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From: Tab Atkins Jr.. via cvs-syncmail <cvsmail@w3.org>
Date: Sat, 24 Sep 2011 01:25:31 +0000
To: public-css-commits@w3.org
Message-Id: <E1R7Gzj-0005h8-G0@lionel-hutz.w3.org>
Update of /sources/public/csswg/css-variables
In directory hutz:/tmp/cvs-serv21876

Added Files:
	Overview.html Overview.src.html 
Log Message:
First draft.

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<!DOCTYPE html>

<html lang=en>
 <head profile="http://www.w3.org/2006/03/hcard"><meta content="text/html;
  charset=utf-8" http-equiv=Content-Type>

  <title>CSS Variables Module Level 1</title>
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  type="text/css">
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  type="text/css">

  <style type="text/css">
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		.replaceme {
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			margin: 0 0.1em;
			background: yellow;
			color: green;
		}
		.replaceme:before {
			 content: "FIXME(";
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	</style>

 <body>
  <div class=head> <!--begin-logo-->
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    src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->

   <h1>CSS Variables Module Level 1</h1>

   <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 24 September
    2011</h2>

   <dl>
    <dt>This version:

    <dd><a
     href="http://www.w3.org/TR/2011/ED-css-variables-20110924/">http://dev.w3.org/csswg/css-variables/</a>
     <!--<dt>Latest version:
		<dd><a href="http://www.w3.org/TR/css-variables/">http://www.w3.org/TR/css-variables/</a>-->
     

    <dt>Editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</a>
     

    <dt>Editors:

    <dd class=vcard> <a href="http://xanthir.com/contact/" rel=url> <span
     class=fn>Tab Atkins Jr.</span>, </a> <span class=org>Google, Inc.</span>
     
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2011 <a
    href="http://www.w3.org/"><acronym title="World Wide Web
    Consortium">W3C</acronym></a><sup>&reg;</sup> (<a
    href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
    of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
    title="European Research Consortium for Informatics and
    Mathematics">ERCIM</acronym></a>, <a
    href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
    <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
    and <a
    href="http://www.w3.org/Consortium/Legal/copyright-documents">document
    use</a> rules apply.</p>
   <!--end-copyright-->
   <hr title="Separator for header">
  </div>

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

  <p>CSS is a language for describing the rendering of structured documents
   (such as HTML and XML) on screen, on paper, in speech, etc. This module
   contains the features of CSS level&nbsp;3 relating to <var
   class=replaceme>DESCRIBE HERE</var>. It includes and extends the
   functionality of CSS level&nbsp;2 <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, which builds on CSS
   level&nbsp;1 <a href="#CSS1" rel=biblioentry>[CSS1]<!--{{CSS1}}--></a>.
   The main extensions compared to level&nbsp;2 are <var
   class=replaceme>SUMMARIZE HERE</var>.

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

  <p>This is a public copy of the editors' draft. It is provided for
   discussion only and may change at any moment. Its publication here does
   not imply endorsement of its contents by W3C. Don't cite this document
   other than as work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss-variables%5D%20PUT%20SUBJECT%20HERE">
   www-style@w3.org</a> (see <a
   href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
   discussion of this specification. When sending e-mail, please put the text
   &#8220;css-variables&#8221; in the subject, preferably like this:
   &#8220;[<!---->css-variables<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

  <p>This document was produced by the <a href="/Style/CSS/members">CSS
   Working Group</a> (part of the <a href="/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
   Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
   W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>The following features are at risk: &hellip;

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

  <ul class=toc>
   <li><a href="#intro"><span class=secno>1. </span> Introduction</a>
    <ul class=toc>
     <li><a href="#placement"><span class=secno>1.1. </span> Module
      Interactions</a>

     <li><a href="#values"><span class=secno>1.2. </span> Values</a>
    </ul>

   <li><a href="#defining-variables"><span class=secno>2. </span> Defining
    Variables</a>
    <ul class=toc>
     <li><a href="#variable-classes"><span class=secno>2.1. </span> Variable
      Classes</a>
    </ul>

   <li><a href="#using-variables"><span class=secno>3. </span> Using
    Variables</a>
    <ul class=toc>
     <li><a href="#using-invalid-variables"><span class=secno>3.1. </span>
      Using Invalid Variables</a>
    </ul>

   <li><a href="#cssom"><span class=secno>4. </span> APIs</a>

   <li><a href="#grammar"><span class=secno>5. </span> The Grammar of
    Variables</a>

   <li><a href="#conformance"><span class=secno>6. </span> Conformance</a>
    <ul class=toc>
     <li><a href="#conventions"><span class=secno>6.1. </span> Document
      Conventions</a>

     <li><a href="#conformance-classes"><span class=secno>6.2. </span>
      Conformance Classes</a>

     <li><a href="#partial"><span class=secno>6.3. </span> Partial
      Implementations</a>

     <li><a href="#experimental"><span class=secno>6.4. </span> Experimental
      Implementations</a>

     <li><a href="#testing"><span class=secno>6.5. </span>Non-Experimental
      Implementations</a>

     <li><a href="#cr-exit-criteria"><span class=secno>6.6. </span> CR Exit
      Criteria</a>
    </ul>

   <li class=no-num><a href="#acknowledgments">Acknowledgments</a>

   <li class=no-num><a href="#references">References</a>
    <ul class=toc>
     <li class=no-num><a href="#normative-references">Normative
      references</a>

     <li class=no-num><a href="#other-references">Other references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>

   <li class=no-num><a href="#property-index">Property index</a>
  </ul>
  <!--end-toc-->

  <h2 id=intro><span class=secno>1. </span> Introduction</h2>

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

  <p>Large documents or applications (and even small ones) can contain quite
   a bit of CSS. Many of the values in the CSS file will be duplicate data;
   for example, a site may establish a color scheme and reuse three or four
   colors throughout the site. Altering this data can be difficult and
   error-prone, since it's scattered throughout the CSS file (and possibly
   across multiple files), and may not be amenable to Find-and-Replace. This
   module introduces <b>Variables</b>, which allow a value to be assigned to
   a name, which may then be used in place of the value elsewhere in the
   document. This makes it easier to read large files, as seemingly-arbitrary
   values now have informative names, and makes editting such files much
   easier and less error-prone, as one only has to change the value once, at
   the variable definition site, and the change will propagate to all uses of
   that variable automatically.

