What should this specification be called? CSS Conditional
Group(ing) Rules? CSS Group(ing) Rules? Something else?
Abstract
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.
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.
The (archived) public
mailing list
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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:
- The inclusion of
@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.
- The addition of support for @-rules inside of conditional grouping
rules is at risk; if interoperable implementations are not found, it may
be removed to advance the other features in this specification to
Proposed Recommendation.
- The
@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.
- The
@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.
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
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 condition 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.
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 [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 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.
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.
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
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 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.
3.1. Error handling
Implementations MUST ignore rules that are not allowed within a group
rule.
Define error handling rules for unknown things.
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 this 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?
5. 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;}
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* declaration ')' S*
;
The rules for evaluating the condition in each of the above grammar
terms are:
- supports_condition
- evaluate the single child term
- supports_negation
- negate the result of evaluating the single child term
- supports_conjunction
- true if all of the (non-‘
and’) child terms are true, otherwise false
- supports_disjunction
- true if any of the (non-‘
or’)
child terms are true, otherwise false
- supports_condition_in_parens
- evaluate the single (non-‘
(’ and
non-‘)’) child term
- supports_declaration_condition
- true if the user agent supports the declaration (which is the same as
the rule for whether the user agent parses the declaration), otherwise
false
and the condition of the @supports rule is the condition
thus described for the supports_rule term.
Describe these examples.
@supports ( display: flex ) {
...
}
@supports ( -moz-box-shadow: 2px 2px 2px black ) or
( -webkit-box-shadow: 2px 2px 2px black ) or
( -o-box-shadow: 2px 2px 2px black ) {
...
}
@supports not ( display: flex ) {
...
}
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. Add examples.
6. Document queries: the
@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?
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:
- <url>
-
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.
- url-prefix(<string>)
-
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/.
- domain(<string>)
-
The domain() function is the domain matching function. It evaluates
to true whenever the URL of the page 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/.
- regexp(<string>)
-
Write me
This definition intentionally matches the behavior of the
pattern
attribute on the input element in [HTML5].
What form of normalization is done on URLs and domains
before matching?
Need to define what URL we care about, perhaps in terms of
browsing context. It should apply to the URL of an
iframe/object/embed/img; it should probably not apply to the URL of an
svg:use.
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?
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
;
Conformance to the CSS Conditional Group Rules Module is defined for
three conformance classes:
- style
sheet
- A CSS
style sheet.
- renderer
- A UA
that interprets the semantics of a style sheet and renders documents that
use them.
- authoring tool
- A UA
that writes a style sheet.
A style sheet is conformant to CSS TEMPLATE
Module if all of its declarations that use properties defined in
this module have values that are valid according to the generic CSS
grammar and the individual grammars of each property as given in this
module.
A renderer is conformant to CSS TEMPLATE Module
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the properties defined by CSS TEMPLATE Module 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 CSS TEMPLATE
Module if it writes syntactically correct style sheets, according to
the generic CSS grammar and the individual grammars of each property in
this module.
7.2. 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.
7.3. 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.
7.4. 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:
- independent
- 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.
- interoperable
- 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.
- implementation
- a user agent which:
- implements the specification.
- 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.
- 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.
Grammar
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 | document_rule alongside ruleset | media |
page.
Acknowledgments
Thanks to the ideas and feedback from Tantek
Çelik, Elika Etemad, Pascal Germroth, Björn
Höhrmann, Chris Moschini, Ben Ward, Zack Weinberg, Boris Zbarsky,
and all the rest of the www-style
community.
References
Normative references
-
- [CSS21]
- Bert Bos; et al. Cascading Style
Sheets Level 2 Revision 1 (CSS 2.1) Specification. 23
April 2009. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2009/CR-CSS2-20090423
- [CSS3-ANIMATIONS]
- Dean Jackson; David Hyatt; Chris Marrin. CSS
Animations Module Level 3. 20 March 2009. W3C Working Draft.
(Work in progress.) URL: http://www.w3.org/TR/2009/WD-css3-animations-20090320
- [CSS3FONT]
- John Daggett. CSS Fonts
Module Level 3. 24 March 2011. W3C Working Draft. (Work in
progress.) URL: http://www.w3.org/TR/2011/WD-css3-fonts-20110324
- [MEDIAQ]
- Håkon Wium Lie; et al. Media
Queries. 27 July 2010. W3C Candidate Recommendation. (Work in
progress.) URL: http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/
- [RFC2119]
- S. Bradner. Key
words for use in RFCs to Indicate Requirement Levels. Internet
RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
- [URI]
- T. Berners-Lee; R. Fielding; L. Masinter. Uniform Resource
Identifiers (URI): generic syntax. January 2005. Internet RFC
3986. URL: http://www.ietf.org/rfc/rfc3986.txt
Other references
-
- [CSS1]
- 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
- [CSS3-TRANSITIONS]
- Dean Jackson; et al. CSS
Transitions Module Level 3. 1 December 2009. W3C Working
Draft. (Work in progress.) URL: http://www.w3.org/TR/2009/WD-css3-transitions-20091201
- [HTML5]
- Ian Hickson. HTML5.
13 January 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-html5-20110113
Index
- authoring tool, 7.1.
- conditional group rules, 2.
@document rule, 6.
- domain matching function, 6.
- exact url matching function, 6.
- group rule body, 3.
@media rule, 4.
- renderer, 7.1.
- style sheet
- as conformance class, 7.1.
@supports rule, 5.
- URL matching functions, 6.
- url prefix matching function, 6.