csswg-drafts/css3-conditional/Overview.src.html at cb54203a02eb924c19b6298841b87c8e748795e6 · w3c/csswg-drafts · GitHub

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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 http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Conditional Rules Module Level 3</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link rel="stylesheet" type="text/css"
        href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
</head>

<div class="head">
<!--logo-->

<h1>CSS Conditional Rules Module Level 3</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:
    <dd><a href="[VERSION]">
    http://www.w3.org/TR/[YEAR]/[STATUS]-[SHORTNAME]-[CDATE]</a>

  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/css3-conditional">http://www.w3.org/TR/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 '@media')</dd>
    <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 '@supports')</dd>
    <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 '@document')</dd>

  <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>

<!--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&nbsp;2 [[!CSS21]],
  which builds on CSS level&nbsp;1 [[CSS1]].
  The main extensions compared to level&nbsp;2 are
  allowing nesting of certain at-rules inside '@media',
  the addition of the '@supports' and '@document'
  rules for conditional processing.

<h2 class="no-num no-toc" id=status>Status of this document</h2>

<!--status-->

<p>The following features are at risk:
<ul>
  <li>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.</li>

  <li>The addition of support for @-rules inside of conditional grouping
  rules is at risk; if interoperable implementations are not found, it
  m
77FB
ay be removed to advance the other features in this specification to
  Proposed Recommendation.</li>

  <li>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.</li>

  <li>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.</li>

</ul>

<h2 class="no-num no-toc" id="contents">Table of contents</h2>

<!--toc-->

<h2 id="introduction">Introduction</h2>

<h3 id="context">Background</h3>

  <p><em>This section is not normative.</em>

  <p>[[!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
  <code class="html">&lt;link&gt;</code>.  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.</p>

  <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>

  <p>This specification also defines additional types of conditional
  group rules, '@supports' and '@document', to
  address author and user requirements.</p>

  <p>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.</p>

  <p>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.</p>

<h3 id="placement">Module Interactions</h3>

  <p>This module replaces and extends the '@media' rule
  feature defined in [[!CSS21]] section <var>7.2.1</var> and
  incorporates the modifications previously made non-normatively by
  [[!MEDIAQ]] section <var>1</var>.</p>

  <p>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.</p>

<h3 id="conventions">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. [[!RFC2119]]</p>

  <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.</p>

<h2 id="processing">Processing of conditional group rules</h2>

<p>This specification defines some CSS @-rules, called <dfn>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.</p>

<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>

<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.</p>

<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 ''#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 <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 [[CSS3-TRANSITIONS]] and
[[!CSS3-ANIMATIONS]]).</p>

<h2 id="contents">Contents of conditional group rules</h2>

<p class="issue">There 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.</p>

<p>The syntax of each conditional group rule consists of some syntax
specific to the type of rule followed by a <dfn>group rule body</dfn>,
which is a block (pair of braces) containing a sequence of rules.</p>

<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 '@charset', '@import',
and '@namespace' rules) are not allowed inside of conditional group
rules.  Conditional group rules can be nested.</p>

<p>In terms of the grammar, this specification defines the following
productions for use in the grammar of conditional group rules:</p>

<pre>nested_statement
  : ruleset | media | page | font_face_rule | keyframes-rule |
    supports_rule | document_rule
  ;

group_rule_body
  : '{' S* nested_statement* '}' S*
  ;</pre>
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 [[!CSS3FONT]], <code>keyframes-rule</code> <span
class="issue">shouldn't have dash?</span> defined in
[[!CSS3-ANIMATIONS]], and <code>media</code>, <code>supports_rule</code>
and <code>document_rule</code> defined in this specification.</p>

<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>

<p>Style sheets <strong>must not</strong> use rules other than the allowed ones inside
conditional group rules.</p>

<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 [[!CSS21]].</p>

<h2 id="use">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>

<p>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 <strong>must
not</strong> place such rules after a conditional group rules, and CSS
processors <strong>must</strong> ignore such rules.</p>

<h2 id="at-media">Media-specific style sheets:  the '@media' rule</h2>

<p>The <dfn>'@media' rule</dfn> 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.</p>

