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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang=en>
<head profile="http://www.w3.org/2006/03/hcard"><meta content="text/html;
charset=utf-8" http-equiv=Content-Type>
<title>CSS Conditional Rules Module Level 3</title>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Conditional Rules Module Level 3</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 31 August 2011</h2>
<dl>
<dt>This version:
<dd><a href="http://www.w3.org/TR/2011/ED-css3-conditional-20110831/">
http://www.w3.org/TR/2011/ED-css3-conditional-20110831/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-conditional/">http://www.w3.org/TR/css3-conditional/</a>
<dt>Editor's draft:
<dd><a
href="http://dev.w3.org/csswg/css3-conditional/">http://dev.w3.org/csswg/css3-conditional/</a>
<dt>Previous version:
<dd><a
href="http://www.w3.org/TR/2011/REC-CSS2-20110607/">http://www.w3.org/TR/2011/REC-CSS2-20110607/</a>
(for ‘<code class=css>@media</code>’)
<dd><a
href="http://lists.w3.org/Archives/Public/www-style/2011Apr/0428.html">http://lists.w3.org/Archives/Public/www-style/2011Apr/0428.html</a>
(for ‘<code class=css>@supports</code>’)
<dd><a
href="http://lists.w3.org/Archives/Public/www-style/2004Aug/0135.html">http://lists.w3.org/Archives/Public/www-style/2004Aug/0135.html</a>
(for ‘<code class=css>@document</code>’)
<dt>Editors:
<dd class=vcard><a class=fn href="http://dbaron.org/">L. David Baron</a>,
<a class=org href="http://www.mozilla.org/">Mozilla</a>, <a class=email
href="mailto:dbaron@dbaron.org">dbaron@dbaron.org</a>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2011 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and
Mathematics">ERCIM</acronym></a>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>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 <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS
level 1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
The main extensions compared to level 2 are allowing nesting of
certain at-rules inside ‘<code class=css>@media</code>’, the
addition of the ‘<code class=css>@supports</code>’ and
‘<code class=css>@document</code>’ rules for conditional
processing.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<!--begin-status-->
<p>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.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-conditional%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) 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<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> 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 <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<p>The following features are at risk:
<ul>
<li>The inclusion of ‘<code class=css>@font-face</code>’ rules
and ‘<code class=css>@keyframes</code>’ 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.
<li>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.
<li>The ‘<code class=css>@supports</code>’ 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.
<li>The ‘<code class=css>@document</code>’ 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.
</ul>
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
<ul class=toc>
<li><a href="#context"><span class=secno>1.1. </span>Background</a>
<li><a href="#placement"><span class=secno>1.2. </span>Module
Interactions</a>
<li><a href="#conventions"><span class=secno>1.3. </span>Document
Conventions</a>
</ul>
<li><a href="#processing"><span class=secno>2. </span>Processing of
conditional group rules</a>
<li><a href="#contents-of"><span class=secno>3. </span>Contents of
conditional group rules</a>
<li><a href="#use"><span class=secno>4. </span>Placement of conditional
group rules</a>
<li><a href="#at-media"><span class=secno>5. </span>Media-specific style
sheets: the ‘<code class=css>@media</code>’ rule</a>
<li><a href="#at-supports"><span class=secno>6. </span>Feature queries:
the ‘<code class=css>@supports</code>’ rule</a>
<ul class=toc>
<li><a href="#support-definition"><span class=secno>6.1.
</span>Definition of support</a>
<li><a href="#partial-implementations"><span class=secno>6.2.
</span>Partial implementations</a>
</ul>
<li><a href="#at-document"><span class=secno>7. </span>Document queries:
the ‘<code class=css>@document</code>’ rule</a>
<li><a href="#apis"><span class=secno>8. </span>APIs</a>
<li><a href="#conformance"><span class=secno>9. </span>Conformance</a>
<ul class=toc>
<li><a href="#base-modules"><span class=secno>9.1. </span>Base
Modules</a>
<li><a href="#conformance-classes"><span class=secno>9.2.