  <h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>

  <p class=replaceme>Explain, normatively, how this module affects the
   definition of CSS.

  <p>This module replaces and extends the <var class=replaceme>SUMMARIZE
   HERE</var> features defined in <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections <var
   class=replaceme>W.X and Y.Z</var>.

  <h3 id=values><span class=secno>1.2. </span> Values</h3>

  <p>This specification follows the <a
   href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
   definition conventions</a> from <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
   this specification are defined in CSS Level 2 Revision 1 <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
   modules may expand the definitions of these value types: for example <a
   href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>,
   when combined with this module, expands the definition of the
   &lt;color&gt; value type as used in this specification.

  <h2 id=defining-variables><span class=secno>2. </span> Defining Variables</h2>

  <p>A &lsquo;<code class=css>@var</code>&rsquo; rule defines a variable and
   assigns a value to it. It consists of the at-keyword &lsquo;<code
   class=css>@var</code>&rsquo; followed by a variable name (an identifier
   starting with the "$" character), followed by the value and finally
   terminated with a semi-colon. The result of this rule is that the given
   value is assigned to the given variable name.

  <p>

  <div class=example>
   <p>This variables declaration:</p>

   <pre>@var $header-color #06c;</pre>

   <p>declares a variable named "$header-color", and assigns to it the value
    "#06c". This can then be used in any place where that value would be
    valid, such as:

   <pre>h1 { background-color: $header-color; }</pre>

   <p>The preceding rule is equivalent to writing &lsquo;<code
    class=css>background-color: #06c;</code>&rsquo;, except that the variable
    name makes the origin of the color clearer, and if $header-color is used
    elsewhere in the stylesheet, all of the uses can be updated at once by
    changing the variable declaration.</p>
  </div>

  <p>Defined variables are available to all stylesheets in the document.
   Thus, using &lsquo;<code class=css>@import</code>&rsquo; to include a
   stylesheet also includes all variables defined in the stylesheet (and
   makes all variables declared outside available within the stylesheet, if
   applicable). &lsquo;<code class=css>@var</code>&rsquo; rules present in
   disabled or alternate stylesheets do not define variables. Scoped
   stylesheets are an exception to this: variables defined in the global
   scope are available within a scoped stylesheet, but variables defined or
   imported within scoped stylesheets are only available within the scoped
   stylesheet and any imported stylesheets.

  <p>A &lsquo;<code class=css>@var</code>&rsquo; rule is <dfn
   id=inactive-var-rule>inactive</dfn> if it's present in a disabled or
   alternate stylesheet, a stylesheet with a media query that evaluates to
   false, or is present within a conditional rule group [[CSS3CONDITIONAL]]
   whose condition evaluates to false. <a
   href="#inactive-var-rule"><i>Inactive</i></a> &lsquo;<code
   class=css>@var</code>&rsquo; rules do not define variables. All other
   &lsquo;<code class=css>@var</code>&rsquo; rules are <dfn
   id=active-var-rule>active</dfn>.

  <p>If new &lsquo;<code class=css>@var</code>&rsquo; rules are added to the
   document through any means, such as direct CSSOM manipulation or new
   stylesheets being added to the document, or if an <a
   href="#inactive-var-rule"><i>inactive</i></a> &lsquo;<code
   class=css>@var</code>&rsquo; rule becomes <a
   href="#active-var-rule"><i>active</i></a>, they define variables.
   Similarly, if &lsquo;<code class=css>@var</code>&rsquo; rules are removed
   from the document or become <a
   href="#inactive-var-rule"><i>inactive</i></a>, they no longer define
   variables.

  <p>If multiple <a href="#active-var-rule"><i>active</i></a> &lsquo;<code
   class=css>@var</code>&rsquo; rules attempt to define variables with the
   same name, the last one defined wins; the preceding &lsquo;<code
   class=css>@var</code>&rsquo; rules do not define a variable. For this
   purpose, user-agent defined rules come before author-defined rules, which
   come before user-defined rules. Within each of the preceding groups, the
   ordering is document order.

  <div class=example>
   <p>Earlier declarations of the same variable have no effect, even if uses
    of the variable come between the earlier and final attempts to define the
    variable:</p>

   <pre>
@var $var red;
p { color: $var; }
@var $var blue;</pre>

   <p>In this example, the color of &lt;p> elements is blue, because the
    second declaration wins over the first declaration in all uses.</p>
  </div>

  <p>Variables can refer to other variables in their value. If a dependency
   cycle is created, all the declarations that directly contribute to the
   cycle define <a href="#invalid-variable"><i>invalid variables</i></a>.

  <div class=example>
   <p>For example, the following set of variable declarations create a
    dependency cycle:</p>

   <pre>
@var $foo red;
@var $bar linear-gradient(transparent, $foo);
@var $foo $bar;</pre>

   <p>The last two declarations create the cycle, and so they define
    &lsquo;<code class=css>$bar</code>&rsquo; and &lsquo;<code
    class=css>$foo</code>&rsquo;, respectively, as <a
    href="#invalid-variable"><i>invalid variables</i></a> instead of as the
    values in the declarations. If the last rule were removed, however, there
    would be no problem: both &lsquo;<code class=css>$foo</code>&rsquo; and
    &lsquo;<code class=css>$bar</code>&rsquo; would then be defined as the
    value &lsquo;<code class=css>red</code>&rsquo;.</p>
  </div>

  <p>Before they are defined, all variables are <a
   href="#invalid-variable"><i>invalid variables</i></a>.

