CSS Conditional Rules Module Level 3

Editor’s Draft,

More details about this document
This version:
https://drafts.csswg.org/css-conditional-3/
Latest published version:
https://www.w3.org/TR/css-conditional-3/
Previous Versions:
Implementation Report:
https://wpt.fyi/results/css/css-conditional?label=master&label=experimental&aligned
Test Suites:
http://test.csswg.org/suites/css-conditional-3_dev/nightly-unstable/
https://wpt.fyi/results/css/css-conditional/
Feedback:
CSSWG Issues Repository
Editors:
L. David Baron (Mozilla)
Elika J. Etemad / fantasai (Invited Expert)
Chris Lilley (W3C)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This module contains the features of CSS for conditional processing of parts of style sheets, conditioned on capabilities of the processor or the document the style sheet is being applied to. It includes and extends the functionality of CSS level 2 [CSS21], which builds on CSS level 1 [CSS1]. The main extensions compared to level 2 are allowing nesting of certain at-rules inside @media, and the addition of the @supports rule for conditional processing.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-conditional” in the title, like this: “[css-conditional] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 2 November 2021 W3C Process Document.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

1.1. Background

This section is not normative.

[CSS21] defines one type of conditional group rule, the @media rule, and allows only style rules (not other @-rules) inside of it. The @media rule provides the ability to have media-specific style sheets, which is also provided by style sheet linking features such as @import and link. The restrictions on the contents of @media rules made them less useful; they have forced authors using CSS features involving @-rules in media-specific style sheets to use separate style sheets for each medium.

This specification extends the rules for the contents of conditional group rules to allow other @-rules, which enables authors to combine CSS features involving @-rules with media specific style sheets within a single style sheet.

This specification also defines an additional type of conditional group rule, @supports, to address author and user requirements.

The @supports rule allows CSS to be conditioned on implementation support for CSS properties and values. This rule makes it much easier for authors to use new CSS features and provide good fallback for implementations that do not support those features. This is particularly important for CSS features that provide new layout mechanisms, and for other cases where a set of related styles needs to be conditioned on property support.

1.2. Module Interactions

This module replaces and extends the @media rule feature defined in [CSS21] section 7.2.1 and incorporates the modifications previously made non-normatively by [MEDIAQUERIES-4] section 1.

2. Processing of conditional group rules

This specification defines some CSS at-rules, called conditional group rules, that associate a condition with a group of other CSS rules. These different rules allow testing different types of conditions, but share common behavior for how their contents are used when the condition is true and when the condition is false.

For example, this rule:
@media print {
  /* hide navigation controls when printing */
  #navigation { display: none }
}

causes a particular CSS rule (making elements with ID “navigation” be display:none) apply only when the style sheet is used for a print medium.

Each conditional group rule has a condition, which at any time evaluates to true or false. When the condition is true, CSS processors must apply the rules inside the group rule as though they were at the group rule’s location; when the condition is false, CSS processors must not apply any of rules inside the group rule. The current state of the condition does not affect the CSS object model, in which the contents of the group rule always remain within the group rule.

Tests

If condition is true, rules applied in place.


If condition is false, rules not applied.


CSSOM contains rules regardless of condition.


This means that when multiple conditional group rules are nested, a rule inside of both of them applies only when all of the rules' conditions are true.

Tests

Nested rules apply when all conditions are true.


For example, with this set of nested rules:
@media print { /* rule (1) */
  /* hide navigation controls when printing */
  #navigation { display: none }
  @media (max-width: 12cm) { /* rule (2) */
    /* keep notes in flow when printing to narrow pages */
    .note { float: none }
  }
}

the condition of the rule marked (1) is true for print media, and the condition of the rule marked (2) is true when the width of the display area (which for print media is the page box) is less than or equal to 12cm. Thus the rule #navigation { display: none } applies whenever this style sheet is applied to print media, and the rule .note { float: none } is applied only when the style sheet is applied to print media and the width of the page box is less than or equal to 12 centimeters.

When the condition for a conditional group rule changes, CSS processors must reflect that the rules now apply or no longer apply, except for properties whose definitions define effects of computed values that persist past the lifetime of that value (such as for some properties in [CSS3-TRANSITIONS] and [CSS3-ANIMATIONS]).

Tests

Change in condition simultaneous with change in condition application (except per transition/animation rules).


