The FCC just voted to kill net neutrality. Congress can still stop it.

CSS Conditional Rules Module Level 3

Editor’s Draft,

Specification Metadata
This version:
Latest published version:
Previous Versions:
Test Suite:
L. David Baron (Mozilla)
Issue Tracking:
GitHub Issues


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, in speech, 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.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-conditional” in the title, preferably like this: “[css-conditional] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 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 [MEDIAQ] section 1.

Its current definition depends on @-rules defined in [CSS3-FONTS] and [CSS3-ANIMATIONS], but that dependency is only on the assumption that those modules will advance ahead of this one. If this module advances faster, then the dependency will be reversed.

In order to allow these new @-rules in CSS style sheets, this specification modifies the stylesheet production in the Appendix G grammar of [CSS21] by replacing the media production defined in [CSS21] with the media production defined in this one, and additionally inserting | supports_rule alongside ruleset | media | page.

2. Processing of conditional group rules

This specification defines some CSS @-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.

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.

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]).

3. Contents of conditional group rules

The syntax of each conditional group rule consists of some syntax specific to the type of rule followed by a group rule body, which is a block (pair of braces) containing a sequence of rules.

A group rule body is allowed to contain style rules and any @-rules that are allowed at the top level of a style sheet before and after a style rule. This means that @-rules that must occur at the beginning of the style sheet (such as '@charset', '@import', and '@namespace' rules) are not allowed inside of conditional group rules. Conditional group rules can be nested.

In terms of the grammar, this specification defines the following productions for use in the grammar of conditional group rules:

Note: Style rules are defined in grammars by the ruleset production.

  : ruleset | media | page | font_face_rule | keyframes_rule |

  : '{' S* nested_statement* '}' S*

in which all the productions are defined in that grammar with the exception of font_face_rule defined in [CSS3-FONTS], keyframes_rule defined in [CSS3-ANIMATIONS], and media and supports_rule defined in this specification.

In general, future CSS specifications that add new @-rules that are not forbidden to occur after some other types of rules should modify this nested_statement production to keep the grammar accurate.

Style sheets must not use rules other than the allowed ones inside conditional group rules.

CSS processors must ignore rules that are not allowed within a group rule, and must handle invalid rules inside of group rules as described in section 4.2 (Rules for handling parsing errors), section 4.1.5 (At-rules), and section 4.1.7 (Rule sets, declaration blocks, and selectors) of [CSS21].

4. Placement of conditional group rules

Conditional group rules are allowed at the top-level of a style sheet, and inside other conditional group rules. CSS processors must process such rules as described above.

Any 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. Therefore, style sheets must not place such rules after a conditional group rules, and CSS processors must ignore such rules.

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

The @media rule is a conditional group rule whose condition is a media query. It consists of the at-keyword '@media' followed by a (possibly empty) media query list (as defined in [MEDIAQ]), followed by a group rule body. 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.

In terms of the grammar, this specification extends the media production in the Grammar of CSS 2.1 ([CSS21], Appendix G) into:

  : MEDIA_SYM S* media_query_list group_rule_body

where the group_rule_body production is defined in this specification, the media_query_list production is defined in [MEDIAQ], and the others are defined in the Grammar of CSS 2.1 ([CSS21], Appendix G).

6. Feature queries: the '@supports' rule

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. 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 slightly more complicated than for the other conditional group rules (though has some similarities to media queries) since:

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

This extends the lexical scanner in the Grammar of CSS 2.1 ([CSS21], Appendix G) by adding:

@{S}{U}{P}{P}{O}{R}{T}{S} {return SUPPORTS_SYM;}
{O}{R}                    {return OR;}

This then extends the grammar in the Grammar of CSS 2.1, using the lexical scanner there, with the additions of AND and NOT tokens defined in the Media Queries specification [MEDIAQ] and the OR and SUPPORTS_SYM tokens defined above, and with declaration, any, and unused productions and the FUNCTION token taken from the core syntax of CSS defined in section 4.1.1 (Tokenization) of [CSS21], by adding:

  : SUPPORTS_SYM S* supports_condition group_rule_body

  : supports_negation | supports_conjunction | supports_disjunction |

  : ( '(' S* supports_condition ')' S* ) | supports_declaration_condition |

  : NOT S* supports_condition_in_parens

  : supports_condition_in_parens ( AND S* supports_condition_in_parens )+

  : supports_condition_in_parens ( OR S* supports_condition_in_parens )+

  : '(' S* declaration ')' S*

  : ( FUNCTION | '(' ) ( any | unused )* ')' S*

The <supports-condition> production is defined as matching the supports_condition grammar, defined above.

Implementations must parse @supports rules based on the above grammar, and when interpreting the above grammar, must match the production before an | operator in preference to the one after it.

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).

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

The result is the result of the single child term.
The result is the result of the single supports_condition or supports_declaration_condition child term.
The result is the negation of the result of the supports_condition_in_parens child term.
The result is true if the result of all of the supports_condition_in_parens child terms is true; otherwise it is false.
The result is true if the result of any of the supports_condition_in_parens child terms is true; otherwise it is false.
The result is whether the CSS processor supports the declaration within the parentheses.
The result is always false. Additionally, style sheets must not write @supports rules that match this grammar production. (In other words, this production exists only for future extensibility, and is not part of the description of a valid style sheet in this level of the specification.) Note that future levels may define functions or other parenthesized expressions that can evaluate to true.