  <h3 id=variable-classes><span class=secno>2.1. </span> Variable Classes</h3>

  <p>Variables fall into a few different classes which affect where and how
   they can be used, depending on the value they're defined as.

  <dl>
   <dt><dfn id=singular-variable title="singular variable|singular
    variables">singular variable</dfn>

   <dd>A variable whose value is a single <a
    href="#cssfoo"><i>CSSFOO</i></a>, such as a keyword (like &lsquo;<code
    class=css>red</code>&rsquo;), a dimension (like &lsquo;<code
    class=css>20px</code>&rsquo;), or a function (like &lsquo;<code
    class=css>linear-gradient(red,blue)</code>&rsquo;).

   <dt><dfn id=compound-variable title="compound variable|compound
    variables">compound variable</dfn>

   <dd>A variable whose value is a space-separated list of <a
    href="#cssfoo"><i>CSSFOO</i></a>s, like &lsquo;<code class=css>20px 30px
    black</code>&rsquo;.

   <dt><dfn id=list-variable title="list variable|list variables">list
    variable</dfn>

   <dd>A variable whose value is multiple <a
    href="#cssfoo"><i>CSSFOO</i></a>s separated by spaces or other
    separators, such as commas or slashes, like &lsquo;<code class=css>red,
    blue, white</code>&rsquo;.
  </dl>

  <p class=issue>What term can I use for <dfn id=cssfoo>CSSFOO</dfn>? It
   looks like it corresponds to "term" in the 2.1 grammar.

  <h2 id=using-variables><span class=secno>3. </span> Using Variables</h2>

  <p>A variable can be used anywhere a value is expected in CSS. Variables
   can not be used as property names, selectors, or anything else besides
   property values - doing so either produces an invalid value or, in some
   situations like the attribute value of an attribute selector, a valid
   value that nonetheless has no relation to the variable of that name.

  <p>A variable is substituted for its value in the property value at
   computed-value time.

  <p><a href="#singular-variable"><i>Singular variables</i></a> can be used
   anywhere as a value. They can be an entire property's value, a component
   value in a larger property value, an argument to a function, etc.. <a
   href="#compound-variable"><i>Compound variables</i></a> are somewhat more
   restricted - they can only be used as an entire property value, a whole
   component of a list-valued property, or a whole function argument. <a
   href="#list-variable"><i>List variables</i></a> are the most restricted -
   they can only be used as an entire property value, as one or more whole
   components of a list-valued property, or as a whole series of function
   arguments.

  <div class=example>
   <p>Here are some examples of valid and invalid use of a <a
    href="#compound-variable"><i>compound variable</i></a>:</p>

   <pre>
@var $compound 20px 30px;
div {
	margin: $compound; 
	/* Valid, as it's used as a whole property value */
	/* Equivalent to "margin: 20px 30px;" */

	text-shadow: $compound blue; 
	/* Invalid, as it's not used as a whole property value */

	background-position: 10px 10px, $compound, 50px; 
	/* Valid, as it's used as a whole component of a list-valued property */
	/* Equivalent to "background-position: 10px 10px, 20px 30px, 50px;" */

	background-image: radial-gradient($compound, red, blue);
	/* Valid, as it's used as a whole function argument */
	/* Equivalent to "background-image: radial-gradient(20px 30px, red, blue);" */
}</pre>

   <p>Here are some examples of valid and invalid uses of a <a
    href="#list-variable"><i>list variable</i></a>:</p>

   <pre>
@var $list 20px, 30px 40px;
div {
	background-position: $list;
	/* Valid, as it's used as a whole property value */
	/* Equivalent to "background-position: 20px, 30px" */

	background-position: 10px $list;
	/* Invalid, as it's not used as a whole property value or component of a list. */
	/* NOT equivalent to "background-position: 10px 20px, 30px 40px;" */

	background-position: 10px, $list, 50px;
	/* Valid, as it's used as a whole sequence of components of a list. */
	/* Equivalent to "background-position: 10px, 20px, 30px 40px, 50px;" */

	background-image: radial-gradient($list, red, blue);
	/* Valid, as it's used as a whole sequence of function arguments. */
	/* Equivalent to "background-image: radial-gradient(20px, 30px 40px, red, blue);" */

	background-image: radial-gradient(10px $list, red, blue);
	/* Invalid, as it's not used as a whole sequence of function arguments */
}</pre>
  </div>

  <p>Using a <a href="#compound-variable"><i>compound variable</i></a> or <a
   href="#list-variable"><i>list variable</i></a> in an invalid way is an <a
   href="#invalid-variable-use"><i>invalid variable use</i></a>.

  <p>Similarly, if a property containing a variable in its property value
   ends up being invalid when the variable's value is substituted in, that is
   an <a href="#invalid-variable-use"><i>invalid variable use</i></a>.

  <div class=example>
   <p>For example, the following usage is fine from a syntax standpoint, but
    results in nonsense when the variable is substituted in:</p>

   <pre>
@var $looks-valid 20px;
p { background-color: $looks-valid; }</pre>

   <p>Since &lsquo;<code class=css>20px</code>&rsquo; is an invalid value for
    &lsquo;<code class=property>background-color</code>&rsquo;, this instance
    of the property computes to &lsquo;<code
    class=property>transparent</code>&rsquo; (the initial value for
    &lsquo;<code class=property>background-color</code>&rsquo;) instead.</p>
  </div>

  <h3 id=using-invalid-variables><span class=secno>3.1. </span> Using Invalid
   Variables</h3>

  <p>An <dfn id=invalid-variable title="invalid variable|invalid
   variables">invalid variable</dfn> results from having variables directly
   or indirectly refer to themselves. Using an <a
   href="#invalid-variable"><i>invalid variable</i></a> results in an <a
   href="#invalid-variable-use"><i>invalid variable use</i></a>.

