CSS Conditional Rules Module Level 3

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.

@media print {
  /* hide navigation controls when printing */
  #navigation { display: none }
}

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.

@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 }
  }
}

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.

nested_statement
  : ruleset | media | page | font_face_rule | keyframes_rule |
    supports_rule
  ;

group_rule_body
  : '{' 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.

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

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

media
  : 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:

• negation is needed so that the new-feature styles and the fallback styles can be separated (within the forward-compatible grammar’s rules for the syntax of @-rules), and not required to override each other
• conjunction (and) is needed so that multiple required features can be tested
• disjunction (or) is needed when there are multiple alternative features for a set of styles, particularly when some of those alternatives are vendor-prefixed properties or values

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_rule
  : SUPPORTS_SYM S* supports_condition S* group_rule_body
  ;

supports_condition
  : supports_negation | supports_conjunction | supports_disjunction |
    supports_condition_in_parens
  ;

supports_condition_in_parens
  : ( '(' S* supports_condition S* ')' ) | supports_declaration_condition |
    general_enclosed
  ;

supports_negation
  : NOT S+ supports_condition_in_parens
  ;

supports_conjunction
  : supports_condition_in_parens ( S+ AND S+ supports_condition_in_parens )+
  ;

supports_disjunction
  : supports_condition_in_parens ( S+ OR S+ supports_condition_in_parens )+
  ;

supports_declaration_condition
  : '(' S* declaration ')'
  ;

general_enclosed
  : ( FUNCTION | '(' ) ( any | unused )* ')'
  ;

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:

supports_condition

The result is the result of the single child term.

supports_condition_in_parens

The result is the result of the single supports_condition or supports_declaration_condition child term.

supports_negation

The result is the negation of the result of the supports_condition_in_parens child term.

supports_conjunction

The result is true if the result of all of the supports_condition_in_parens child terms is true; otherwise it is false.

supports_disjunction

The result is true if the result of any of the supports_condition_in_parens child terms is true; otherwise it is false.

supports_declaration_condition

The result is whether the CSS processor supports the declaration within the parentheses.

general_enclosed

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.

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

@supports not ( display: flex ) {
  body { width: 100%; height: 100%; background: white; color: black; }
  #navigation { width: 25%; }
  #article { width: 75%; }
}

.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.

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

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

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

Furthermore, whitespace is required after a not and on both sides of an and or or.

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

@supports display: flex {
  // ...
}

@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.

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

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

@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 (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, 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 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 (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 DOMString (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 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. (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 parsed and evaluated as a supports_condition, would return true. Otherwise, it must return false.

8. Changes

The following (non-editorial) changes were made to this specification since the 13 December 2012 Working Draft:

• Require whitespace around and and or and after not.
• Add note explaining that user preferences that effectively disable a property (e.g., high-contrast mode disabling colors) do not effect the definition of support.
• Describe requirements for conditionText getter on CSSSupportsRule.
• Clarify the definition of “literal match” in CSS.supports().
• Specify behavior of CSSGroupingRule.insertRule when given an empty string or more than one syntactically valid rule.

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.