</span>Conformance Classes</a>
<li><a href="#partial"><span class=secno>9.3. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>9.4. </span>Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>9.5. </span>CR Exit
Criteria</a>
</ul>
<li class=no-num><a href="#grammar">Grammar</a>
<li class=no-num><a href="#acknowledgments">Acknowledgments</a>
<li class=no-num><a href="#references">References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#index">Index</a>
</ul>
<!--end-toc-->
<h2 id=introduction><span class=secno>1. </span>Introduction</h2>
<h3 id=context><span class=secno>1.1. </span>Background</h3>
<p><em>This section is not normative.</em>
<p><a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> defines
one type of conditional group rule, the ‘<code
class=css>@media</code>’ rule, and allows only rulesets (not other
@-rules) inside of it. The ‘<code class=css>@media</code>’
rule provides the ability to have media-specific style sheets, which is
also provided by style sheet linking features such as ‘<code
class=css>@import</code>’ and <code class=html><link></code>.
The restrictions on the contents of ‘<code
class=css>@media</code>’ 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.
<p>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.
<p>This specification also defines additional types of conditional group
rules, ‘<code class=css>@supports</code>’ and ‘<code
class=css>@document</code>’, to address author and user
requirements.
<p>The ‘<code class=css>@supports</code>’ 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.
<p>The ‘<code class=css>@document</code>’ 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.
<h3 id=placement><span class=secno>1.2. </span>Module Interactions</h3>
<p>This module replaces and extends the ‘<code
class=css>@media</code>’ rule feature defined in <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> section <var>7.2.1</var> and
incorporates the modifications previously made non-normatively by <a
href="#MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a> section
<var>1</var>.
<p>Its current definition depends on @-rules defined in <a href="#CSS3FONT"
rel=biblioentry>[CSS3FONT]<!--{{!CSS3FONT}}--></a> and <a
href="#CSS3-ANIMATIONS"
rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>, 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.
<h3 id=conventions><span class=secno>1.3. </span>Document Conventions</h3>
<p>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.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>
<p>Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with <code
class=html>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word “Note” and are set apart from
the normative text with <code class=html>class="note"</code>, like this:
<p class=note>Note, this is an informative note.
<h2 id=processing><span class=secno>2. </span>Processing of conditional
group rules</h2>
<p>This specification defines some CSS @-rules, called <dfn
id=conditional-group-rules>conditional group rules</dfn>, 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.
<div class=example>
<p>For example, this rule:</p>
<pre>@media print {
#navigation { display: none }
}</pre>
<p>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:</p>
<pre>@document url("http://www.example.com/") {
#example1 { display: none }
}</pre>
<p>does the same type of conditional application, but using a different
condition: whether the style sheet is being applied to the page
<code>http://www.example.com/</code>.</p>
</div>
<p>Each conditional group rule has a condition, which at any time evaluates
to true or false. When the condition is true, CSS processors
<strong>must</strong> apply the rules inside the group rule as though they
were at the group rule's location; when the condition is false, CSS
processors <strong>must</strong> 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.
<p>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.
<div class=example>For example, with this set of nested rules:
<pre>@media print { // rule (1)
#navigation { display: none }
@media (max-width: 12cm) { // rule (2)
.note { float: none }
}
}</pre>
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 ‘<code class=css>#navigation { display: none
}</code>’ applies whenever this style sheet is applied to print
media, and the rule ‘<code class=css>.note { float: none
}</code>’ is applied only when the style sheet is applied to print
media <em>and</em> the width of the page box is less than or equal to 12
centimeters.</div>
<p>When the condition for a conditional group rule changes, CSS processors
<strong>must</strong> 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 <a href="#CSS3-TRANSITIONS"
rel=biblioentry>[CSS3-TRANSITIONS]<!--{{CSS3-TRANSITIONS}}--></a> and <a
href="#CSS3-ANIMATIONS"
rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>).
<h2 id=contents-of><span class=secno>3. </span>Contents of conditional
group rules</h2>
<p class=issue>There is also likely demand for using these conditions with
‘<code class=css>@import</code>’. We should see if we can come
up with sensible syntax for that, perhaps functional notation at the end
of the ‘<code class=css>@import</code>’ rule.
<p>The syntax of each conditional group rule consists of some syntax
specific to the type of rule followed by a <dfn id=group-rule-body>group
rule body</dfn>, which is a block (pair of braces) containing a sequence
of rules.
<p>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 ‘<code class=css>@charset</code>’, ‘<code
class=css>@import</code>’, and ‘<code
class=css>@namespace</code>’ rules) are not allowed inside of
conditional group rules. Conditional group rules can be nested.