  <p>An <dfn id=invalid-variable-use>invalid variable use</dfn> results from
   either using an <a href="#invalid-variable"><i>invalid variable</i></a>,
   or using a <a href="#compound-variable"><i>compound variable</i></a> or <a
   href="#list-variable"><i>list variable</i></a> in an invalid way. When
   this happens, the property the variable is used in must compute to the
   property's initial value.

  <div class=example>
   <p>For example, in the following code:</p>

   <pre>
@var $invalid $invalid; /* Invalid, due to the trivial cycle. */
p { background-color: red; }
p { background-color: $invalid; }</pre>

   <p>the &lt;p> elements will have transparent backgrounds (the initial
    value for &lsquo;<code class=property>background-color</code>&rsquo;),
    rather than red text.</p>
  </div>

  <p class=note>The <a href="#invalid-variable-use"><i>invalid variable
   use</i></a> concept exists because variables can't "fail early" like other
   syntax errors can, so by the time the user agent realizes a variable is
   invalid, it's already thrown away the other cascaded values.

  <h2 id=cssom><span class=secno>4. </span> APIs</h2>

  <p class=issue>Define the "basic" CSSOM APIs from <a
   href="http://disruptive-innovations.com/zoo/cssvariables/#mozTocId847334">Daniel's
   and Hyatt's spec</a>.

  <p class=issue>Define the more convenient CSSOM API from <a
   href="http://www.xanthir.com/blog/b4AD0">my blog post</a>.

  <h2 id=grammar><span class=secno>5. </span> The Grammar of Variables</h2>

  <p class=issue>I'm not sure if I've done this section correctly. For now,
   I'll try my best to copypasta what Conditionals is doing, since dbaron
   usually know what's what.

  <p>This specification extends the lexical scanner in the <a
   href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by
   adding:

  <pre>
@{V}{A}{R}    {return VARIABLE_SYM;}
"$"name       {return VARIABLE;}</pre>

  <p>and the grammar by adding:

  <pre>
variable_declaration
  : VARIABLE_SYM S+ variable_name S+ expr ':';
  ;</pre>

  <p>and by amending:

  <pre>
stylesheet
  : [ CHARSET_SYM STRING ';' ]?
    [S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
    [ [ ruleset | media | page | variable_declaration ] [ CDO S* | CDC S* ]* ]*
  ;
media
  : MEDIA_SYM S* media_list '{' S* [ ruleset | variable_declaration ]* '}' S*
  ;
term
  : unary_operator?
    [ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
      TIME S* | FREQ S* ]
  | STRING S* | IDENT S* | URI S* | hexcolor | function | VARIABLE S*
  ;</pre>

  <p class=issue>This should instead amend the "nested_statement" production
   in the Conditionals spec. Should I do so and reference Conditionals, or
   are we going to move that to a Syntax module?

  <p class=issue>Should variables be usable elsewhere, like in the value of a
   MQ?

  <h2 id=conformance><span class=secno>6. </span> Conformance</h2>

  <h3 id=conventions><span class=secno>6.1. </span> Document Conventions</h3>

  <p>Conformance requirements are expressed with a combination of descriptive
   assertions and RFC 2119 terminology. The key words “MUST”, “MUST
   NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
   “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
   normative parts of this document are to be interpreted as described in RFC
   2119. However, for readability, these words do not appear in all uppercase
   letters in this specification.

  <p>All of the text of this specification is normative except sections
   explicitly marked as non-normative, examples, and notes. <a
   href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>

  <p>Examples in this specification are introduced with the words “for
   example” or are set apart from the normative text with
   <code>class="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.

  <h3 id=conformance-classes><span class=secno>6.2. </span> Conformance
   Classes</h3>

  <p>Conformance to CSS Variables Module is defined for three conformance
   classes:

  <dl>
   <dt><dfn id=style-sheet title="style sheet!!as conformance class">style
    sheet</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
    style sheet</a>.

   <dt><dfn id=renderer>renderer</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
    that interprets the semantics of a style sheet and renders documents that
    use them.

   <dt><dfn id=authoring-tool>authoring tool</dfn>

   <dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
    that writes a style sheet.
  </dl>

  <p>A style sheet is conformant to CSS Variables Module if all of its
   declarations that use properties defined in this module have values that
   are valid according to the generic CSS grammar and the individual grammars
   of each property as given in this module.

  <p>A renderer is conformant to CSS Variables Module if, in addition to
   interpreting the style sheet as defined by the appropriate specifications,
   it supports all the features defined by CSS Variables Module by parsing
   them correctly and rendering the document accordingly. However, the
   inability of a UA to correctly render a document due to limitations of the
   device does not make the UA non-conformant. (For example, a UA is not
   required to render color on a monochrome monitor.)

  <p>An authoring tool is conformant to CSS Variables Module if it writes
   style sheets that are syntactically correct according to the generic CSS
   grammar and the individual grammars of each feature in this module, and
   meet all other conformance requirements of style sheets as described in
   this module.

  <h3 id=partial><span class=secno>6.3. </span> Partial Implementations</h3>

  <p>So that 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.

  <h3 id=experimental><span class=secno>6.4. </span> Experimental
   Implementations</h3>

  <p>To avoid clashes with future CSS features, the CSS2.1 specification
   reserves a <a
   href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
   syntax</a> for proprietary and experimental extensions to CSS.