3. Contents of conditional group rules

All conditional group rules are defined to take a <stylesheet> in their block, which means they can accept any rule that is normally allowed at the top-level of a stylesheet, and not otherwise restricted. (For example, an @import rule must appear at the actual beginning of a stylesheet, and so is not valid inside of another rule.)

Tests

Valid to nest all unrestricted top-level rules.


Invalid or unknown rules inside the <stylesheet> must be considered invalid and ignored, but do not invalidate the conditional group rule.

Tests

Invalid rules do not invalidate conditional group rule.


Any namespace prefixes used in a conditional group rule must have been declared, otherwise they are invalid.

For example, this rule containing a CSS qualified name is valid, because the namespace prefix has been bound to a namespace url:
@namespace x url(http://www.w3.org/1999/xlink);
@supports (content: attr(x|href)) {
  // do something }
For example, to determine whether this rule is valid:
@supports (content: attr(n|tooltip)) {
  // do something }

The user agent will consult the namespace map to see whether a namespace url exists corresponding to the "n" prefix.

Tests

Validity of namespace prefixes depends on namespace map.


4. Placement of conditional group rules

Conditional group rules are allowed wherever style rules are allowed (at the top-level of a style sheet, as well as within other conditional group rules). CSS processors must process such rules as described above.

Tests

Conditional group rules allowed wherever style rules are allowed.


Any at-rules that are not allowed after a style rule (e.g., @charset, @import, or @namespace rules) are also not allowed after a conditional group rule, and are therefore invalid when so placed.

Tests

At rules not allowed after style rule invalid after conditional group rule.


5. Media-specific style sheets: the @media rule

@media

In all current engines.

Firefox1+Safari3+Chrome1+
Opera9.2+Edge79+
Edge (Legacy)12+IE6+
Firefox for Android?iOS Safari1+Chrome for Android?Android WebView37+Samsung Internet?Opera Mobile10.1+

The @media rule is a conditional group rule whose condition is a media query. Its syntax is:

@media <media-query-list> {
  <stylesheet>
}

It consists of the at-keyword @media followed by a (possibly empty) media query list (as defined in [MEDIAQUERIES-4]), followed by a block containing arbitrary rules. The condition of the rule is the result of the media query.

This @media rule:
@media screen and (min-width: 35em),
       print and (min-width: 40em) {
  #section_navigation { float: left; width: 10em; }
}

has the condition screen and (min-width: 35em), print and (min-width: 40em), which is true for screen displays whose viewport is at least 35 times the initial font size and for print displays whose viewport is at least 40 times the initial font size. When either of these is true, the condition of the rule is true, and the rule #section_navigation { float: left; width: 10em; } is applied.

Tests

Media rule condition can be empty


Media rule condition is media query


White space is optional where not required for tokenization.


6. Feature queries: the @supports rule

@supports

In all current engines.

Firefox22+Safari9+Chrome28+
Opera12.1+Edge79+
Edge (Legacy)12+IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile12.1+

The @supports rule is a conditional group rule whose condition tests whether the user agent supports CSS property:value pairs. Authors can use it to write style sheets that use new features when available but degrade gracefully when those features are not supported. These queries are called CSS feature queries or (colloquially) supports queries.

Note: CSS has existing mechanisms for graceful degradation, such as ignoring unsupported properties or values, but these are not always sufficient when large groups of styles need to be tied to the support for certain features, as is the case for use of new layout system features.

The syntax of the condition in the @supports rule is similar to that defined for <media-condition> in [MEDIAQUERIES-4], but without the "unknown" value logic:

Therefore, the syntax of the @supports rule allows testing for property:value pairs, and arbitrary conjunctions (and), disjunctions (or), and negations (not) of them.

The syntax of the @supports rule is:

@supports <supports-condition> {
  <stylesheet>
}

with <supports-condition> defined as:

<supports-condition> = not <supports-in-parens>
                     | <supports-in-parens> [ and <supports-in-parens> ]*
                     | <supports-in-parens> [ or <supports-in-parens> ]*
<supports-in-parens> = ( <supports-condition> ) | <supports-feature> | <general-enclosed>
<supports-feature> = <supports-decl>
<supports-decl> = ( <declaration> )

The above grammar is purposely very loose for forwards-compatibility reasons, since the <general-enclosed> production allows for substantial future extensibility. Any @supports rule that does not parse according to the grammar above (that is, a rule that does not match this loose grammar which includes the <general-enclosed> production) is invalid. Style sheets must not use such a rule and processors must ignore such a rule (including all of its contents).

Tests

White space is optional where not required for tokenization.


