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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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This document was produced by the CSS Working Group as a Proposed Recommendation.
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A CSS Namespace Test Suite has been developed during the Candidate Recommendation phase of this CSS Namespaces specification. An implementation report is also available.
This document is the same as the previous, Candidate Recommendation version, except for editorial changes.
W3C asks that members of the W3C Advisory Committee review this document and fill in the corresponding online form [member-only]. The review closes on 8 September 2011.
@namespace
rule
This section is non-normative.
This CSS Namespaces module defines syntax for using namespaces in CSS.
It defines the @namespace
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.
Note that a CSS client that does not support this module will (if it
properly conforms to CSS's
forward-compatible parsing rules) ignore all @namespace
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.
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.
Conformance to CSS Namespaces is defined for two classes:
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]
Examples in this specification are introduced with the words "for
example" or are set apart from the normative text with
class="example"
, like this:
This is an example of an informative example.
Informative notes begin with the word "Note" and are set apart from the
normative text with class="note"
, like this:
Note, this is an informative note.
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.
In CSS Namespaces a namespace name consisting of the empty string is taken to represent the null namespace or lack of a namespace.
For example, given the namespace declarations:
@namespace empty ""; @namespace "";
The type
selectors elem
, |elem
, and
empty|elem
are equivalent.
@namespace
ruleThe @namespace
at-rule
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 CSS qualified names defined
below.
@namespace "http://www.w3.org/1999/xhtml"; @namespace svg "http://www.w3.org/2000/svg";
The first rule declares a default namespace
http://www.w3.org/1999/xhtml
to be applied to names that
have no explicit namespace component.
The second rule declares a namespace prefix svg
that is
used to apply the namespace http://www.w3.org/2000/svg
where the svg
namespace prefix is used.
In CSS Namespaces, as in Namespaces in XML 1.0, the prefix is merely a syntactic construct; it is the expanded name (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.
For example, given the following XML document:
<qml:elem xmlns:qml="http://example.com/q-markup"></qml:elem>
and the following @namespace
declarations at the
beginning of a CSS file:
@namespace Q "http://example.com/q-markup"; @namespace lq "http://example.com/q-markup";
The selectors Q|elem
and lq|elem
in that CSS
file would both match the element <qml:elem>
. (The
selector qml|elem
would be invalid.)
The syntax for the @namespace
rule is as follows (using
the notation from the Grammar appendix of CSS
2.1 [CSS21]):
namespace : NAMESPACE_SYM S* [namespace_prefix S*]? [STRING|URI] S* ';' S* ; namespace_prefix : IDENT ;
with the new token:
@{N}{A}{M}{E}{S}{P}{A}{C}{E} {return NAMESPACE_SYM;}
Any @namespace
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 [ namespace [S|CDO|CDC]* ]*
immediately after [ import [S|CDO|CDC]* ]*
in the
stylesheet
grammar.
A syntactically invalid @namespace
rule (whether malformed
or misplaced) must be ignored. A CSS
style
sheet containing an invalid @namespace
rule is not a valid
style sheet.
A URI string parsed from the URI
syntax must be treated as
a literal string: as with the STRING
syntax, no URI-specific
normalization is applied.
All strings—including the empty string and strings representing
invalid URIs—are valid namespace names in @namespace
declarations.
The namespace prefix is declared only within the style sheet in which
its @namespace
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.
A namespace prefix, 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, like CSS counter names, case-sensitive.
If in the namespace declaration the namespace prefix is omitted, then the namespace so declared is the default namespace. The default namespace 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—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.
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.
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.
A CSS qualified name 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" (|
, 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 (*
, U+002A) as a wildcard prefix to indicate a name
in any namespace, including no namespace.
Given the namespace declarations:
@namespace toto "http://toto.example.org"; @namespace "http://example.com/foo";
In a context where the default namespace applies
toto|A
A
in the
http://toto.example.org
namespace.
|B
B
that belongs to no namespace.
*|C
C
in any namespace, including no
namespace.
D
D
in the
http://example.com/foo
namespace.
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 (wqname
) and for qualified names that disallow
wildcard prefixes (qname
). (The syntax uses notation from
the Grammar appendix of
CSS 2.1. [CSS21] Note this means that
comments, but not white space, are implicitly allowed between tokens.):
qname_prefix : [namespace_prefix]? '|' ; wqname_prefix : namespace_prefix? '|' | '*' '|' ;
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, ignored.
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 Håkon 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, Bjöern Höhrmann, and Lachlan Hunt for their comments.