  <p>Prior to a specification reaching the Candidate Recommendation stage in
   the W3C process, all implementations of a CSS feature are considered
   experimental. The CSS Working Group recommends that implementations use a
   vendor-prefixed syntax for such features, including those in W3C Working
   Drafts. This avoids incompatibilities with future changes in the draft.

  <h3 id=testing><span class=secno>6.5. </span>Non-Experimental
   Implementations</h3>

  <p>Once a specification reaches the Candidate Recommendation stage,
   non-experimental implementations are possible, and implementors should
   release an unprefixed implementation of any CR-level feature they can
   demonstrate to be correctly implemented according to spec.

  <p>To establish and maintain the interoperability of CSS across
   implementations, the CSS Working Group requests that non-experimental CSS
   renderers submit an implementation report (and, if necessary, the
   testcases used for that implementation report) to the W3C before releasing
   an unprefixed implementation of any CSS features. Testcases submitted to
   W3C are subject to review and correction by the CSS Working Group.

  <p>Further information on submitting testcases and implementation reports
   can be found from on the CSS Working Group's website at <a
   href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
   Questions should be directed to the <a
   href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
   mailing list.

  <h3 id=cr-exit-criteria><span class=secno>6.6. </span> CR Exit Criteria</h3>

  <p class=issue>[Change or remove the following CR exit criteria if the spec
   is not a module, but, e.g., a Note or a profile. This text was <a
   href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on
   2008-06-04.</a>]

  <p> 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=acknowledgments>Acknowledgments</h2>

  <p><var class=replaceme>[acknowledgments]</var>

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

  <h3 class=no-num id=normative-references>Normative references</h3>
  <!--begin-normative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS21>[CSS21]

   <dd>Bert Bos; et al. <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
    Sheets Level 2 Revision 1 (CSS&#160;2.1) Specification.</cite></a> 7 June
    2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607">http://www.w3.org/TR/2011/REC-CSS2-20110607</a>
    </dd>
   <!---->

   <dt id=RFC2119>[RFC2119]

   <dd>S. Bradner. <a href="http://www.ietf.org/rfc/rfc2119.txt"><cite>Key
    words for use in RFCs to Indicate Requirement Levels.</cite></a> Internet
    RFC 2119. URL: <a
    href="http://www.ietf.org/rfc/rfc2119.txt">http://www.ietf.org/rfc/rfc2119.txt</a>
    </dd>
   <!---->
  </dl>
  <!--end-normative-->

  <h3 class=no-num id=other-references>Other references</h3>
  <!--begin-informative-->
  <!-- Sorted by label -->

  <dl class=bibliography>
   <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
    <!---->

   <dt id=CSS1>[CSS1]

   <dd>H&#229;kon Wium Lie; Bert Bos. <a
    href="http://www.w3.org/TR/2008/REC-CSS1-20080411"><cite>Cascading Style
    Sheets (CSS1) Level 1 Specification.</cite></a> 11 April 2008. W3C
    Recommendation. URL: <a
    href="http://www.w3.org/TR/2008/REC-CSS1-20080411">http://www.w3.org/TR/2008/REC-CSS1-20080411</a>
    </dd>
   <!---->

   <dt id=CSS3COLOR>[CSS3COLOR]

   <dd>Tantek &#199;elik; Chris Lilley; L. David Baron. <a
    href="http://www.w3.org/TR/2011/REC-css3-color-20110607"><cite>CSS Color
    Module Level 3.</cite></a> 7 June 2011. W3C Recommendation. URL: <a
    href="http://www.w3.org/TR/2011/REC-css3-color-20110607">http://www.w3.org/TR/2011/REC-css3-color-20110607</a>
    </dd>
   <!---->
  </dl>
  <!--end-informative-->

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

  <ul class=indexlist>
   <li>active, <a href="#active-var-rule"
    title=active><strong>2.</strong></a>

   <li>authoring tool, <a href="#authoring-tool" title="authoring
    tool"><strong>6.2.</strong></a>

   <li>compound variable, <a href="#compound-variable" title="compound
    variable"><strong>2.1.</strong></a>

   <li>compound variables, <a href="#compound-variable" title="compound
    variables"><strong>2.1.</strong></a>

   <li>CSSFOO, <a href="#cssfoo" title=CSSFOO><strong>2.1.</strong></a>

   <li>inactive, <a href="#inactive-var-rule"
    title=inactive><strong>2.</strong></a>

   <li>invalid variable, <a href="#invalid-variable" title="invalid
    variable"><strong>3.1.</strong></a>

   <li>invalid variables, <a href="#invalid-variable" title="invalid
    variables"><strong>3.1.</strong></a>

   <li>invalid variable use, <a href="#invalid-variable-use" title="invalid
    variable use"><strong>3.1.</strong></a>

   <li>list variable, <a href="#list-variable" title="list
    variable"><strong>2.1.</strong></a>

   <li>list variables, <a href="#list-variable" title="list
    variables"><strong>2.1.</strong></a>

   <li>renderer, <a href="#renderer" title=renderer><strong>6.2.</strong></a>
    

   <li>singular variable, <a href="#singular-variable" title="singular
    variable"><strong>2.1.</strong></a>

   <li>singular variables, <a href="#singular-variable" title="singular
    variables"><strong>2.1.</strong></a>

   <li>style sheet
    <ul>
     <li>as conformance class, <a href="#style-sheet" title="style sheet, as
      conformance class"><strong>6.2.</strong></a>
    </ul>
  </ul>
  <!--end-index-->

  <h2 class=no-num id=property-index>Property index</h2>
  <!--begin-properties-->

  <table class=proptable>
   <thead>
    <tr>
     <th>Property

     <th>Values

     <th>Initial

     <th>Applies&nbsp;to

     <th>Inh.