Grammatically-invalid @supports rule is ignored.


Each of these grammar terms is associated with a boolean result, as follows:

<supports-condition>
<supports-in-parens>

The result is the result of the child subexpression.

not <supports-in-parens>

The result is the negation of the <supports-in-parens> term.

Tests

Not negates supports condition.


Not must be followed by space.


Not requires parentheses.


<supports-in-parens> [ and <supports-in-parens> ]*

The result is true if all of the <supports-in-parens> child terms are true, and false otherwise.

Tests

And condition is true iff all linked conditions are true.


And requires parentheses.


<supports-in-parens> [ or <supports-in-parens> ]*

The result is false if all of the <supports-in-parens> child terms are false, and true otherwise.

Tests

Or condition is true iff any of linked conditions is true.


Or must be followed by space.


Or requires parentheses.


<supports-decl>

The result is true if the UA supports the declaration within the parentheses.

Tests

Supports condition is true iff declaration is supported.


Declaration cannot include semicolon.


Declaration value can be empty.


Declaration cannot include invalid !tokens.


<general-enclosed>

The result is false.

Authors must not use <general-enclosed> in their stylesheets. It exists only for future-compatibility, so that new syntax additions do not invalidate too much of a <supports-condition> in older user agents.

Tests

Unrecognized but grammatically-valid condition is false, not invalid.


Brackets/parenthesis must be balanced


The condition of the @supports rule is the result of the <supports-condition> in its prelude.

For example, the following rule
@supports ( display: flex ) {
  body, #navigation, #content { display: flex; }
  #navigation { background: blue; color: white; }
  #article { background: white; color: black; }
}

applies the rules inside the @supports rule only when display: flex is supported.

The following example shows an additional @supports rule that can be used to provide an alternative for when display: flex is not supported:
@supports not ( display: flex ) {
  body { width: 100%; height: 100%; background: white; color: black; }
  #navigation { width: 25%; }
  #article { width: 75%; }
}

Note that the width declarations may be harmful to the flex-based layout, so it is important that they be present only in the non-flex styles.

The following example checks for support for the box-shadow property, including checking for support for vendor-prefixed versions of it. When the support is present, it specifies both box-shadow (with the prefixed versions) and border in a way what would cause the box to become invisible were box-shadow not supported.
.noticebox {
  border: 1px solid black;
  padding: 1px;
}
@supports ( box-shadow: 0 0 2px black inset ) or
          ( -moz-box-shadow: 0 0 2px black inset ) or
          ( -webkit-box-shadow: 0 0 2px black inset ) or
          ( -o-box-shadow: 0 0 2px black inset ) {
  .noticebox {
    -moz-box-shadow: 0 0 2px black inset;
    -webkit-box-shadow: 0 0 2px black inset;
    -o-box-shadow: 0 0 2px black inset;
    box-shadow: 0 0 2px black inset; /* unprefixed last */
    /* override the rule above the @supports rule */
    border: none;
    padding: 2px;
  }
}

To avoid confusion between and and or, the syntax requires that both and and or be specified explicitly (rather than, say, using commas or spaces for one of them). Likewise, to avoid confusion caused by precedence rules, the syntax does not allow and, or, and not operators to be mixed without a layer of parentheses.

For example, the following rule is not valid:
@supports (transition-property: color) or
          (animation-name: foo) and
          (transform: rotate(10deg)) {
  /* ... */
}

Instead, authors must write one of the following:

@supports ((transition-property: color) or
           (animation-name: foo)) and
          (transform: rotate(10deg)) {
  /* ... */
}
@supports (transition-property: color) or
          ((animation-name: foo) and
           (transform: rotate(10deg))) {
  /* ... */
}
Tests

Parentheses are required to mix opeartors.


The declaration being tested must always occur within parentheses, when it is the only thing in the expression.

For example, the following rule is not valid:
@supports display: flex {
  /* ... */
}

Instead, authors must write:

@supports (display: flex) {
  /* ... */
}
Tests

Parentheses are required around declaration test.


The syntax allows extra parentheses when they are not needed. This flexibility is sometimes useful for authors (for example, when commenting out parts of an expression) and may also be useful for authoring tools.

For example, authors may write:
@supports ((display: flex)) {
  /* ... */
}
Tests

Extra parentheses are allowed.


A trailing !important on a declaration being tested is allowed, though it won’t change the validity of the declaration.

For example, the following rule is valid:
@supports (display: flex !important) {
  /* ... */
}
Tests

!important is allowed.