<p>In terms of the grammar, this specification defines the following
productions for use in the grammar of conditional group rules:
<pre>nested_statement
: ruleset | media | page | font_face_rule | keyframes-rule |
supports_rule | document_rule
;
group_rule_body
: '{' S* nested_statement* '}' S*
;</pre>
<p> in which all the productions are defined in that grammar with the
exception of <code>font_face_rule</code> <span class=issue>not</span>
defined in <a href="#CSS3FONT"
rel=biblioentry>[CSS3FONT]<!--{{!CSS3FONT}}--></a>,
<code>keyframes-rule</code> <span class=issue>shouldn't have dash?</span>
defined in <a href="#CSS3-ANIMATIONS"
rel=biblioentry>[CSS3-ANIMATIONS]<!--{{!CSS3-ANIMATIONS}}--></a>, and
<code>media</code>, <code>supports_rule</code> and
<code>document_rule</code> defined in this specification.
<p>In general, future CSS specifications that add new @-rules that are not
forbidden to occur after some other types of rules should modify this
<code>nested_statement</code> production to keep the grammar accurate.
<p>Style sheets <strong>must not</strong> use rules other than the allowed
ones inside conditional group rules.
<p>CSS processors <strong>must</strong> ignore rules that are not allowed
within a group rule, and <strong>must</strong> handle invalid rules inside
of group rules as described in <a
href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">section 4.2
(Rules for handling parsing errors)</a>, <a
href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">section 4.1.5
(At-rules)</a>, and <a
href="http://www.w3.org/TR/CSS21/syndata.html#rule-sets">section 4.1.7
(Rule sets, declaration blocks, and selectors)</a> of <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<h2 id=use><span class=secno>4. </span>Placement of conditional group rules</h2>
<p>Conditional group rules are allowed at the top-level of a style sheet,
and inside other conditional group rules. CSS processors
<strong>must</strong> process such rules as <a
href="#processing">described above</a>.
<p>Any rules that are not allowed after a ruleset (e.g., ‘<code
class=css>@charset</code>’, ‘<code
class=css>@import</code>’, or ‘<code
class=css>@namespace</code>’ rules) are also not allowed after a
conditional group rule. Therefore, style sheets <strong>must not</strong>
place such rules after a conditional group rules, and CSS processors
<strong>must</strong> ignore such rules.
<h2 id=at-media><span class=secno>5. </span>Media-specific style sheets:
the ‘<code class=css>@media</code>’ rule</h2>
<p>The <dfn id=media-rule>‘<code class=css>@media</code>’
rule</dfn> is a conditional group rule whose condition is a media query.
It consists of the at-keyword ‘<code class=css>@media</code>’
followed by a (possibly empty) media query (as defined in <a
href="#MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>), followed
by a group rule body. The condition of the rule is the result of the media
query.
<div class=example>
<p>This ‘<code class=css>@media</code>’ rule:</p>
<pre>@media print, (max-width: 600px) {
#extra_navigation { display: none }
}</pre>
<p>has the condition ‘<code class=css>print, (max-width:
600px)</code>’, 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 ‘<code class=css>#extra_navigation {
display: none }</code>’ is applied.
</div>
<p>In terms of the grammar, this specification extends the
<code>media</code> production in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G)
into:
<pre>media
: MEDIA_SYM S+ media_query_list group_rule_body
;</pre>
<p>where the <code>group_rule_body</code> production is defined in this
specification, the <code>media_query_list</code> production is defined in
<a href="#MEDIAQ" rel=biblioentry>[MEDIAQ]<!--{{!MEDIAQ}}--></a>, and the
others are defined in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G).
<p class=issue>This changes the <code>S*</code> in CSS 2.1 into
<code>S+</code>. Is that correct?
<h2 id=at-supports><span class=secno>6. </span>Feature queries: the
‘<code class=css>@supports</code>’ rule</h2>
<p>The <dfn id=supports-rule>‘<code class=css>@supports</code>’
rule</dfn> 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.
<p>The syntax of the condition in the ‘<code
class=css>@supports</code>’ rule is slightly more complicated than
for the other conditional group rules (though has some similarities to
media queries) since:
<ul>
<li>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
<li>conjunction (and) is needed so that multiple required features can be
tested
<li>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
</ul>
<p>Therefore, the syntax of the ‘<code
class=css>@supports</code>’ rule allows testing for property:value
pairs, and arbitrary conjunctions (and), disjunctions (or), and negations
(not) of them.
