csswg-drafts/css3-namespace/Overview.src.html at 5e0a5311c204679cd753042b32f7e61216ba1026 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Namespaces Module</title>
  <link rel="stylesheet" type="text/css" href="../default.css">
  <link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-ED">
</head>

<body>

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

<h1>CSS Namespaces Module</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:</dt>
    <dd><a href="http://www.w3.org/TR/[YEAR]/CR-css3-namespace-[CDATE]/">http://www.w3.org/TR/[YEAR]/CR-css3-namespace-[CDATE]/</a></dd>
    <!-- <dd><a href="http://dev.w3.org/csswg/css3-namespace/">http://dev.w3.org/csswg/css3-namespace/</a> -->
  <dt>Latest version:</dt>
    <dd><a href="http://www.w3.org/TR/css3-namespace/">http://www.w3.org/TR/css3-namespace/</a></dd>
  <dt>Previous version:</dt>
    <dd><a href="http://www.w3.org/TR/2008/CR-css3-namespace-20080523/">http://www.w3.org/TR/2008/CR-css3-namespace-20080523/</a></dd>
  <dt>Editors:</dt>
    <dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a></dd>
    <dd><a href="http://annevankesteren.nl/contact">Anne van Kesteren</a>, Opera Software ASA</dd>
  <dt>Previous Editors:</dt>
    <dd>Peter Linss, Netscape Communications</dd>
    <dd>Chris Lilley, W3C</dd>
</dl>
<!--begin-copyright-->
<p>[Here will be included the file "../copyright.inc"]</p>
<!--end-copyright-->

<hr title="Separator for header">
</div>

<h2 class="no-num no-toc" id="abstract">Abstract</h2>

  <p>This CSS Namespaces module defines the syntax for using namespaces in CSS.
    It defines the <code>@namespace</code> rule for declaring the
    default namespace and binding namespaces to namespace prefixes, and
    it also defines a syntax that other specifications can adopt for using
    those prefixes in namespace-qualified names.</p>

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

<p>For this specification to exit the CR stage, the following
conditions shall be met:</p>

<ol>
  <li><p>There must be at least two interoperable implementations. For
  the purposes of this criterion, we define the following terms:</p>

  <dl>
    <dt>interoperable</dt>

    <dd><p>passing the respective test case(s) in the <a href="http://www.w3.org/Style/CSS/Test/">CSS test suite</a>,
    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. </p></dd>

    <dt>implementation</dt>

    <dd><p>a user agent which:</p>

    <ol>
      <li>implements the specification.</li>

      <li>is available (i.e. publicly downloadable or available
      through some other public point of sale mechanism). This is the
      "show me" requirement.</li>

      <li>is shipped, or is a "nightly build" (i.e., a development version
      for the next release), but is not experimental (i.e., a version
      specifically designed to pass the test suite and not intended for
      daily usage going forward).</li></ol></dd></dl></li>

  <li><p>A minimum of another three months of the CR period must elapse.
    That is, this specification will not exit CR before 23 August 2008.
    When the specification exits CR, an implementation report will be
    published. At this point, no such report exists.</p></li>

  <li><p>The specified technology must not be harmful for
  accessibility.</p></li></ol>

<p>A CSS Namespace <a href="http://www.w3.org/Style/CSS/Test/">Test Suite</a>
 will be developed during the Candidate Recommendation phase of this CSS
 Namespaces specification.</p>

<p>Comments on and subsequent changes since the last Working Draft are
  listed in the separate <a
  href="http://dev.w3.org/csswg/css3-namespace/issues-2"> Disposition
  of Comments.</a></p>

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

<div>
  <h2 id="intro">Introduction</h2>

  <p><em>This section is non-normative.</em></p>

  <p>This CSS Namespaces module defines syntax for using namespaces in CSS.
    It defines the <code>@namespace</code> rule for declaring a
    default namespace and for binding namespaces to namespace prefixes.
    It also defines a syntax for using those prefixes to represent
    namespace-qualified names. It does not define where such names are
    valid or what they mean: that depends on their context and is defined
    by a host language, such as Selectors ([[SELECT]]), that references the
    syntax defined in the CSS Namespaces module.

