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[css-namespaces] Initial bikeshedding of the spec.
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<h1>CSS Namespaces Module Level 3</h1>
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<pre class='metadata'>
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Group: csswg
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Shortname: css-namespace
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Level: 3
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Status: ED
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ED: http://dev.w3.org/csswg/css-namespaces/
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TR: http://www.w3.org/TR/css3-namespace/
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Editor: Elika J. Etemad, http://fantasai.inkedblade.net/contact
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Former Editor: Anne van Kesteren
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Former Editor: Peter Linss
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Former Editor: Chris Lilley
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Previous Version: http://www.w3.org/TR/2008/CR-css3-namespace-20080523/
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Abstract: This CSS Namespaces module defines the syntax for using namespaces in CSS. It defines the ''@namespace'' 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.
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Link Defaults: selectors-4 (dfn) type selectors, css-syntax-3 (dfn) at-rule
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</pre>
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<!--
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<h2 class="no-num no-toc" id="status">Status of this document</h2>
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A CSS Namespace <a href="http://www.w3.org/Style/CSS/Test/">Test Suite</a>
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has been developed during the Candidate Recommendation phase of this CSS
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Namespaces specification. An <a
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href="/Style/CSS/Test/CSS3/Namespace/20090210/reports/implement-report.html"
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>implementation report</a> is also available.
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This document is the same as the previous, Candidate Recommendation
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version, except for editorial changes.
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<em>W3C asks that members of the W3C Advisory Committee review this
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document and fill in the corresponding <a
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href="/2002/09/wbs/33280/PR-css3-namespace/" >online form
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[member-only].</a> The review closes on 8 September 2011.</em>
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-->
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<h2 id="intro">
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Introduction</h2>
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<em>This section is non-normative.</em>
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This CSS Namespaces module defines syntax for using namespaces in CSS.
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It defines the ''@namespace'' rule for declaring a default namespace
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and for binding namespaces to namespace prefixes.
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It also defines a syntax for using those prefixes to represent namespace-qualified names.
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It does not define where such names are valid or what they mean:
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that depends on their context and is defined by a host language,
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such as Selectors ([[SELECT]]),
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that references the syntax defined in the CSS Namespaces module.
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Note that a CSS client that does not support this module will
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(if it properly conforms to <a href="http://www.w3.org/TR/CSS21/syndata.html#parsing-errors">CSS's forward-compatible parsing rules</a>)
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ignore all ''@namespace'' rules,
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as well as all style rules that make use of namespace qualified names.
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The syntax of delimiting namespace prefixes in CSS was deliberately chosen
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so that these CSS clients would ignore the style rules
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rather than possibly match them incorrectly.
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<h3 id="terminology">
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Terminology</h3>
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Besides terms introduced by this specification,
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CSS Namespaces uses the terminology defined in Namespaces in XML 1.0. [[!XML-NAMES]]
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However, the syntax defined here is not restricted to representing XML element and attribute names
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and may represent other kinds of namespaces as defined by the host language.
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In CSS Namespaces a namespace name consisting of the empty string
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is taken to represent the null namespace
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or lack of a namespace.
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<div class="example">
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For example, given the namespace declarations:
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<pre>
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@namespace empty "";
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@namespace "";
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</pre>
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The <a>type selectors</a>
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<code>elem</code>,
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<code>|elem</code>,
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and <code>empty|elem</code>
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are equivalent.
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</div>
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<div>
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<h2 id="declaration">Declaring namespaces: the ''@namespace'' rule</h2>
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The ''@namespace'' <a>at-rule</a> declares a namespace prefix
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and associates it with a given namespace name (a string).
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This namespace prefix can then be used in namespace-qualified names
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such as the <a>CSS qualified names</a> defined below.
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<div class="example">
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<pre>
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@namespace "http://www.w3.org/1999/xhtml";
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@namespace svg "http://www.w3.org/2000/svg";
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</pre>
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The first rule declares a default namespace
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<code>http://www.w3.org/1999/xhtml</code>
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to be applied to names that have no explicit namespace component.