6.1. Definition of support

For forward-compatibility, section 4.1.8 (Declarations and properties) of [CSS21] defines rules for handling invalid properties and values. CSS processors that do not implement or partially implement a specification must treat any part of a value that they do not implement, or do not have a usable level of support for, as invalid according to this rule for handling invalid properties and values, and therefore must discard the declaration as a parse error.

A CSS processor is considered to support a declaration (consisting of a property and value) if it accepts that declaration (rather than discarding it as a parse error) within a style rule. If a processor does not implement, with a usable level of support, the value given, then it must not accept the declaration or claim support for it.

Note: Note that properties or values whose support is effectively disabled by user preferences are still considered as supported by this definition. For example, if a user has enabled a high-contrast mode that causes colors to be overridden, the CSS processor is still considered to support the color property even though declarations of the color property may have no effect. On the other hand, a developer-facing preference whose purpose is to enable or disable support for an experimental CSS feature does affect this definition of support.

These rules (and the equivalence between them) allow authors to use fallback (either in the [CSS1] sense of declarations that are overridden by later declarations or with the new capabilities provided by the @supports rule in this specification) that works correctly for the features implemented. This applies especially to compound values; implementations must implement all parts of the value in order to consider the declaration supported, either inside a style rule or in the declaration condition of an @supports rule.

Tests

Supports queries are true iff property declaration (including all values) is parsed/supported.


7. APIs

Tests

7.1. Extensions to the CSSRule interface

The CSSRule interface is extended as follows:

partial interface CSSRule {
    const unsigned short SUPPORTS_RULE = 12;
};
Tests

CSSRule.SUPPORTS_RULE = 12


7.2. The CSSConditionRule interface

CSSConditionRule

In all current engines.

Firefox20+Safari14.1+Chrome56+
Opera?Edge79+
Edge (Legacy)12+IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile?

The CSSConditionRule interface represents all the “conditional” at-rules, which consist of a condition and a statement block.

CSSConditionRule/conditionText

In all current engines.

Firefox20+Safari14.1+Chrome56+
Opera?Edge79+
Edge (Legacy)12+IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile?
[Exposed=Window]
interface CSSConditionRule : CSSGroupingRule {
    readonly attribute CSSOMString conditionText;
};
Tests

CSSConditionRule inherits from CSSGroupingRule.


CSSConditionRule has .conditionText attribute.


conditionText of type CSSOMString
The conditionText attribute represents the condition of the rule. Since what this condition does varies between the derived interfaces of CSSConditionRule, those derived interfaces may specify different behavior for this attribute (see, for example, CSSMediaRule below). In the absence of such rule-specific behavior, the following rules apply:

The conditionText attribute, on getting, must return the result of serializing the associated condition.

Tests

.conditionText returns serialization of condition.


    7.3. The CSSMediaRule interface

    CSSMediaRule

    In all current engines.

    Firefox1+Safari3+Chrome1+
    Opera12.1+Edge79+
    Edge (Legacy)12+IE9+
    Firefox for Android?iOS Safari1+Chrome for Android?Android WebView?Samsung Internet?Opera Mobile12.1+

    The CSSMediaRule interface represents a @media at-rule:

    CSSMediaRule/media

    In all current engines.

    Firefox1+Safari3+Chrome1+
    Opera12.1+Edge79+
    Edge (Legacy)12+IE9+
    Firefox for Android?iOS Safari1+Chrome for Android?Android WebView?Samsung Internet?Opera Mobile12.1+
    [Exposed=Window]
    interface CSSMediaRule : CSSConditionRule {
        [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
    };
    
    media of type MediaList, readonly
    The media attribute must return a MediaList object for the list of media queries specified with the @media at-rule.
    Tests

    .media returns a MediaList matching the @media condition.


      conditionText of type CSSOMString (CSSMediaRule-specific definition for attribute on CSSConditionRule)
      The conditionText attribute (defined on the CSSConditionRule parent rule), on getting, must return the value of media.mediaText on the rule.
      Tests

      Value of CSSMediaRule.conditionText matches value of media.mediaText.


        Setting the conditionText attribute must set the media.mediaText attribute on the rule.

        Tests

        Setting CSSMediaRule.conditionText also sets .media.mediaText.


          7.4. The CSSSupportsRule interface

          CSSSupportsRule

          In all current engines.

          Firefox22+Safari9+Chrome28+
          Opera12.1+Edge79+
          Edge (Legacy)12+IENone
          Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile15+

          The CSSSupportsRule interface represents a @supports rule.

