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’, the addition of the ‘@supports’ and ‘@document’ rules for conditional processing.
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This document was produced by the CSS Working Group (part of the Style Activity).
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This is the First Public Working Draft of the CSS Conditional Rules Module Level 3.
The following features are at risk:
@font-face’ rules and ‘@keyframes’ rules as allowed within all of the @-rules in this specification is at risk, though only because of the relative rates of advancement of specifications. If this specification is able to advance faster than one or both of the specifications defining those rules, then the inclusion of those rules will move from this specification to the specification defining those rules.@supports’ rule is at risk; if interoperable implementations are not found, it may be removed to advance the other features in this specification to Proposed Recommendation.@document’ rule is at risk; if interoperable implementations are not found, it may be removed to advance the other features in this specification to Proposed Recommendation.@media’ rule@supports’ rule
@document’ ruleThis 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 additional types of conditional group rules, ‘@supports’ and ‘@document’, 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.
The ‘@document’ rule allows CSS to be conditioned on the page to which the style sheet is being applied. This allows users to apply styles to a particular page or group of pages, which greatly increases the power of user style sheets.
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 [CSS3FONT] 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 ConventionsConformance 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 rulesThis 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. Likewise, this CSS rule:
@document url("http://www.example.com/") {
#example1 { display: none }
}
does the same type of conditional application, but using a different condition: whether the style sheet is being applied to the page http://www.example.com/.
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
andthe 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 rulesThere is also likely demand for using these conditions with ‘@import’. We should see if we can come up with sensible syntax for that, perhaps functional notation at the end of the ‘@import’ rule.
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:
nested_statement
: ruleset | media | page | font_face_rule | keyframes-rule |
supports_rule | document_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 not defined in [CSS3FONT], keyframes-rule shouldn't have dash? defined in [CSS3-ANIMATIONS], and media, supports_rule and document_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 rulesConditional 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.
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 (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 : 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).
This changes the S* in CSS 2.1 into S+. Is that correct?
@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;}
and the grammar by adding
supports_rule
: SUPPORTS_SYM S+ supports_condition group_rule_body
;
supports_condition
: supports_negation | supports_conjunction | supports_disjunction |
supports_declaration_condition
;
supports_negation
: 'not' S* supports_condition_in_parens
;
supports_conjunction
: supports_condition_in_parens ( 'and' S* supports_condition_in_parens )+
;
supports_disjunction
: supports_condition_in_parens ( 'or' S* supports_condition_in_parens )+
;
supports_condition_in_parens
: ( '(' supports_condition ')' S* ) | supports_declaration_condition
;
supports_declaration_condition
: '(' S* core_declaration ')' S*
;
in which core_declaration is the production declaration in the core syntax of CSS defined in section 4.1.1 (Tokenization) of [CSS21].
Any ‘@supports’ rule that does not parse according to the grammar above is invalid. Style sheets must not use such a rule and processors must ignore such a rule.
Note that this means that declarations that meet the forward-compatible syntax for declarations are permitted (and support for them is then tested by the ‘@supports’ rule), but declarations that do not meet the forward-compatible syntax for declarations cause the entire ‘@supports’ rule to be ignored.
Is any further allowance for forward-compatible parsing needed, for example, to allow additional features (such as, say, selector tests) to be added to the ‘@supports’ rule? Or are these forward-compatible parsing rules the best solution for such future expansion anyway?
Each of these grammar terms is associated with a boolean result, as follows:
supports_condition_in_parens child term.
supports_condition_in_parens child terms is true; otherwise it is false.
supports_condition_in_parens child terms is true; otherwise it is false.
supports_condition or supports_declaration_condition child term.
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;
box-shadow: 2px 2px 2px black;
-moz-box-shadow: 2px 2px 2px black;
-webkit-box-shadow: 2px 2px 2px black;
-o-box-shadow: 2px 2px 2px black;
}
}
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))) {
// ...
}6.1. Definition of support
A CSS processor is considered to support a declaration (consisting of a property and value) if it implements the given value of the given property.
6.2. Partial implementationsFor forward-compatibility, section 4.1.8 (Declarations and properties) of [CSS21] defines rules for handling invalid properties and values. CSS processors that partially implement a specification must treat any part of a value that they:
as invalid according to this rule, and must not accept or support a declaration containing such a value. This allows 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.
@document’ rule
The ‘@document’ rule is a conditional group rule whose condition depends on the URL of the document being styled. This allows style sheets, particularly user style sheets, to have styles that only apply to a set of pages rather than to all pages using the style sheet.
Given that this @-rule is intended primarily for user style sheets, what should this specification say about its use in author style sheets? Should it be forbidden? Should use instead be discouraged? Or should this specification remain neutral on the topic, since there are valid uses in author style sheets?
