CSS Conditional Rules Module Level 3

W3C Working Draft 13 December 2012

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

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-conditional” in the subject, preferably like this: “[css3-conditional] …summary of comment…

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

This document was produced by a group operating under the 5 February 2004 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.

The following features are at risk:

This is a Last Call Working Draft. The deadline for comments is 10 January 2013.

Table of contents

1. Introduction

1.1. Background

This section is not normative.

[CSS21] defines one type of conditional group rule, the ‘@media’ rule, and allows only rulesets (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.

1.3. 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.

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 {
  #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)
  #navigation { display: none }
  @media (max-width: 12cm) { // rule (2)
    .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 rulesets and any @-rules that are allowed at the top level of a style sheet before and after a ruleset. 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:

  : 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 ruleset (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 print, (max-width: 600px) {
  #extra_navigation { display: none }

has the condition ‘print, (max-width: 600px)’, which is true for print media and for devices whose width is at most 600px. When either of these is true, the condition of the rule is true, and the rule ‘#extra_navigation { display: none }’ 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;}

and the grammar 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*

in which declaration, any, and unused are the productions in the core syntax of CSS defined in section 4.1.1 (Tokenization) of [CSS21], and the AND and NOT tokens are defined in the Media Queries specification [MEDIAQ].

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: flexbox ) {
  body, #navigation, #content { display: flexbox; }
  #navigation { background: blue; color: white; }
  #article { background: white; color: black; }

applies the rules inside the ‘@supports’ rule only when ‘display: flexbox’ is supported.

The following example shows an additional ‘@supports’ rule that can be used to provide an alternative for when ‘display: flexbox’ is not supported:

@supports not ( display: flexbox ) {
  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 flexbox-based layout, so it is important that they be present only in the non-flexbox 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 ‘color’ in a way what would cause the text to become invisible were ‘box-shadow’ not supported.

@supports ( box-shadow: 2px 2px 2px black ) or
          ( -moz-box-shadow: 2px 2px 2px black ) or
          ( -webkit-box-shadow: 2px 2px 2px black ) or
          ( -o-box-shadow: 2px 2px 2px black ) {
  .outline {
    color: white;
    -moz-box-shadow: 2px 2px 2px black;
    -webkit-box-shadow: 2px 2px 2px black;
    -o-box-shadow: 2px 2px 2px black;
    box-shadow: 2px 2px 2px black; /* unprefixed last */

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: flexbox {
  // ...

Instead, authors must write:

@supports (display: flexbox) {
  // ...

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: flexbox)) {
  // ...

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: flexbox !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.

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 ruleset 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 (DOMString 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(DOMString 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 the 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. 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. This needs to specify what to do if rule is the empty string, if it contains more than one CSS rule, or if it contains garbage after a valid rule.
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 DOMString conditionText;
conditionText of type DOMString

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’ rule:

interface CSSMediaRule : CSSConditionRule {
    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’ rule.
conditionText of type DOMString
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 {

7.6. The CSS interface, and the supports() function

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

interface CSS {
  static boolean supports(DOMString property, DOMString value);
  static boolean supports(DOMString conditionText);
supports(DOMString property, DOMString value), returns boolean
supports(DOMString 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. Otherwise, it must return false.

When invoked with a single conditionText argument, it must return true if conditionText, when parsed and evaluated as a supports_condition, would return true. Otherwise, it must return false.


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.

8. Conformance

8.1. Base Modules

This specification defines conformance in terms of base modules, which are modules that this specification builds on top of. The base modules of this module are:

All of the conformance requirements of all base modules are incorporated as conformance requirements of this module, except where overridden by this module.

Additionally, all conformance requirements related to validity of syntax in this module and all of its base modules are to be interpreted as though all syntax in all of those modules is valid.

For example, this means that grammar presented in modules other than [CSS21] must obey the requirements that [CSS21] defines for the parsing of properties, and that requirements for handling invalid syntax in [CSS21] do not treat syntax added by other modules as invalid.

Additionally, the set of valid syntax can be increased by the conformance of a style sheet or processor to additional modules; use of such syntax does not make a style sheet nonconformant and failure to treat such syntax as invalid does not make a processor nonconformant.

8.2. Conformance Classes

Conformance to the CSS Conditional Rules Module is defined for three conformance classes:

style sheet
A CSS style sheet.
A tool that reads CSS style sheets: it may be a renderer or user-agent that interprets the semantics of a style sheet and renders documents that use style sheets, or it may be a validator that checks style sheets.
authoring tool
A tool that writes a style sheet.

A style sheet is conformant to the CSS Conditional Rules Module if it meets all of the conformance requirements in the module that are described as requirements of style sheets.

A processor is conformant to the CSS Conditional Rules Module if it meets all applicable conformance requirements in the module that are described as requirements of processors. In general, all requirements are applicable to renderers. Requirements concerning a part of CSS not performed by a processor are not applicable, e.g., requirements related to rendering are not applicable to a validator. The inability of a processor to correctly render a document due to limitations of the device does not make it non-conformant. (For example, a renderer is not required to render color on a monochrome monitor.)

An authoring tool is conformant to the CSS Conditional Rules Module if it writes style sheets that conform to the module and (if it reads CSS) it is a conformant processor.

8.3. 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.

8.4. Experimental Implementations

To avoid clashes with future CSS features, the CSS specifications reserve a prefixed syntax for proprietary property and value extensions to CSS. The CSS Working Group recommends that experimental implementations of features in CSS Working Drafts also use vendor-prefixed property or value names. This avoids any incompatibilities with future changes in the draft. Once a specification reaches the Candidate Recommendation stage, implementors should implement the non-prefixed syntax for any feature they consider to be correctly implemented according to spec.

8.5. CR Exit Criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.
a user agent which:
  1. implements the specification.
  2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
  3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

9. Changes

The following (non-editorial) changes were made to this specification since the 11 September 2012 Working Draft:


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


Normative references

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607/
Dean Jackson; et al. CSS Animations. 3 April 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-animations-20120403/
John Daggett. CSS Fonts Module Level 3. 23 August 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-fonts-20120823/
Florian Rivoal. Media Queries. 19 June 2012. W3C Recommendation. URL: http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

Other references

Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS1-20080411
Dean Jackson; et al. CSS Transitions. 3 April 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-transitions-20120403/