     <th>Percentages

     <th>Media

   <tbody>
  </table>
  <!--end-properties-->
</html>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-declaration:"~/SGML/HTML4.decl"
sgml-default-doctype-name:"html"
sgml-minimize-attributes:t
sgml-nofill-elements:("pre" "style" "br")
sgml-live-element-indicator:t
sgml-omittag:nil
sgml-shorttag:nil
sgml-namecase-general:t
sgml-general-insert-case:lower
sgml-always-quote-attributes:t
sgml-indent-step:nil
sgml-indent-data:t
sgml-parent-document:nil
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
-->

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<head profile="http://www.w3.org/2006/03/hcard">
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	<title>CSS Variables Module Level 1</title>
	<link rel="stylesheet" type="text/css" href="http://dev.w3.org/csswg/css-module/default.css">
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<div class="head">
<!--logo-->

<h1>CSS Variables Module Level 1</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
	<dt>This version:
		<dd><a href="[VERSION]">http://dev.w3.org/csswg/css-variables/</a>

	<!--<dt>Latest version:
		<dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/[SHORTNAME]/</a>-->

	<dt>Editor's draft:
		<dd><a href="http://dev.w3.org/csswg/css-variables/">http://dev.w3.org/csswg/css-variables/</var></a>

	<dt>Editors:
		<dd class=vcard>
			<a href="http://xanthir.com/contact/" rel=url>
				<span class=fn>Tab Atkins Jr.</span>,
			</a>
			<span class=org>Google, Inc.</span>
</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 level&nbsp;3 relating to <var class="replaceme">DESCRIBE HERE</var>.
	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 <var class="replaceme">SUMMARIZE HERE</var>.

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

<!--status-->

<p>The following features are at risk: &hellip;

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

<!--toc-->

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

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

	<p>Large documents or applications (and even small ones) can contain quite a bit of CSS.  Many of the values in the CSS file will be duplicate data; for example, a site may establish a color scheme and reuse three or four colors throughout the site.  Altering this data can be difficult and error-prone, since it's scattered throughout the CSS file (and possibly across multiple files), and may not be amenable to Find-and-Replace.  This module introduces <b>Variables</b>, which allow a value to be assigned to a name, which may then be used in place of the value elsewhere in the document.  This makes it easier to read large files, as seemingly-arbitrary values now have informative names, and makes editting such files much easier and less error-prone, as one only has to change the value once, at the variable definition site, and the change will propagate to all uses of that variable automatically.</p>

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

	<p class="replaceme">Explain, normatively, how this module affects the definition of CSS.</p>

	<p>This module replaces and extends the <var class="replaceme">SUMMARIZE HERE</var>
	features defined in [[!CSS21]] sections <var class="replaceme">W.X and Y.Z</var>.

<h3 id="values">
Values</h3>

	<p>This specification follows the
	<a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
	definition conventions</a> from [[!CSS21]]. Value types not defined in
	this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
	Other CSS modules may expand the definitions of these value types: for
	example [[CSS3COLOR]], when combined with this module, expands the
	definition of the &lt;color&gt; value type as used in this specification.</p>
	
<h2 id="defining-variables">
Defining Variables</h2>

	<p>A ''@var'' rule defines a variable and assigns a value to it.  It consists of the at-keyword ''@var'' followed by a variable name (an identifier starting with the "$" character), followed by the value and finally terminated with a semi-colon.  The result of this rule is that the given value is assigned to the given variable name.<p>

	<div class=example>
		<p>This variables declaration:</p>

		<pre>@var $header-color #06c;</pre>

		<p>declares a variable named "$header-color", and assigns to it the value "#06c".  This can then be used in any place where that value would be valid, such as:

		<pre>h1 { background-color: $header-color; }</pre>
	
		<p>The preceding rule is equivalent to writing ''background-color: #06c;'', except that the variable name makes the origin of the color clearer, and if $header-color is used elsewhere in the stylesheet, all of the uses can be updated at once by changing the variable declaration.</p>
	</div>

	<p>Defined variables are available to all stylesheets in the document.  Thus, using ''@import'' to include a stylesheet also includes all variables defined in the stylesheet (and makes all variables declared outside available within the stylesheet, if applicable).  ''@var'' rules present in disabled or alternate stylesheets do not define variables.  Scoped stylesheets are an exception to this: variables defined in the global scope are available within a scoped stylesheet, but variables defined or imported within scoped stylesheets are only available within the scoped stylesheet and any imported stylesheets.</p>

	<p>A ''@var'' rule is <dfn id='inactive-var-rule'>inactive</dfn> if it's present in a disabled or alternate stylesheet, a stylesheet with a media query that evaluates to false, or is present within a conditional rule group [[CSS3CONDITIONAL]] whose condition evaluates to false.  <i>Inactive</i> ''@var'' rules do not define variables.  All other ''@var'' rules are <dfn id='active-var-rule'>active</dfn>.</p>

	<p>If new ''@var'' rules are added to the document through any means, such as direct CSSOM manipulation or new stylesheets being added to the document, or if an <i>inactive</i> ''@var'' rule becomes <i>active</i>, they define variables.  Similarly, if ''@var'' rules are removed from the document or become <i>inactive</i>, they no longer define variables.</p>

	<p>If multiple <i>active</i> ''@var'' rules attempt to define variables with the same name, the last one defined wins; the preceding ''@var'' rules do not define a variable.  For this purpose, user-agent defined rules come before author-defined rules, which come before user-defined rules.  Within each of the preceding groups, the ordering is document order.</p>

	<div class='example'>
		<p>Earlier declarations of the same variable have no effect, even if uses of the variable come between the earlier and final attempts to define the variable:</p>

		<pre>
@var $var red;
p { color: $var; }
@var $var blue;</pre>

		<p>In this example, the color of &lt;p> elements is blue, because the second declaration wins over the first declaration in all uses.</p>
	</div>