  <p>Note that a CSS client that does not support this module will (if it
    properly conforms to
    <a href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">CSS's
    forward-compatible parsing rules</a>)
    ignore all <code>@namespace</code> rules, as well as all style rules that
    make use of namespace qualified names. The syntax of delimiting namespace
    prefixes in CSS was deliberately chosen so that these CSS clients would
    ignore the style rules rather than possibly match them incorrectly.</p>
</div>

<div>
  <h2 id="conformance">Conformance</h2>

  <p>A document or implementation cannot conform to CSS Namespaces alone,
    but can claim conformance to CSS Namespaces if it satisfies the
    conformance requirements in this specification when implementing CSS or
    another host language that normatively references this specification.</p>

  <p>Conformance to CSS Namespaces is defined for two classes:
  <dl>
    <dt><dfn>style sheet</dfn></dt>
      <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS style
        sheet</a> (or a complete unit of another host language that normatively
        references CSS Namespaces).
    <dt><dfn>interpreter</dfn></dt>
      <dd>Someone or something that interprets the semantics of a style sheet.
        (CSS <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">user
        agents</a> fall under this category.)</dd>
  </dl>

  <p>The 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. 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="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="note"</code>, like this:

  <p class="note">Note, this is an informative note.</p>

  <h3 id="terminology">Terminology</h3>

  <p>Besides terms introduced by this specification, CSS Namespaces uses the
    terminology defined in Namespaces in XML 1.0. [[!XML-NAMES]] However,
    the syntax defined here is not restricted to representing XML element and
    attribute names and may represent other kinds of namespaces as defined by
    the host language.
    </p>

  <p>In CSS Namespaces a namespace name consisting of the empty string is
    taken to represent the null namespace or lack of a namespace.</p>

  <div class="example">
    <p>For example, given the namespace declarations:</p>

    <pre>@namespace empty "";
<!--  -->@namespace "";</pre>

    <p>The <a href="http://www.w3.org/TR/css3-selectors/#type-selectors">type
      selectors</a> <code>elem</code>, <code>|elem</code>, and
      <code>empty|elem</code> are equivalent.</p>
  </div>
</div>

<div>
<h2 id="declaration">Declaring namespaces: the <code>@namespace</code> rule</h2>

  <p>The <code>@namespace</code> <a href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">at-rule</a>
    declares a namespace prefix and associates it with a given namespace name
    (a string). This namespace prefix can then be used in namespace-qualified
    names such as the <a href="#css-qnames">CSS qualified names</a> defined below.</p>

  <div class="example">
    <pre>@namespace "http://www.w3.org/1999/xhtml";
<!--  -->@namespace svg "http://www.w3.org/2000/svg";</pre>

    <p>The first rule declares a default namespace
      <code>http://www.w3.org/1999/xhtml</code> to be applied to names that
      have no explicit namespace component.</p>

    <p>The second rule declares a namespace prefix <code>svg</code> that
      is used to apply the namespace <code>http://www.w3.org/2000/svg</code>
      where the <code>svg</code> namespace prefix is used.</p>
  </div>

  <p id="expanded-name-match">In CSS Namespaces, as in Namespaces in XML 1.0,
    the prefix is merely a syntactic construct; it is
    the <em><dfn id="expanded-name">expanded name</dfn></em> (the tuple of
    local name and namespace name) that is significant.
    Thus the actual prefixes used in a CSS style sheet, and whether they are
    defaulted or not, are independent of the namespace prefixes used in the
    markup and whether these are defaulted or not.</p>