The condition of the '@supports' rule is the result of the supports_condition term that is a child of the supports_rule term.

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))) {
  // ...

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) {
  // ...

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)) {
  // ...

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) {
  // ...

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). 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 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.

7. APIs

7.1. Extensions to the CSSRule interface

The CSSRule interface is extended as follows:

partial interface CSSRule {
    const unsigned short SUPPORTS_RULE = 12;

7.2. The CSSGroupingRule interface

The CSSGroupingRule interface represents an at-rule that contains other rules nested inside itself.

interface CSSGroupingRule : CSSRule {
    readonly attribute CSSRuleList cssRules;
    unsigned long insertRule (CSSOMString rule, unsigned long index);
    void deleteRule (unsigned long index);
cssRules of type CSSRuleList, readonly
The cssRules attribute must return a CSSRuleList object for the list of CSS rules nested inside the grouping rule.
insertRule(CSSOMString rule, unsigned long index), returns unsigned long
The insertRule operation must insert a CSS rule rule into the CSS rule list returned by cssRules, such that the inserted rule will be at position index, and any rules previously at index or higher will increase their index by one. It must throw INDEX_SIZE_ERR if index is greater than cssRules.length. It must throw SYNTAX_ERR if rule has a syntax error and is unparseable; this does not include syntax errors handled by error handling rules for constructs inside of the rule, but this does include cases where the string given does not parse into a single CSS rule (such as when the string is empty) or where there is anything other than whitespace or comments after that single CSS rule. It must throw HIERARCHY_REQUEST_ERR if the rule cannot be inserted at the location specified, for example, if an @import rule is inserted inside a group rule.

The return value is the index parameter.

deleteRule (unsigned long index), return void
The deleteRule operation must remove a CSS rule from the CSS rule list returned by cssRules at index. It must throw INDEX_SIZE_ERR if index is greater than or equal to cssRules.length.

7.3. The CSSConditionRule interface

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

interface CSSConditionRule : CSSGroupingRule {
    attribute CSSOMString conditionText;
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.

On setting the conditionText attribute these steps must be run:

  1. Trim the given value of white space.
  2. If the given value matches the grammar of the appropriate condition production for the given rule, replace the associated CSS condition with the given value.
  3. Otherwise, do nothing.

7.4. The CSSMediaRule interface

The CSSMediaRule interface represents a @media at-rule:

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.
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.

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

7.5. The CSSSupportsRule interface

The CSSSupportsRule interface represents a @supports rule.

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 exensibility 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.

7.6. 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
supports(CSSOMString conditionText), returns boolean
When the supports() method is invoked with two arguments property and value, it must return true if property is a literal match for the name of a CSS property that the UA supports, and value would be successfully parsed as a supported value for that property. (Literal match means that no CSS escape processing is performed, and leading and trailing whitespace are not stripped, so any leading whitespace, trailing whitespace, or CSS escapes equivalent to the name of a property would cause the method to return false.) Otherwise, it must return false.

When invoked with a single conditionText argument, it must return true if conditionText, when either parsed and evaluated as a supports_condition or parsed as a declaration, wrapped in implied parentheses, and evaluated as a supports_condition, would return true. Otherwise, it must return false.

Privacy and Security Considerations

This spec introduces no new security 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 [MEDIAQ] 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 4 April 2013 Candidate Recommendation:


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.


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.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
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.

Requirements for Responsible Implementation of CSS

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

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 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.

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.

Implementations of CR-level Features

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.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at Questions should be directed to the mailing list.


Terms defined by this specification

Terms defined by reference


Normative References

Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 14 January 2016. CR. URL:
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 20 February 2014. CR. URL:
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL:
Dean Jackson; et al. CSS Animations Level 1. 30 November 2017. WD. URL:
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 3. 26 June 2018. CR. URL:
Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL:
Florian Rivoal; Tab Atkins Jr.. Media Queries Level 4. 5 September 2017. CR. URL:
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. 15 December 2016. ED. URL:

Informative References

Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 17 October 2017. CR. URL:
Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 19 June 2018. REC. URL:
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 20 April 2018. WD. URL:
Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. REC. URL:
David Baron; Dean Jackson; Brian Birtles. CSS Transitions. 30 November 2017. WD. URL:

IDL Index

partial interface CSSRule {
    const unsigned short SUPPORTS_RULE = 12;

interface CSSGroupingRule : CSSRule {
    readonly attribute CSSRuleList cssRules;
    unsigned long insertRule (CSSOMString rule, unsigned long index);
    void deleteRule (unsigned long index);

interface CSSConditionRule : CSSGroupingRule {
    attribute CSSOMString conditionText;

interface CSSMediaRule : CSSConditionRule {
    [SameObject, PutForwards=mediaText] readonly attribute MediaList media;

interface CSSSupportsRule : CSSConditionRule {

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