The URL of the document being styled is the URI at which the document is located, excluding any fragment identifiers. (This means, for example, that HTTP redirects have been followed.) If the styles are being applied inside a complete document embedded into the presentation of another (e.g., [HTML5]'s iframe, object, or img elements), the relevant URI is that of the frame, not of its container. However, if content from other documents is mixed in via mechanisms that mix content from one document into another (e.g., [SVG11]'s use element), then the address of the container document is used.
Note: In [HTML5], this is the document's address of a document in a browsing context.
What form of normalization is done on URLs and domains before matching? In particular, this specification needs to describe:
The ‘@document’ rule's condition is written as a comma-separated list of URL matching functions, and the condition evaluates to true whenever any one of those functions evaluates to true. The following URL matching functions are permitted:
The ‘url()’ function is the exact url matching function. It evaluates to true whenever the URL of the document being styled is exactly the URL given.
The ‘url()’ function, since it is a core syntax element in CSS, is allowed (subject to different character limitations and thus escaping requirements) to contain an unquoted value (in addition to the string values that are allowed as arguments for all four functions).
For example, this rule:
@document url("http://www.w3.org/Style/CSS/") {
#summary { background: yellow; color: black}
}
styles the summary element on the page http://www.w3.org/Style/CSS/, but not on any other pages.
The ‘url-prefix()’ function is the url prefix matching function. It evaluates to true whenever the URL of the document being styled has the argument to the function as an initial substring (which is true when the two strings are equal). When the argument is the empty string, it evaluates to true for all documents.
For example, this rule:
@document url-prefix("http://www.w3.org/Style/CSS/") {
#summary { background: yellow; color: black}
}
styles the summary element on the page http://www.w3.org/Style/CSS/ and on the page http://www.w3.org/Style/CSS/Test, but it does not affect the page http://www.w3.org/ or the page http://www.example.com/Style/CSS/.
The ‘domain()’ function is the domain matching function. It evaluates to true whenever the URL of the document being styled has a host subcomponent (as defined in [URI]) and that host subcomponent is exactly the argument to the ‘domain()’ function or a final substring of the host component is a period (U+002E) immediately followed by the argument to the ‘domain()’ function.
For example, this rule:
@document domain("w3.org") {
body { font-size: 16px ! important }
}
changes the font size of the body element for pages such as http://www.w3.org/Style/CSS/ and http://w3.org/Style/CSS/ and http://lists.w3.org/Archives/Public/www-style/ but it does not affect the page http://www.example.com/Style/CSS/.
The contents of the <string> argument must match the JavaScript Pattern production ([ECMA-262-5.1], section 15.10.1). However, failing to do so is not a CSS syntax error and does not trigger any error handling for CSS syntax errors.
The ‘regexp()’ function evaluates to true whenever the string argument compiled as a JavaScript regular expression with the global, ignoreCase and multiline flags disabled (see [ECMA-262-5.1], sections 15.10.7.2 through 15.10.7.4) compiles successfully and the resulting regular expression matches the entirety of the URL of the document being styled.
Note that regular expression must match the entire URL, not just a part of it.
Note that this definition intentionally matches the behavior of the pattern attribute on the input element in [HTML5].
For example, this rule:
@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
body { font-size: 20px ! important }
}
changes the font size of the body element for pages such as http://www.w3.org/TR/2011/PR-CSS2-20110412/.
Note that the backslashes in the regular expression require CSS escaping as ‘\\’.
Implementations must treat any unknown URL matching functions as a syntax error, and thus ignore the ‘@document’ rule. Should we instead have more complicated error handling rules to make forward-compatibility work differently, or is this rule the best solution for such future expansion anyway?
This extends the lexical scanner in the Grammar of CSS 2.1 ([CSS21], Appendix G) by adding:
@{D}{O}{C}{U}{M}{E}{N}{T} {return DOCUMENT_SYM;}
and the grammar by adding
document_rule : DOCUMENT_SYM S+ url_match_fn ( "," S* url_match_fn )* group_rule_body ; url_match_fn : (URI | FUNCTION) S* ;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 ClassesConformance to the CSS Conditional Rules Module is defined for three conformance classes:
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 ImplementationsSo 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 ImplementationsTo 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 CriteriaFor 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:
The specification will remain Candidate Recommendation for at least six months.
GrammarIn 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 | document_rule alongside ruleset | media | page.
Thanks to the ideas and feedback from Tantek Çelik, Elika Etemad, Pascal Germroth, Björn Höhrmann, Alex Mogilevsky, Chris Moschini, Ben Ward, Zack Weinberg, Boris Zbarsky, and all the rest of the www-style community.
References Normative references@document’ rule, 7.@media’ rule, 5.@supports’ rule, 6.RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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