          [Exposed=Window]
          interface CSSSupportsRule : CSSConditionRule {
          };
          
          conditionText of type CSSOMString (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
          The conditionText attribute (defined on the CSSConditionRule parent rule), on getting, must return the condition that was specified, without any logical simplifications, so that the returned condition will evaluate to the same result as the specified condition in any conformant implementation of this specification (including implementations that implement future extensions allowed by the <general-enclosed> extensibility mechanism in this specification). In other words, token stream simplifications are allowed (such as reducing whitespace to a single space or omitting it in cases where it is known to be optional), but logical simplifications (such as removal of unneeded parentheses, or simplification based on evaluating results) are not allowed.
          Tests

          CSSSupportsRule.conditionText can have tokenization simplifications.


            CSSSupportsRule.conditionText cannot have other simplifications.


              7.5. The CSS namespace, and the supports() function

              The CSS namespace holds useful CSS-related functions that do not belong elsewhere.

              partial namespace CSS {
                boolean supports(CSSOMString property, CSSOMString value);
                boolean supports(CSSOMString conditionText);
              };
              
              supports(CSSOMString property, CSSOMString value), returns boolean

              CSS/supports

              In all current engines.

              Firefox55+Safari9+Chrome61+
              Opera12.1+Edge79+
              Edge (Legacy)12+IENone
              Firefox for Android?iOS Safari?Chrome for Android?Android WebView61+Samsung Internet8.0+Opera Mobile12.1+
              When the supports(property, value) method is invoked with two arguments property and value:
              1. If property is an ASCII case-insensitive match for any defined CSS property that the UA supports, and value successfully parses according to that property’s grammar, return true.

              2. Otherwise, if property is a custom property name string, return true.

              3. Otherwise, return false.

              Note: No CSS escape or whitespace processing is performed on the property name, so CSS.supports(" width", "5px") will return false, as " width" isn’t the name of any property due to the leading space.

              Note: !important flags are not part of property grammars, and will cause value to parse as invalid, just as they would in the value argument to element.style.setProperty().

              Tests

              CSS.supports(arg1, arg2) evaluates support of property arg1 with value arg2.


                supports(CSSOMString conditionText), returns boolean
                When the supports(conditionText) method is invoked with a single conditionText argument:
                1. If conditionText, parsed and evaluated as a <supports-condition>, would return true, return true.

                2. Otherwise, If conditionText, wrapped in parentheses and then parsed and evaluated as a <supports-condition>, would return true, return true.

                3. Otherwise, return false.

                All namespaces in the conditionText argument are considered invalid, just as they are in document.querySelector("a|b").

                Tests

                CSS.supports(arg1) evaluates supports condition arg1.


                  CSS.supports(arg1) implies parentheses.


                    Tests

                    Security Considerations

                    This spec introduces no new security considerations.

                    Privacy Considerations

                    Various features in this specification, associated mainly with the @media rule but also to some degree with the @supports rule, provide information to Web content about the user’s hardware and software and their configuration and state. Most of the information is provided through the features in [MEDIAQUERIES-4] rather than through the features in this specification. However, the @supports rule may provide some additional details about the user’s software and whether it is running with non-default settings that may enable or disable certain features.

                    Most of this information can also be determined through other APIs. However, the features in this specification are one of the ways this information is exposed on the Web.

                    This information can also, in aggregate, be used to improve the accuracy of fingerprinting of the user.

                    8. Changes

                    The following (non-editorial) changes were made to this specification since the 13 January 2022 Candidate Recommendation Snapshot:

                    The following (non-editorial) changes were made to this specification since the 8 December 2020 Candidate Recommendation Snapshot:

                    The following (non-editorial) changes were made to this specification since the 4 April 2013 Candidate Recommendation:

                    Acknowledgments

                    Thanks to the ideas and feedback from Tab Atkins, Arthur Barstow, Ben Callahan, Tantek Çelik, Alex Danilo, Elika Etemad, Pascal Germroth, Björn Höhrmann, Paul Irish, Brad Kemper, Anne van Kesteren, Vitor Menezes, Alex Mogilevsky, Chris Moschini, James Nurthen, Simon Pieters, Florian Rivoal, Simon Sapin, Nicholas Shanks, Ben Ward, Zack Weinberg, Estelle Weyl, Boris Zbarsky, and all the rest of the www-style community.

