csswg-drafts/css-values/Overview.html at cfc2865f0304ba4983b25a2a113f3b694962d4a6 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

<html lang=en>
 <head profile="http://dublincore.org/documents/2008/08/04/dc-html/ ">
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>
  <title>CSS Values and Units Module Level 3</title>

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  <meta content=text name=dcterms.type>
  <meta content=2013-12-06 name=dcterms.date>
  <meta content="Håkon Wium Lie" name=dcterms.creator>
  <meta content="Tab Atkins" name=dcterms.creator>
  <meta content="Elika J. Etemad" name=dcterms.creator>
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   <p><a href="http://www.w3.org/"><img alt=W3C height=48
    src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->

   <h1 class=p-name>CSS Values and Units Module Level 3</h1>

   <h2 class="no-num no-toc">Editor's Draft <span class=dt-updated><span
    class=value title=20131206> 6 December 2013</span></span></h2>

   <dl>
    <dt>This version:</dt>
    <!--<dd><a class="u-url" href="http://dev.w3.org/csswg/css-values/">http://www.w3.org/TR/2013/CR-css3-values-20131206/</a></dd>-->

    <dd><a class=u-url
     href="http://dev.w3.org/csswg/css-values/">http://dev.w3.org/csswg/css-values/</a>

    <dt>Latest version:

    <dd><a
     href="http://www.w3.org/TR/css-values/">http://www.w3.org/TR/css-values/</a>

    <dt>Editor's draft:

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

    <dt>Previous version:

    <dd><a href="http://www.w3.org/TR/2013/CR-css3-values-20130730/"
     rel=previous>http://www.w3.org/TR/2013/CR-css3-values-20130730/</a>

    <dt>Issue Tracking:

    <dd><a href="http://www.w3.org/Style/CSS/Tracker/products/33"
     rel=issues>http://www.w3.org/Style/CSS/Tracker/products/33</a>

    <dt>Feedback:

    <dd><a
     href="mailto:www-style@w3.org?subject=%5Bcss3-values%5D%20feedback">www-style@w3.org</a>
     with subject line “<kbd>[css3-values] <var>… message topic
     …</var></kbd>” (<a
     href="http://lists.w3.org/Archives/Public/www-style/"
     rel=discussion>archives</a>)

    <dt>Editors:

    <dd class="p-author h-card vcard"> <a class="p-name fn u-url url"
     href="mailto:howcome@opera.com" rel=author>Håkon Wium Lie</a> (<span
     class="p-org org">Opera Software</span>)

    <dd class="p-author h-card vcard"> <a class="p-name fn u-url url"
     href="http://www.xanthir.com/contact" rel=author>Tab Atkins</a> (<span
     class="p-org org">Google</span>)

    <dd class="p-author h-card vcard"> <a class="p-name fn u-url url"
     href="http://fantasai.inkedblade.net/contact" rel=author>Elika J.
     Etemad</a> (<span class="p-org org">Mozilla</span>)
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    title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
    href="http://www.csail.mit.edu/"><abbr
    title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
    href="http://www.ercim.eu/"><abbr
    title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>, <a
    href="http://ev.buaa.edu.cn/">Beihang</a>), All Rights Reserved. W3C <a
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   <hr title="Separator for header">
  </div>

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

  <p><span class=p-summary>This CSS3 module describes the common values and
   units that CSS properties accept and the syntax used for describing them
   in CSS property definitions.</span>

  <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-values%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
   “css-values” in the subject, preferably like this:
   “[<!---->css-values<!---->] <em>…summary of comment…</em>”

  <p>This document was produced by the <a
   href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
   the <a href="http://www.w3.org/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
   2004 W3C Patent Policy</a>. W3C maintains a <a
   href="http://www.w3.org/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="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
   6 of the W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>All features described in this specification that also exist in CSS 2.1
   <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> are
   intended to be backwards compatible. If you notice a conflict between this
   draft and CSS 2.1 <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, please inform the editors!

  <p>The CR period will last at least until 4 May 2013. At the time of
   publication, no test suite and implementation report have yet been made.
   They will be made available from the <a href="/Style/CSS/Test/">CSS test
   suites page.</a> See the section <a href="#cr-exit-criteria">“CR exit
   criteria”</a> for details.

  <p>See the section <a href="#changes">“Changes”</a> for changes made to
   this specification since the last Candidate Recommendation.

  <p>The following features are at-risk and may be dropped during the CR
   period: ‘<a href="#calc"><code class=css>calc()</code></a>’, ‘<a
   href="#toggle"><code class=css>toggle()</code></a>’, ‘<a
   href="#attr"><code class=css>attr()</code></a>’.