	<p>Variables can refer to other variables in their value.  If a dependency cycle is created, all the declarations that directly contribute to the cycle define <i>invalid variables</i>.</p>

	<div class='example'>
		<p>For example, the following set of variable declarations create a dependency cycle:</p>

		<pre>
@var $foo red;
@var $bar linear-gradient(transparent, $foo);
@var $foo $bar;</pre>

		<p>The last two declarations create the cycle, and so they define ''$bar'' and ''$foo'', respectively, as <i>invalid variables</i> instead of as the values in the declarations.  If the last rule were removed, however, there would be no problem: both ''$foo'' and ''$bar'' would then be defined as the value ''red''.</p>
	</div>

	<p>Before they are defined, all variables are <i>invalid variables</i>.</p>

<h3 id='variable-classes'>
Variable Classes</h3>

	<p>Variables fall into a few different classes which affect where and how they can be used, depending on the value they're defined as.</p>

	<dl>
		<dt><dfn title="singular variable|singular variables">singular variable</dfn></dt>
		<dd>A variable whose value is a single <i>CSSFOO</i>, such as a keyword (like ''red''), a dimension (like ''20px''), or a function (like ''linear-gradient(red,blue)'').</dd>

		<dt><dfn title="compound variable|compound variables">compound variable</dfn></dt>
		<dd>A variable whose value is a space-separated list of <i>CSSFOO</i>s, like ''20px 30px black''.</dd>

		<dt><dfn title="list variable|list variables">list variable</dfn></dt>
		<dd>A variable whose value is multiple <i>CSSFOO</i>s separated by spaces or other separators, such as commas or slashes, like ''red, blue, white''.</dd>
	</dl>

	<p class='issue'>What term can I use for <dfn>CSSFOO</dfn>?  It looks like it corresponds to "term" in the 2.1 grammar.</p>

<h2 id='using-variables'>
Using Variables</h2>

	<p>A variable can be used anywhere a value is expected in CSS.  Variables can not be used as property names, selectors, or anything else besides property values - doing so either produces an invalid value or, in some situations like the attribute value of an attribute selector, a valid value that nonetheless has no relation to the variable of that name.</p>

	<p>A variable is substituted for its value in the property value at computed-value time.</p>

	<p><i>Singular variables</i> can be used anywhere as a value.  They can be an entire property's value, a component value in a larger property value, an argument to a function, etc..  <i>Compound variables</i> are somewhat more restricted - they can only be used as an entire property value, a whole component of a list-valued property, or a whole function argument.  <i>List variables</i> are the most restricted - they can only be used as an entire property value, as one or more whole components of a list-valued property, or as a whole series of function arguments.</p>

	<div class='example'>
		<p>Here are some examples of valid and invalid use of a <i>compound variable</i>:</p>

		<pre>
@var $compound 20px 30px;
div {
	margin: $compound; 
	/* Valid, as it's used as a whole property value */
	/* Equivalent to "margin: 20px 30px;" */

	text-shadow: $compound blue; 
	/* Invalid, as it's not used as a whole property value */

	background-position: 10px 10px, $compound, 50px; 
	/* Valid, as it's used as a whole component of a list-valued property */
	/* Equivalent to "background-position: 10px 10px, 20px 30px, 50px;" */

	background-image: radial-gradient($compound, red, blue);
	/* Valid, as it's used as a whole function argument */
	/* Equivalent to "background-image: radial-gradient(20px 30px, red, blue);" */
}</pre>

		<p>Here are some examples of valid and invalid uses of a <i>list variable</i>:</p>

		<pre>
@var $list 20px, 30px 40px;
div {
	background-position: $list;
	/* Valid, as it's used as a whole property value */
	/* Equivalent to "background-position: 20px, 30px" */

	background-position: 10px $list;
	/* Invalid, as it's not used as a whole property value or component of a list. */
	/* NOT equivalent to "background-position: 10px 20px, 30px 40px;" */

	background-position: 10px, $list, 50px;
	/* Valid, as it's used as a whole sequence of components of a list. */
	/* Equivalent to "background-position: 10px, 20px, 30px 40px, 50px;" */

	background-image: radial-gradient($list, red, blue);
	/* Valid, as it's used as a whole sequence of function arguments. */
	/* Equivalent to "background-image: radial-gradient(20px, 30px 40px, red, blue);" */

	background-image: radial-gradient(10px $list, red, blue);
	/* Invalid, as it's not used as a whole sequence of function arguments */
}</pre>
	</div>

	<p>Using a <i>compound variable</i> or <i>list variable</i> in an invalid way is an <i>invalid variable use</i>.</p>

	<p>Similarly, if a property containing a variable in its property value ends up being invalid when the variable's value is substituted in, that is an <i>invalid variable use</i>.</p>

	<div class='example'>
		<p>For example, the following usage is fine from a syntax standpoint, but results in nonsense when the variable is substituted in:</p>

		<pre>
@var $looks-valid 20px;
p { background-color: $looks-valid; }</pre>

		<p>Since ''20px'' is an invalid value for 'background-color', this instance of the property computes to 'transparent' (the initial value for 'background-color') instead.</p>
	</div>

<h3 id='using-invalid-variables'>
Using Invalid Variables</h3>

	<p>An <dfn title="invalid variable|invalid variables">invalid variable</dfn> results from having variables directly or indirectly refer to themselves.  Using an <i>invalid variable</i> results in an <i>invalid variable use</i>.</p>

	<p>An <dfn>invalid variable use</dfn> results from either using an <i>invalid variable</i>, or using a <i>compound variable</i> or <i>list variable</i> in an invalid way.  When this happens, the property the variable is used in must compute to the property's initial value.</p>