<div class="example">
<p>This '@media' rule:</p>
<pre>@media print, (max-width: 600px) {
  #extra_navigation { display: none }
}</pre>
<p>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.
</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>
([[!CSS21]], 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 [[!MEDIAQ]], and the others are defined in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G).
<p class="issue">This changes the <code>S*</code> in CSS 2.1 into
<code>S+</code>.  Is that correct?</p>

<h2 id="at-supports">Feature queries: the '@supports' rule</h2>

<p>The <dfn>'@supports' 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>

<p>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:</p>
<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>
  <li>conjunction (and) is needed so that multiple required features can
  be tested</li>
  <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</li>
</ul>

<p>Therefore, the syntax of the '@supports' rule allows
testing for property:value pairs, and arbitrary conjunctions (and),
disjunctions (or), and negations (not) of them.</p>

<p>This extends the lexical scanner in the
<a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) by adding:
<pre>@{S}{U}{P}{P}{O}{R}{T}{S}	{return SUPPORTS_SYM;}</pre>
<p>and the grammar by adding</p>
<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 [[!CSS21]].</p>

<p>Any ''@supports'' 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>

<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 ''@supports'' rule), but declarations
that do not meet the forward-compatible syntax for declarations cause
the entire ''@supports'' rule to be ignored.</p>

<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 ''@supports'' rule?  Or are these
forward-compatible parsing rules the best solution for such future
expansion anyway?</p>

<p>Each of these grammar terms is associated with a boolean result, as
follows:</p>
<dl>
<dt>supports_condition</dt>
<dd>
  The result is the result of the single child term.
</dd>

<dt>supports_negation</dt>
<dd>
  The result is the <em>negation</em> of the result of the
  <code>supports_condition_in_parens</code> child term.
</dd>

<dt>supports_conjunction</dt>
<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.
</dd>

<dt>supports_disjunction</dt>
<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.
</dd>

<dt>supports_condition_in_parens</dt>
<dd>
  The result is the result of the single <code>supports_condition</code>
  or <code>supports_declaration_condition</code> child term.
</dd>

<dt>supports_declaration_condition</dt>
<dd>
  The result is whether the CSS processor <a
  href="#support-definition">supports</a> the declaration.
</dd>
</dl>

<p>The condition of the '@supports' rule is the result of the
<code>supports_condition</code> term that is a child of the
<code>supports_rule</code> term.</p>

<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 '@supports' rule only when
''display: flexbox'' is supported.</p>
</div>

<div class="example">
<p>The following example shows an additional '@supports' rule that can
be used to provide an alternative for when ''display: flexbox'' 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 'width' 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 '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.</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 ''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.</p>

<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">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.</p>

<h3 id="partial-implementations">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 [[!CSS21]] 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:</p>
<ul>
  <li>do not implement, or</li>
  <li>do not have a usable level of support for,</li>
</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 [[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.</p>

<h2 id="at-document">Document queries: the '@document' rule</h2>

<p>The <dfn>'@document' 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 se
1337
t of pages rather than to all pages using the
style sheet.</p>

<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>

<p id="url-of-doc">The <dfn>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., [[HTML5]]&apos;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 
F889
into
another (e.g., [[SVG11]]&apos;s <code>use</code> element), then the
address of the container document is used.</p>

<p class="note">Note:  In [[HTML5]], 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>.</p>

<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>
<li>what normalization (if any) happens to the argument of each of the match
functions before the comparison that they describe and</li>
<li>whether the
comparison algorithm used is string comparison or some other URL
comparison algorithm.</li></ul></div>

<p>The '@document' rule's condition is written as a
comma-separated list of <dfn>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:</p>

<dl>
  <dt><dfn id="url-exact" title="url()|URL matching functions::exact">&lt;url&gt;</dfn></dt>

  <dd>
    <p>The 'url()' function is the <dfn>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 '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).</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>
  </dd>

  <dt><dfn id="url-prefix" title="url-prefix()|URL matching functions::prefix">url-prefix(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The 'url-prefix()' function is the <dfn>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>
  </dd>

  <dt><dfn id="url-domain" title="domain()|URL matching functions::domain">domain(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The 'domain()' function is the <dfn>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 [[!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.</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>
  </dd>

  <dt><dfn id="url-regexp" title="regexp()|URL matching functions::regular expression">regexp(&lt;string&gt;)</dfn></dt>

