property’s computed value on the parent element.
If the cascaded value of a property is the inherit keyword, the property’s specified and computed values are the inherited value.
7.3.3. Erasing All Declarations: the unset keywordThe unset CSS-wide keyword acts as either inherit or initial, depending on whether the property is inherited or not.
If the cascaded value of a property is the unset keyword, then if it is an inherited property, this is treated as inherit, and if it is not, this is treated as initial. This keyword effectively erases all declared values occurring earlier in the cascade, correctly inheriting or not as appropriate for the property (or all longhands of a shorthand).
7.3.4. Rolling Back Cascade Origins: the revert keywordThe revert CSS-wide keyword rolls back the cascade to the cascaded value of the earlier origin.
If the cascaded value of a property is the revert keyword, the behavior depends on the cascade origin to which the declaration belongs:
Note: Effectively the revert keyword is substituted with the value from the earlier cascade origin. Thus, reverting a shorthand property reverts all its longhands; reverting any property alias of a property reverts all of them; reverting one of the paired properties in a logical property group also reverts the other one; etc.
8. Changes 8.1. Changes since the 13 Jan 2022 Candidate Recommendation SnapshotNon-trivial changes since the 13 Jan 2022 Candidate Recommendation Snapshot:
Clarify that all “aliases” of a property are reverted by revert.
Clarify that style sheets are ordered in [[final CSS style sheets]] order.
Non-trivial changes since the 15 October 2021 Working Draft:
Updated @import grammar for media queries and supports conditions
Allowed functional notation parse-time aliases
Defined fetching an @import, in terms of Fetch
Added § 4.1.1 Value Aliasing section. (Issue 6193)
Non-trivial changes since the 19 March 2021 Working Draft include:
Non-trivial changes since the 18 August 2020 Working Draft include:
Elements that are not connected or are not part of the document’s flattened element tree do not participate in CSS value processing, and do not have declared, cascaded, specified, computed, used, or actual values, even if they potentially have style declarations assigned to them (for example, by a
styleattribute).
Non-trivial changes since the 28 August 2018 Candidate Recommendation include:
Non-trivial changes since the 14 January 2016 Working Draft include:
- user origin
- Rolls back the cascade cascaded value to the user-agent level, so that the specified value is calculated as if no author-level or user-level rules were specified for this property on this element .
- author origin
- Rolls back the cascade cascaded value to the user level, so that the specified value is calculated as if no author-level rules were specified for this property on this element .
The all property is a shorthand that resets all CSS properties except direction and unicode-bidi. … It does not reset custom properties [css-variables-1].
If an @import rule refers to a valid stylesheet, user agents must treat the contents of the stylesheet as if they were written in place of the @import rule , with two exceptions:
- If a feature (such as the @namespace rule) explicitly defines that it only applies to a particular stylesheet, and not any imported ones, then it doesn’t apply to the imported stylesheet.
- If a feature relies on the relative ordering of two or more constructs in a stylesheet (such as the requirement that @charset must not have any other content preceding it), it only applies between constructs in the same stylesheet.
For the purpose of this specification, text nodes are treated as element children of their associated element, and possess the full set of properties; since they cannot be targeted by selectors all of their computed values are assigned by defaulting.
A Disposition of Comments is available.
8.5. Changes Since the 21 April 2015 Working DraftChanges since the 21 April 2015 Working Draft include:
The following features have been added since Level 3:
Introduced revert keyword, for rolling back the cascade.
Introduced supports() syntax for supports-conditional @import rules.
Added encapsulation context to the cascade sort criteria to accommodate Shadow DOM. [DOM]
Defined the property two aliasing mechanisms CSS uses to support legacy syntaxes. See § 3.1 Property Aliasing.
The following features have been added since Level 2:
David Baron, Tantek Çelik, Noam Rosenthal, Simon Sapin, and Boris Zbarsky contributed to this specification.
Privacy and Security ConsiderationsThe cascade process does not distinguish between same-origin and cross-origin stylesheets, enabling the content of cross-origin stylesheets to be inferred from the computed styles they apply to a document.
User preferences and UA defaults expressed via application of style rules are exposed by the cascade process, and can be inferred from the computed styles they apply to a document.
The @import rule does not apply the CORS protocol to loading cross-origin stylesheets, instead allowing them to be freely imported and applied.
The @import rule assumes that resources without Content-Type metadata (or any same-origin file if the host document is in quirks mode) are text/css, potentially allowing arbitrary files to be imported into the page and interpreted as CSS, potentially allowing sensitive data to be inferred from the computed styles they apply to a document.
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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