<p>This extends the lexical scanner in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by
adding:
<pre>@{S}{U}{P}{P}{O}{R}{T}{S} {return SUPPORTS_SYM;}</pre>
<p>and the grammar by adding
<pre>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*
;</pre>
<p>in which <code>core_declaration</code> is the production
<code>declaration</code> in the core syntax of CSS defined in <a
href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">section 4.1.1
(Tokenization)</a> of <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<p>Any ‘<code class=css>@supports</code>’ rule that does not
parse according to the grammar above is invalid. Style sheets <strong>must
not</strong> use such a rule and processors <strong>must</strong> ignore
such a rule.
<p class=note>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 ‘<code class=css>@supports</code>’
rule), but declarations that do not meet the forward-compatible syntax for
declarations cause the entire ‘<code
class=css>@supports</code>’ rule to be ignored.
<p class=issue>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 ‘<code class=css>@supports</code>’
rule? Or are these forward-compatible parsing rules the best solution for
such future expansion anyway?
<p>Each of these grammar terms is associated with a boolean result, as
follows:
<dl>
<dt>supports_condition
<dd> The result is the result of the single child term.
<dt>supports_negation
<dd> The result is the <em>negation</em> of the result of the
<code>supports_condition_in_parens</code> child term.
<dt>supports_conjunction
<dd> The result is true if the result of <em>all</em> of the
<code>supports_condition_in_parens</code> child terms is true; otherwise
it is false.
<dt>supports_disjunction
<dd> The result is true if the result of <em>any</em> of the
<code>supports_condition_in_parens</code> child terms is true; otherwise
it is false.
<dt>supports_condition_in_parens
<dd> The result is the result of the single
<code>supports_condition</code> or
<code>supports_declaration_condition</code> child term.
<dt>supports_declaration_condition
<dd> The result is whether the CSS processor <a
href="#support-definition">supports</a> the declaration.
</dl>
<p>The condition of the ‘<code class=css>@supports</code>’ rule
is the result of the <code>supports_condition</code> term that is a child
of the <code>supports_rule</code> term.
<div class=example>
<p>For example, the following rule</p>
<pre>@supports ( display: flexbox ) {
body, #navigation, #content { display: flexbox; }
#navigation { background: blue; color: white; }
#article { background: white; color: black; }
}</pre>
<p>applies the rules inside the ‘<code
class=css>@supports</code>’ rule only when ‘<code
class=css>display: flexbox</code>’ is supported.</p>
</div>
<div class=example>
<p>The following example shows an additional ‘<code
class=css>@supports</code>’ rule that can be used to provide an
alternative for when ‘<code class=css>display:
flexbox</code>’ is not supported:</p>
<pre>@supports not ( display: flexbox ) {
body { width: 100%; height: 100%; background: white; color: black; }
#navigation { width: 25%; }
#article { width: 75%; }
}</pre>
<p>Note that the ‘<code class=property>width</code>’
declarations may be harmful to the flexbox-based layout, so it is
important that they be present only in the non-flexbox styles.</p>
</div>
<div class=example>
<p>The following example checks for support for the ‘<code
class=property>box-shadow</code>’ property, including checking for
support for vendor-prefixed versions of it. When the support is present,
it specifies both ‘<code class=property>box-shadow</code>’
(with the prefixed versions) and ‘<code
class=property>color</code>’ in a way what would cause the text to
become invisible were ‘<code
class=property>box-shadow</code>’ not supported.</p>
<pre>@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;
}
}</pre>
</div>
<p>To avoid confusion between ‘<code class=css>and</code>’ and
‘<code class=css>or</code>’, the syntax requires that both
‘<code class=css>and</code>’ and ‘<code
class=css>or</code>’ 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 ‘<code
class=css>and</code>’, ‘<code class=css>or</code>’, and
‘<code class=css>not</code>’ operators to be mixed without a
layer of parentheses.
<div class=example>
<p>For example, the following rule is not valid:
<pre class=illegal-example>@supports (transition-property: color) or
(animation-name: foo) and
(transform: rotate(10deg)) {
// ...
}</pre>
<p>Instead, authors must write one of the following:</p>
<pre>@supports ((transition-property: color) or
(animation-name: foo)) and
(transform: rotate(10deg)) {
// ...
}</pre>
<pre>@supports (transition-property: color) or
((animation-name: foo)) and
(transform: rotate(10deg))) {
// ...
}</pre>
</div>
<h3 id=support-definition><span class=secno>6.1. </span>Definition of
support</h3>
<p>A CSS processor is considered to <dfn id=dfn-support>support</dfn> a
declaration (consisting of a property and value) if it implements the
given value of the given property.