  <div class="example">
    <p>For example, given the following XML document:</p>
    <pre><!--
   -->&lt;qml:elem xmlns:qml="http://example.com/q-markup">&lt;/qml:elem><!--
 --></pre>
    <p>and the following <code>@namespace</code> declarations at the
      beginning of a CSS file:</p>
    <pre><!--
    -->@namespace Q "http://example.com/q-markup";
<!---->@namespace lq "http://example.com/q-markup";<!--
 --></pre>
    <p>The selectors <code>Q|elem</code> and <code>lq|elem</code> in that CSS
      file would both match the element <code>&lt;qml:elem&gt;</code>. (The
      selector <code>qml|elem</code> would be invalid.)</p>
  </div>

<h3 id="syntax">Syntax</h3>
  <p>The syntax for the <code>@namespace</code> rule is as follows (using the
    notation from the <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar
    appendix of CSS 2.1</a> [[!CSS21]]):</p>

  <pre><!--
     -->namespace
<!-- -->  : NAMESPACE_SYM S* [namespace_prefix S*]? [STRING|URI] S* ';' S*
<!-- -->  ;
<!-- -->namespace_prefix
<!-- -->  : IDENT
<!-- -->  ;<!--
  --></pre>

  <p>with the new token:</p>

  <pre>@{N}{A}{M}{E}{S}{P}{A}{C}{E} {return NAMESPACE_SYM;}</pre>

  <p>Any <code>@namespace</code> rules must follow all @charset and @import
    rules and precede all other non-ignored at-rules and rule sets in a style
    sheet. For CSS syntax this adds <code>[ namespace [S|CDO|CDC]* ]*</code>
    immediately after <code>[ import [S|CDO|CDC]* ]*</code> in the
    <code>stylesheet</code> grammar.</p>

  <p>A syntactically invalid <code>@namespace</code> rule (whether malformed or
    misplaced) must be <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
    A CSS <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">style sheet</a>
    containing an invalid <code>@namespace</code> rule is itself
    <a href="http://www.w3.org/TR/CSS21/conform.html#valid-style-sheet">invalid</a>.</p>

  <p>A URI string parsed from the <code>URI</code> syntax must be treated as
    a literal string: as with the <code>STRING</code> syntax, no URI-specific
    normalization is applied.</p>

  <p>All strings&#8212;including the empty string and strings representing
    invalid URIs&#8212;are valid namespace names in <code>@namespace</code>
    declarations.</p>

<h3 id="scope">Scope</h3>

  <p>The namespace prefix is declared only within the style sheet in which its
    <code>@namespace</code> rule appears. It is not declared in any style sheets
    importing or imported by that style sheet, nor in any other style sheets
    applying to the document.</p>

<h3 id="prefixes">Declaring Prefixes</h3>

  <p>A <dfn>namespace prefix</dfn>, once declared, represents the namespace for
    which it was declared and can be used to indicate the namespace of a
    namespace-qualified name. Namespace prefixes are,
    <a href="http://www.w3.org/TR/CSS21/syndata.html#counter">like CSS counter
    names</a>, case-sensitive.</p>
    <!-- They do not, however, distinguish between equivalent
    canonical Unicode representations. Essentially, an <code>@namespace</code>
    declaration declares all prefixes that map to the same NFC representation as
    the given prefix to the same namespace name. (See [[!UNICODE]] for the definition
    of NFC normalization.) -->

  <p>If in the namespace declaration the namespace prefix is omitted, then the
    namespace so declared is the default namespace. The <dfn>default
    namespace</dfn> may apply to names that have no explicit namespace prefix:
    modules that employ namespace prefixes must define in which contexts the
    default namespace applies. For example, following [[!XML-NAMES]], in
    Selectors [[SELECT]] the default namespace applies to type
    selectors&#8212;but it does not apply to attribute selectors. There is no
    default value for the default namespace: modules that assign unqualified
    names to the default namespace must define how those unqualified names are
    to be interpreted when no default namespace is declared.</p>

  <p class="note">Note that using default namespaces in conjunction with type
    selectors can cause UAs that support default namespaces and UAs that don't
    support default namespaces to interpret selectors differently. See
    <a href="http://www.w3.org/TR/css3-selectors/#downlevel">Namespaces and
    down-level clients</a> in the Selectors module [[SELECT]] for details.</p>