	<div class='example'>
		<p>For example, in the following code:</p>

		<pre>
@var $invalid $invalid; /* Invalid, due to the trivial cycle. */
p { background-color: red; }
p { background-color: $invalid; }</pre>

		<p>the &lt;p> elements will have transparent backgrounds (the initial value for 'background-color'), rather than red text.</p>
	</div>

	<p class='note'>The <i>invalid variable use</i> concept exists because variables can't "fail early" like other syntax errors can, so by the time the user agent realizes a variable is invalid, it's already thrown away the other cascaded values.</p>

<h2 id='cssom'>
APIs</h2>

	<p class='issue'>Define the "basic" CSSOM APIs from <a href="http://disruptive-innovations.com/zoo/cssvariables/#mozTocId847334">Daniel's and Hyatt's spec</a>.</p>

	<p class='issue'>Define the more convenient CSSOM API from <a href="http://www.xanthir.com/blog/b4AD0">my blog post</a>.</p>


<h2 id='grammar'>
The Grammar of Variables</h2>

	<p class='issue'>I'm not sure if I've done this section correctly.  For now, I'll try my best to copypasta what Conditionals is doing, since dbaron usually know what's what.</p>

	<p>This specification extends the lexical scanner in the <a href="http://www.w3.org/TR/CSS21/grammar.html">Grammar of CSS 2.1</a> (<a href="#CSS21" rel="biblioentry">[CSS21]<!--{{!CSS21}}--></a>, Appendix G) by adding:</p>

	<pre>
@{V}{A}{R}    {return VARIABLE_SYM;}
"$"name       {return VARIABLE;}</pre>

	<p>and the grammar by adding:</p>

	<pre>
variable_declaration
  : VARIABLE_SYM S+ variable_name S+ expr ':';
  ;</pre>

	<p>and by amending:</p>

	<pre>
stylesheet
  : [ CHARSET_SYM STRING ';' ]?
    [S|CDO|CDC]* [ import [ CDO S* | CDC S* ]* ]*
    [ [ ruleset | media | page | variable_declaration ] [ CDO S* | CDC S* ]* ]*
  ;
media
  : MEDIA_SYM S* media_list '{' S* [ ruleset | variable_declaration ]* '}' S*
  ;
term
  : unary_operator?
    [ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
      TIME S* | FREQ S* ]
  | STRING S* | IDENT S* | URI S* | hexcolor | function | VARIABLE S*
  ;</pre>

	<p class='issue'>This should instead amend the "nested_statement" production in the Conditionals spec.  Should I do so and reference Conditionals, or are we going to move that to a Syntax module?</p>

	<p class='issue'>Should variables be usable elsewhere, like in the value of a MQ?</p>

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

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

	<p>Conformance to CSS Variables Module
	is defined for three conformance classes:
	<dl>
		<dt><dfn title="style sheet!!as conformance class">style sheet</dfn>
			<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
			style sheet</a>.
		<dt><dfn>renderer</dfn></dt>
			<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
			that interprets the semantics of a style sheet and renders
			documents that use them.
		<dt><dfn id="authoring-tool">authoring tool</dfn></dt>
			<dd>A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
			that writes a style sheet.
	</dl>
	
	<p>A style sheet is conformant to CSS Variables Module
	if all of its declarations that use properties defined in this module
	have values that are valid according to the generic CSS grammar and the
	individual grammars of each property as given in this module.
	
	<p>A renderer is conformant to CSS Variables Module
	if, in addition to interpreting the style sheet as defined by the
	appropriate specifications, it supports all the features defined
	by CSS Variables Module by parsing them correctly
	and rendering the document accordingly. However, the inability of a
	UA to correctly render a document due to limitations of the device
	does not make the UA non-conformant. (For example, a UA is not
	required to render color on a monochrome monitor.)
	
	<p>An authoring tool is conformant to CSS Variables Module
	if it writes style sheets that are syntactically correct according to the
	generic CSS grammar and the individual grammars of each feature in
	this module, and meet all other conformance requirements of style sheets
	as described in this module.

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

	<p>So that 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 CSS2.1 specification
	reserves a <a href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
	syntax</a> for proprietary and experimental extensions to CSS.
	
	<p>Prior to a specification reaching the Candidate Recommendation stage
	in the W3C process, all implementations of a CSS feature are considered
	experimental. The CSS Working Group recommends that implementations
	use a vendor-prefixed syntax for such features, including those in
	W3C Working Drafts. This avoids incompatibilities with future changes
	in the draft.
	</p>
 
<h3 id="testing">Non-Experimental Implementations</h3>

	<p>Once a specification reaches the Candidate Recommendation stage,
	non-experimental implementations are possible, and implementors should
	release an unprefixed implementation of any CR-level feature they
	can demonstrate to be correctly implemented according to spec.
	
	<p>To establish and maintain the interoperability of CSS across
	implementations, the CSS Working Group requests that non-experimental
	CSS renderers submit an implementation report (and, if necessary, the
	testcases used for that implementation report) to the W3C before
	releasing an unprefixed implementation of any CSS features. Testcases
	submitted to W3C are subject to review and correction by the CSS
	Working Group.
	
	<p>Further information on submitting testcases and implementation reports
	can be found from on the CSS Working Group's website at
	<a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
	Questions should be directed to the
	<a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a>
	mailing list.

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

	<p class=issue>[Change or remove the following CR exit criteria if
	the spec is not a module, but, e.g., a Note or a profile. This text was <a
	href="http://www.w3.org/Style/CSS/Tracker/actions/44"> decided on 2008-06-04.</a>]</p>

	<p>
	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="acknowledgments">Acknowledgments</h2>

	<p><var class="replaceme">[acknowledgments]</var></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-->


<h2 class="no-num" id="property-index">Property index</h2>
<!-- properties -->
</body>
</html>
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Received on Saturday, 24 September 2011 01:25:40 GMT

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