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The second rule declares a namespace prefix <code>svg</code>
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that is used to apply the namespace <code>http://www.w3.org/2000/svg</code>
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where the <code>svg</code> namespace prefix is used.
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</div>
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In CSS Namespaces, as in Namespaces in XML 1.0,
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the prefix is merely a syntactic construct;
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it is the <dfn export>expanded name</dfn>
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(the tuple of local name and namespace name)
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that is significant.
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Thus the actual prefixes used in a CSS style sheet,
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and whether they are defaulted or not,
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are independent of the namespace prefixes used in the markup
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and whether these are defaulted or not.
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<div class="example">
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For example, given the following XML document:
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<pre>
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&lt;qml:elem xmlns:qml="http://example.com/q-markup">&lt;/qml:elem>
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</pre>
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and the following ''@namespace'' declarations at the
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beginning of a CSS file:
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<pre>
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@namespace Q "http://example.com/q-markup";
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@namespace lq "http://example.com/q-markup";
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</pre>
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The selectors ''Q|elem'' and ''lq|elem'' in that CSS file
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would both match the element <code>&lt;qml:elem&gt;</code>.
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(The selector ''qml|elem'' would be invalid,
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because CSS namespaces only recognize prefixes declared in CSS,
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not those declared by the document language.)
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</div>
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<h3 id="syntax">
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Syntax</h3>
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The syntax for the ''@namespace'' rule is as follows
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(using the notation from the <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar appendix of CSS 2.1</a> [[!CSS21]]):
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<pre>
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namespace
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: NAMESPACE_SYM S* [namespace_prefix S*]? [STRING|URI] S* ';' S*
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;
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namespace_prefix
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: IDENT
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;
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</pre>
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with the new token:
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<pre>@{N}{A}{M}{E}{S}{P}{A}{C}{E} {return NAMESPACE_SYM;}</pre>
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Any ''@namespace'' rules must follow all @charset and @import rules
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and precede all other non-ignored at-rules and style rules in a style sheet.
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For CSS syntax this adds <code>[ namespace [S|CDO|CDC]* ]*</code>
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immediately after <code>[ import [S|CDO|CDC]* ]*</code> in the <code>stylesheet</code> grammar.
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A syntactically invalid ''@namespace'' rule
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(whether malformed or misplaced)
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must be <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
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A CSS <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">style sheet</a> containing an invalid ''@namespace'' rule
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is not a <a href="http://www.w3.org/TR/CSS21/conform.html#valid-style-sheet">valid style sheet</a>.
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A URI string parsed from the <code>URI</code> syntax must be treated as a literal string:
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as with the <code>STRING</code> syntax,
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no URI-specific normalization is applied.
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All strings--
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including the empty string and strings representing invalid URIs--
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are valid namespace names in ''@namespace'' declarations.
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<h3 id="scope">
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Scope</h3>
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The namespace prefix is declared only within the style sheet in which its
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''@namespace'' rule appears. It is not declared in any style sheets
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importing or imported by that style sheet, nor in any other style sheets
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applying to the document.
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<h3 id="prefixes">
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Declaring Prefixes</h3>
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A <dfn export>namespace prefix</dfn>, once declared,
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represents the namespace for which it was declared
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and can be used to indicate the namespace of a namespace-qualified name.
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Namespace prefixes are,
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<a href="http://www.w3.org/TR/CSS21/syndata.html#counter">like CSS counter names</a>,
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case-sensitive.
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<!-- They do not, however, distinguish between equivalent
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canonical Unicode representations. Essentially, an ''@namespace''
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declaration declares all prefixes that map to the same NFC representation as
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the given prefix to the same namespace name. (See [[!UNICODE]] for the definition
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of NFC normalization.) -->
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If in the namespace declaration the namespace prefix is omitted,
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then the namespace so declared is the default namespace.
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The <dfn export>default namespace</dfn> may apply to names that have no explicit namespace prefix:
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modules that employ namespace prefixes must define in which contexts the default namespace applies.