  <p>If a namespace prefix or default namespace is declared more than once only
    the last declaration shall be used. Declaring a namespace prefix or default
    namespace more than once is nonconforming.</p>
</div>

<div>
<h2 id="css-qnames">CSS Qualified Names</h2>

  <p>A <dfn>CSS qualified name</dfn> is a name explicitly located within (associated
    with) a namespace. To form a qualified name in CSS syntax, a namespace
    prefix that has been declared within scope is prepended to a local name
    (such as an element or attribute name), separated by a "vertical bar"
    (<code>|</code>, U+007C). The prefix, representing the namespace for which
    it has been declared, indicates the namespace of the local name.
    The prefix of a qualified name may be omitted to indicate that the name
    belongs to no namespace, i.e. that the namespace name part of the expanded
    name has no value.
    Some contexts (as defined by the host language) may allow the use of an asterisk
    (<code>*</code>, U+002A) as a wildcard prefix to indicate a name in any namespace,
    including no namespace.</p>

  <div class="example">
    <p>Given the namespace declarations:</p>

    <pre>@namespace toto "http://toto.example.org";
<!--  -->@namespace "http://example.com/foo";</pre>

    <p>In a context where the default namespace applies</p>

    <dl>
    <dt><code>toto|A</code></dt>
      <dd>represents the name <code>A</code> in the <code>http://toto.example.org</code>
        namespace.</dd>
    <dt><code>|B</code></dt>
      <dd>represents the name <code>B</code> that belongs to no namespace.</dd>
    <dt><code>*|C</code></dt>
      <dd>represents the name <code>C</code> in any namespace, including no
        namespace.</dd>
    <dt><code>D</code></dt>
         namespace.</dd>
    </dl>
  </div>

  <p>The syntax for the portion of a CSS qualified name before the local name
    is given below, both for qualified names that allow wildcard prefixes
    (<code>wqname</code>) and for qualified names that disallow wildcard prefixes
    (<code>qname</code>). (The syntax uses notation from the
    <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar appendix of
    CSS 2.1</a> [[!CSS21]]):</p>

  <pre><!--
     -->qname_prefix
<!-- -->  : [namespace_prefix]? '|'
<!-- -->  ;
<!-- -->wqname_prefix
<!-- -->  : namespace_prefix? '|'
<!-- -->  | '*' '|'
<!-- -->  ;<!--
--></pre>

  <p>CSS qualified names can be used in (for example) selectors and property
    values as described in other modules. Those modules must define handling
    of namespace prefixes that have not been properly declared. Such handling
    should treat undeclared namespace prefixes as a parsing error that will
    cause the selector or declaration (etc.) to be considered invalid and, in
    CSS, <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.</p>

  <div class="example">
    <p>For example, the Selectors module [[SELECT]] defines a type selector with
    an undeclared namespace prefix to be an invalid selector, and CSS
    [[!CSS21]] requires rule sets with an invalid selector to be completely
    ignored.</p>
  </div>
</div>

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

<p>This draft borrows heavily from earlier drafts on CSS namespace support
  by Chris Lilley and by Peter Linss and early (unpublished)
  drafts on CSS and XML by H&aring;kon Lie and Bert Bos, and XML Namespaces
  and CSS by Bert Bos and Steven Pemberton. Many current and former members of
  the CSS Working Group have contributed to this document. Discussions on
  www-style@w3.org and in other places have also contributed ideas to this
  specification. Special thanks goes to L. David Baron, Karl Dubost, Ian Hickson,
  Bj&ouml;ern H&ouml;hrmann, and Lachlan Hunt for their comments.</p>

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


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


<p>[Here will be inserted the file "normative.inc"]</p>
<!--end-normative-->


<h3 class="no-num" id="informative-references">Informative references</h3>
<!--begin-informative-->


<p>[Here will be inserted the file "informative.inc"]</p>
<!--end-informative-->


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