<h3 id=partial-implementations><span class=secno>6.2. </span>Partial
implementations</h3>
<p>For forward-compatibility, <a
href="http://www.w3.org/TR/CSS21/syndata.html#declaration">section 4.1.8
(Declarations and properties)</a> of <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> defines rules for handling
invalid properties and values. CSS processors that partially implement a
specification <strong>must</strong> treat any part of a value that they:
<ul>
<li>do not implement, or
<li>do not have a usable level of support for,
</ul>
<p>as invalid according to this rule, and <strong>must not</strong> accept
or support a declaration containing such a value. This allows authors to
use fallback (either in the <a href="#CSS1"
rel=biblioentry>[CSS1]<!--{{CSS1}}--></a> sense of declarations that are
overridden by later declarations or with the new capabilities provided by
the ‘<code class=css>@supports</code>’ 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 ‘<code
class=css>@supports</code>’ rule.
<h2 id=at-document><span class=secno>7. </span>Document queries: the
‘<code class=css>@document</code>’ rule</h2>
<p>The <dfn id=document-rule>‘<code class=css>@document</code>’
rule</dfn> is a conditional group rule whose condition depends on the <a
href="#url-of-doc">URL of the document being styled</a>. 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.
<p class=issue>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?
<p id=url-of-doc>The <dfn id=url-of-the-document-being-styled>URL of the
document being styled</dfn> 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., <a
href="#HTML5" rel=biblioentry>[HTML5]<!--{{HTML5}}--></a>'s <code
class=html>iframe</code>, <code class=html>object</code>, or <code
class=html>img</code> 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., <a
href="#SVG11" rel=biblioentry>[SVG11]<!--{{SVG11}}--></a>'s
<code>use</code> element), then the address of the container document is
used.
<p class=note>Note: In <a href="#HTML5"
rel=biblioentry>[HTML5]<!--{{HTML5}}--></a>, this is the <a
href="http://dev.w3.org/html5/spec/dom.html#the-document-s-address">document's
address</a> of a document in a <a
href="http://dev.w3.org/html5/spec/browsers.html#browsing-context">browsing
context</a>.
<div class=issue>What form of normalization is done on URLs and domains
before matching? In particular, this specification needs to describe:
<ul>
<li>what form is used for the <a href="#url-of-doc">URL of the document
being styled</a> (and what has been normalized in that form)
<li>what normalization (if any) happens to the argument of each of the
match functions before the comparison that they describe and
<li>whether the comparison algorithm used is string comparison or some
other URL comparison algorithm.
</ul>
</div>
<p>The ‘<code class=css>@document</code>’ rule's condition is
written as a comma-separated list of <dfn id=url-matching-functions>URL
matching functions</dfn>, and the condition evaluates to true whenever any
one of those functions evaluates to true. The following URL matching
functions are permitted:
<dl>
<dt><dfn id=url-exact title="url()|URL matching
functions::exact"><url></dfn>
<dd>
<p>The ‘<a href="#url-exact"><code
class=css>url()</code></a>’ function is the <dfn
id=exact-url-matching-function>exact url matching function</dfn>. It
evaluates to true whenever the <a href="#url-of-doc">URL of the document
being styled</a> is exactly the URL given.</p>
<p class=Note>The ‘<a href="#url-exact"><code
class=css>url()</code></a>’ 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).</p>
<div class=example>
<p>For example, this rule:</p>
<pre>@document url("http://www.w3.org/Style/CSS/") {
#summary { background: yellow; color: black}
}</pre>
<p>styles the <code class=html>summary</code> element on the page
<code>http://www.w3.org/Style/CSS/</code>, but not on any other pages.</p>
</div>
<dt><dfn id=url-prefix title="url-prefix()|URL matching
functions::prefix">url-prefix(<string>)</dfn>
<dd>
<p>The ‘<a href="#url-prefix"><code
class=css>url-prefix()</code></a>’ function is the <dfn
id=url-prefix-matching-function>url prefix matching function</dfn>. It
evaluates to true whenever the <a href="#url-of-doc">URL of the document
being styled</a> 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.</p>
<div class=example>
<p>For example, this rule:</p>
<pre>@document url-prefix("http://www.w3.org/Style/CSS/") {