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For example, following [[!XML-NAMES]],
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in Selectors [[SELECT]] the default namespace applies to type selectors--
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but it does not apply to attribute selectors.
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There is no default value for the default namespace:
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modules that assign unqualified names to the default namespace
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must define how those unqualified names are to be interpreted
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when no default namespace is declared.
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Note: Note that using default namespaces in conjunction with type selectors
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can cause UAs that support default namespaces
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and UAs that don't support default namespaces
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to interpret selectors differently.
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If a namespace prefix or default namespace is declared more than once
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only the last declaration shall be used.
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Declaring a namespace prefix or default namespace more than once is nonconforming.
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<h2 id="css-qnames">
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CSS Qualified Names</h2>
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A <dfn export>CSS qualified name</dfn> is a name explicitly located within (associated with) a namespace.
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To form a qualified name in CSS syntax,
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a namespace prefix that has been declared within scope is prepended to a local name
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(such as an element or attribute name),
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separated by a "vertical bar"(<code>|</code>, U+007C).
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The prefix, representing the namespace for which it has been declared,
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indicates the namespace of the local name.
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The prefix of a qualified name may be omitted to indicate that the name belongs to no namespace,
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i.e. that the namespace name part of the expanded name has no value.
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Some contexts (as defined by the host language)
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may allow the use of an asterisk (<code>*</code>, U+002A)
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as a wildcard prefix to indicate a name in any namespace,
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including no namespace.
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<div class="example">
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Given the namespace declarations:
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<pre>
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@namespace toto "http://toto.example.org";
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@namespace "http://example.com/foo";
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</pre>
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In a context where the default namespace applies
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<dl>
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<dt><code>toto|A</code>
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<dd>
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represents the name <code>A</code>
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in the <code>http://toto.example.org</code> namespace.
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<dt><code>|B</code>
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<dd>
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represents the name <code>B</code>
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that belongs to no namespace.
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<dt><code>*|C</code>
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<dd>
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represents the name <code>C</code>
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in any namespace,
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including no namespace.
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<dt><code>D</code>
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<dd>
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represents the name <code>D</code>
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in the <code>http://example.com/foo</code> namespace.
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</dl>
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</div>
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The syntax for the portion of a CSS qualified name before the local name is given below,
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both for qualified names that allow wildcard prefixes (<code>wqname</code>)
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and for qualified names that disallow wildcard prefixes (<code>qname</code>).
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(The syntax uses notation from the <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar appendix of CSS 2.1</a>. [[!CSS21]]
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Note this means that comments, but not white space, are implicitly allowed between tokens.):
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<pre>
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qname_prefix
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: namespace_prefix? '|'
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;
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wqname_prefix
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: [ namespace_prefix? | '*' ] '|'
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;
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qname
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: qname_prefix? ident
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;
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wqname
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: wqname_prefix? ident
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;
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</pre>
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CSS qualified names can be used in (for example)
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selectors and property values as described in other modules.
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Those modules must define handling of namespace prefixes that have not been properly declared.
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Such handling should treat undeclared namespace prefixes as a parsing error
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that will cause the selector or declaration (etc.) to be considered invalid
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and, in CSS, <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.
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<div class="example">
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For example, the Selectors module [[SELECT]] defines a type selector
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with an undeclared namespace prefix
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to be an invalid selector,
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and CSS [[!CSS21]] requires style rules with an invalid selector to be completely ignored.
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</div>
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<h2 class="no-num" id="acks">
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Acknowledgments</h2>
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This draft borrows heavily from earlier drafts on CSS namespace support
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by Chris Lilley and by Peter Linss
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and early (unpublished) drafts on CSS and XML by Håkon Lie and Bert Bos,
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and XML Namespaces and CSS by Bert Bos and Steven Pemberton.
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Many current and former members of the CSS Working Group have contributed to this document.
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Discussions on www-style@w3.org and in other places have also contributed ideas to this specification.
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Special thanks goes to
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L. David Baron,
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Karl Dubost,
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Ian Hickson,
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Björn Höhrmann,
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and Lachlan Hunt
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for their comments.

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