                    Conformance

                    Document conventions

                    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:

                    This is an example of an informative example.

                    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.

                    Tests

                    Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


                    Conformance classes

                    Conformance to this specification is defined for three conformance classes:

                    style sheet
                    A CSS style sheet.
                    renderer
                    A UA that interprets the semantics of a style sheet and renders documents that use them.
                    authoring tool
                    A UA that writes a style sheet.

                    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.

                    Partial implementations

                    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.

                    Implementations of Unstable and Proprietary Features

                    To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

                    Non-experimental implementations

                    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.

                    Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

                    Index

                    Terms defined by this specification

                    Terms defined by reference

                    References

                    Normative References

                    [CSS-CASCADE-5]
                    Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. 13 January 2022. CR. URL: https://www.w3.org/TR/css-cascade-5/
                    [CSS-CONDITIONAL-5]
                    David Baron; Elika Etemad; Chris Lilley. CSS Conditional Rules Module Level 5. 21 December 2021. WD. URL: https://www.w3.org/TR/css-conditional-5/
                    [CSS-SYNTAX-3]
                    Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 24 December 2021. CR. URL: https://www.w3.org/TR/css-syntax-3/
                    [CSS-TYPED-OM-1]
                    Shane Stephens; Tab Atkins Jr.; Naina Raisinghani. CSS Typed OM Level 1. 10 April 2018. WD. URL: https://www.w3.org/TR/css-typed-om-1/
                    [CSS-VALUES-4]
                    Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 16 December 2021. WD. URL: https://www.w3.org/TR/css-values-4/
                    [CSS21]
                    Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
                    [CSS3-ANIMATIONS]
                    Dean Jackson; et al. CSS Animations Level 1. 11 October 2018. WD. URL: https://www.w3.org/TR/css-animations-1/
                    [CSSOM-1]
                    Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). 26 August 2021. WD. URL: https://www.w3.org/TR/cssom-1/
                    [HTML]
                    Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
                    [INFRA]
                    Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
                    [MEDIAQUERIES-4]
                    Florian Rivoal; Tab Atkins Jr.. Media Queries Level 4. 25 December 2021. CR. URL: https://www.w3.org/TR/mediaqueries-4/
                    [MEDIAQUERIES-5]
                    Dean Jackson; et al. Media Queries Level 5. 18 December 2021. WD. URL: https://www.w3.org/TR/mediaqueries-5/
                    [RFC2119]
                    S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
                    [WEBIDL]
                    Edgar Chen; Timothy Gu. Web IDL Standard. Living Standard. URL: https://webidl.spec.whatwg.org/

                    Informative References

                    [CSS-BACKGROUNDS-3]
                    Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 26 July 2021. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
                    [CSS-COLOR-4]
                    Tab Atkins Jr.; Chris Lilley; Lea Verou. CSS Color Module Level 4. 28 April 2022. WD. URL: https://www.w3.org/TR/css-color-4/
                    [CSS-DISPLAY-3]
                    Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 3 September 2021. CR. URL: https://www.w3.org/TR/css-display-3/
                    [CSS-NAMESPACES-3]
                    Elika Etemad. CSS Namespaces Module Level 3. 20 March 2014. REC. URL: https://www.w3.org/TR/css-namespaces-3/
                    [CSS-SIZING-3]
                    Tab Atkins Jr.; Elika Etemad. CSS Box Sizing Module Level 3. 17 December 2021. WD. URL: https://www.w3.org/TR/css-sizing-3/
                    [CSS1]
                    Håkon Wium Lie; Bert Bos. Cascading Style Sheets, level 1. 13 September 2018. REC. URL: https://www.w3.org/TR/CSS1/
                    [CSS3-TRANSITIONS]
                    David Baron; et al. CSS Transitions. 11 October 2018. WD. URL: https://www.w3.org/TR/css-transitions-1/

                    IDL Index

                    partial interface CSSRule {
                        const unsigned short SUPPORTS_RULE = 12;
                    };
                    
                    [Exposed=Window]
                    interface CSSConditionRule : CSSGroupingRule {
                        readonly attribute CSSOMString conditionText;
                    };
                    
                    [Exposed=Window]
                    interface CSSMediaRule : CSSConditionRule {
                        [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
                    };
                    
                    [Exposed=Window]
                    interface CSSSupportsRule : CSSConditionRule {
                    };
                    
                    partial namespace CSS {
                      boolean supports(CSSOMString property, CSSOMString value);
                      boolean supports(CSSOMString conditionText);
                    };