  <dd>
    <p>The contents of the &lt;string&gt; argument <strong>must</strong>
    match the JavaScript <code>Pattern</code> 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.</p>

    <p>The ''regexp()'' 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 [[!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
    <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 [[HTML5]].</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 element 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 ''\\''.</p>
    </div>
  </dd>

</dl>

<p>Implementations <strong>must</strong> treat any unknown URL matching
functions as a syntax error, and thus ignore the '@document' 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>

<p>This extends the lexical scanner in the
<a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a>
([[!CSS21]], Appendix G) by adding:
<pre>@{D}{O}{C}{U}{M}{E}{N}{T}	{return DOCUMENT_SYM;}</pre>
<p>and the grammar by adding</p>
<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="conformance">Conformance</h2>

<h3 id="base-modules">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:</p>

<ul>
<li>[[!CSS21]]</li>
</ul>

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

<p>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.</p>

<div class="example"><p>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.</p></div>

<p>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.</p>

<h3 id="conformance-classes">Conformance Classes</h3>

  <p>Conformance to the CSS Conditional Rules Module is defined for three
  conformance classes:
  <dl>
    <dt><dfn title="conformance::style sheet" id="conform-style-sheet">style sheet</dfn>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
      style sheet</a>.</dd>
    <dt><dfn title="conformance::processor" id="conform-processor">processor</dfn></dt>
      <dd>A tool that reads CSS style sheets:  it may be a renderer or
      <a
      href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user-agent</a>
      that interprets the semantics of a style sheet and renders
      documents that use style sheets, or it may be a validator that
      checks style sheets.</dd>
    <dt><dfn title="conformance::authoring tool" id="conform-authoring-tool">authoring tool</dfn></dt>
      <dd>A tool that writes a style sheet.</dd>
  </dl>

  <p>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.</p>

  <p>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.)</p>

  <p>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.</p>

<h3 id="partial">
Partial Implementations</h3>

  <p>So th
1337
at authors can exploit the forward-compatible parsing rules to
  assign fallback values, CSS renderers <strong>must</strong>
  treat as invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignore
  as appropriate</a>) any at-rules, properties, property values, keywords,
  and other syntactic constructs for which they have no usable level of
  support. In particular, user agents <strong>must not</strong> 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.</p>

<h3 id="experimental">Experimental Implementations</h3>

  <p>To avoid clashes with future CSS features, the CSS specifications
  reserve a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
  syntax</a> 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.</p>

<h3 id="cr-exit-criteria">CR Exit Criteria</h3>

  <p id=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:

  <dl>
    <dt>independent <dd>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.

    <dt>interoperable <dd>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.

    <dt>implementation <dd>a user agent which:

    <ol class=inline>
      <li>implements the specification.

      <li>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.

      <li>is not experimental (i.e., a version specifically designed
      to pass the test suite and is not intended for normal usage
      going forward).
    </ol>
  </dl>

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

<h2 class=no-num id=grammar>Grammar</h2>

<p>In order to allow these new @-rules in CSS style sheets, this
specification modifies the <code>stylesheet</code> production in the <a
href="http://www.w3.org/TR/CSS21/grammar.html">Appendix G</a> grammar of
[[!CSS21]] by replacing the <code>media</code> production defined in
[[!CSS21]] with the <code>media</code> production defined in this one,
and additionally inserting <code>| supports_rule | document_rule</code>
alongside <code>ruleset | media | page</code>.</p>

<h2 class=no-num id=acknowledgments>Acknowledgments</h2>

<p>
Thanks to the ideas and feedback from
<span lang="tr">Tantek Çelik</span>,
Elika Etemad,
Pascal Germroth,
<span lang="de">Björn Höhrmann</span>,
Alex Mogilevsky,
Chris Moschini,
Ben Ward,
Zack Weinberg,
Boris Zbarsky,
and all the rest of the <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.

</p>

<h2 class=no-num id=references>References</h2>


<h3 class="no-num" id="normative-references">Normative references</h3>
<!--normative-->

<h3 class="no-num" id="other-references">Other references</h3>
<!--informative-->

<h2 class="no-num" id="index">Index</h2>
<!--index-->

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