#summary { background: yellow; color: black}
}</pre>
<p>styles the <code class=html>summary</code> element on the page
<code>http://www.w3.org/Style/CSS/</code> and on the page
<code>http://www.w3.org/Style/CSS/Test</code>, but it does not affect
the page <code>http://www.w3.org/</code> or the page
<code>http://www.example.com/Style/CSS/</code>.</p>
</div>
<dt><dfn id=url-domain title="domain()|URL matching
functions::domain">domain(<string>)</dfn>
<dd>
<p>The ‘<a href="#url-domain"><code
class=css>domain()</code></a>’ function is the <dfn
id=domain-matching-function>domain matching function</dfn>. It evaluates
to true whenever the <a href="#url-of-doc">URL of the document being
styled</a> has a host subcomponent (as defined in <a href="#URI"
rel=biblioentry>[URI]<!--{{!URI}}--></a>) and that host subcomponent is
exactly the argument to the ‘<a href="#url-domain"><code
class=css>domain()</code></a>’ function or a final substring of
the host component is a period (U+002E) immediately followed by the
argument to the ‘<a href="#url-domain"><code
class=css>domain()</code></a>’ function.</p>
<div class=example>
<p>For example, this rule:</p>
<pre>@document domain("w3.org") {
body { font-size: 16px ! important }
}</pre>
<p>changes the font size of the body element for pages such as
<code>http://www.w3.org/Style/CSS/</code> and
<code>http://w3.org/Style/CSS/</code> and
<code>http://lists.w3.org/Archives/Public/www-style/</code> but it does
not affect the page <code>http://www.example.com/Style/CSS/</code>.</p>
</div>
<dt><dfn id=url-regexp title="regexp()|URL matching functions::regular
expression">regexp(<string>)</dfn>
<dd>
<p>The contents of the <string> argument <strong>must</strong>
match the JavaScript <code>Pattern</code> production (<a
href="#ECMA-262-5.1"
rel=biblioentry>[ECMA-262-5.1]<!--{{!ECMA-262-5.1}}--></a>, 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.</p>
<p>The ‘<a href="#url-regexp"><code
class=css>regexp()</code></a>’ function evaluates to true whenever
the string argument compiled as a JavaScript regular expression with the
<code>global</code>, <code>ignoreCase</code> and <code>multiline</code>
flags <em>disabled</em> (see <a href="#ECMA-262-5.1"
rel=biblioentry>[ECMA-262-5.1]<!--{{!ECMA-262-5.1}}--></a>, sections
15.10.7.2 through 15.10.7.4) compiles successfully and the resulting
regular expression matches the entirety of the <a href="#url-of-doc">URL
of the document being styled</a>.</p>
<p class=note>Note that regular expression must match the entire URL, not
just a part of it.</p>
<p class=note>Note that this definition intentionally matches the
behavior of the <a
href="http://dev.w3.org/html5/spec/common-input-element-attributes.html#attr-input-pattern"><code
class=html>pattern</code> attribute</a> on the <code
class=html>input</code> element in <a href="#HTML5"
rel=biblioentry>[HTML5]<!--{{HTML5}}--></a>.</p>
<div class=example>
<p>For example, this rule:</p>
<pre>@document regexp("http://www.w3.org/TR/\\d{4}/[^/]*-CSS2-\\d{8}/") {
body { font-size: 20px ! important }
}</pre>
<p>changes the font size of the body el
61FE
ement for pages such as
<code>http://www.w3.org/TR/2011/PR-CSS2-20110412/</code>.</p>
<p class=note>Note that the backslashes in the regular expression
require CSS escaping as ‘<code class=css>\\</code>’.</p>
</div>
</dl>
<p>Implementations <strong>must</strong> treat any unknown URL matching
functions as a syntax error, and thus ignore the ‘<code
class=css>@document</code>’ rule. <span class=issue>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?</span>
<p>This extends the lexical scanner in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by
adding:
<pre>@{D}{O}{C}{U}{M}{E}{N}{T} {return DOCUMENT_SYM;}</pre>
<p>and the grammar by adding
<pre>document_rule
: DOCUMENT_SYM S+ url_match_fn ( "," S* url_match_fn )* group_rule_body
;
url_match_fn
: (URI | FUNCTION) S*
;</pre>
<h2 id=apis><span class=secno>8. </span>APIs</h2>
<p class=issue>This specification needs to define the CSSOM interfaces for
the new rule types.
<p class=issue>Should this specification also define APIs for querying
@supports, like <code>matchMedium</code> allows querying of media?
<h2 id=conformance><span class=secno>9. </span>Conformance</h2>
<h3 id=base-modules><span class=secno>9.1. </span>Base Modules</h3>
<p>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:
<ul>
<li><a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>