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

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

   <li><a href="#value-defs"><span class=secno>2. </span> Value Definition
    Syntax</a>
    <ul class=toc>
     <li><a href="#component-types"><span class=secno>2.1. </span> Component
      value types</a>

     <li><a href="#component-combinators"><span class=secno>2.2. </span>
      Component value combinators</a>

     <li><a href="#component-multipliers"><span class=secno>2.3. </span>
      Component value multipliers</a>

     <li><a href="#component-whitespace"><span class=secno>2.4. </span>
      Component values and white space</a>

     <li><a href="#value-examples"><span class=secno>2.5. </span> Property
      value examples</a>
    </ul>

   <li><a href="#textual-values"><span class=secno>3. </span> Textual Data
    Types</a>
    <ul class=toc>
     <li><a href="#keywords"><span class=secno>3.1. </span> Pre-defined
      Keywords</a>
      <ul class=toc>
       <li><a href="#common-keywords"><span class=secno>3.1.1. </span>
        CSS-wide keywords: ‘<code class=css>initial</code>’ and ‘<code
        class=css>inherit</code>’</a>
      </ul>

     <li><a href="#custom-idents"><span class=secno>3.2. </span>
      Author-defined Identifiers: the ‘<code
      class=css>&lt;custom-ident></code>’ type</a>

     <li><a href="#strings"><span class=secno>3.3. </span> Quoted Strings:
      the ‘<code class=css>&lt;string></code>’ type</a>

     <li><a href="#urls"><span class=secno>3.4. </span> Resource Locators:
      the ‘<code class=css>&lt;url></code>’ type</a>
    </ul>

   <li><a href="#numeric-types"><span class=secno>4. </span> Numeric Data
    Types</a>
    <ul class=toc>
     <li><a href="#integers"><span class=secno>4.1. </span> Integers: the
      ‘<code class=css>&lt;integer></code>’ type</a>

     <li><a href="#numbers"><span class=secno>4.2. </span> Numbers: the
      ‘<code class=css>&lt;number></code>’ type</a>

     <li><a href="#percentages"><span class=secno>4.3. </span> Percentages:
      the ‘<code class=css>&lt;percentage></code>’ type</a>

     <li><a href="#dimensions"><span class=secno>4.4. </span> Numbers with
      Units: the ‘<code class=css>&lt;dimension></code>’ type</a>
    </ul>

   <li><a href="#lengths"><span class=secno>5. </span> Distance Units: the
    ‘<code class=css>&lt;length></code>’ type</a>
    <ul class=toc>
     <li><a href="#relative-lengths"><span class=secno>5.1. </span> Relative
      lengths</a>
      <ul class=toc>
       <li><a href="#font-relative-lengths"><span class=secno>5.1.1. </span>
        Font-relative lengths: the ‘<code class=css>em</code>’, ‘<code
        class=css>ex</code>’, ‘<code class=css>ch</code>’, ‘<code
        class=css>rem</code>’ units</a>

       <li><a href="#viewport-relative-lengths"><span class=secno>5.1.2.
        </span> Viewport-percentage lengths: the ‘<code
        class=css>vw</code>’, ‘<code class=css>vh</code>’, ‘<code
        class=css>vmin</code>’, ‘<code class=css>vmax</code>’ units</a>
      </ul>

     <li><a href="#absolute-lengths"><span class=secno>5.2. </span> Absolute
      lengths: the ‘<code class=css>cm</code>’, ‘<code
      class=css>mm</code>’, ‘<code class=css>in</code>’, ‘<code
      class=css>pt</code>’, ‘<code class=css>pc</code>’, ‘<code
      class=css>px</code>’ units</a>
    </ul>

   <li><a href="#other-units"><span class=secno>6. </span> Other Units</a>
    <ul class=toc>
     <li><a href="#angles"><span class=secno>6.1. </span> Angles: the
      ‘<code class=css>&lt;angle></code>’ type and ‘<code
      class=css>deg</code>’, ‘<code class=css>grad</code>’, ‘<code
      class=css>rad</code>’, ‘<code class=css>turn</code>’ units</a>

     <li><a href="#time"><span class=secno>6.2. </span> Times: the ‘<code
      class=css>&lt;time></code>’ type and ‘<code class=css>s</code>’,
      ‘<code class=css>ms</code>’ units</a>

     <li><a href="#frequency"><span class=secno>6.3. </span> Frequencies: the
      ‘<code class=css>&lt;frequency></code>’ type and ‘<code
      class=css>Hz</code>’, ‘<code class=css>kHz</code>’ units</a>

     <li><a href="#resolution"><span class=secno>6.4. </span> Resolutions:
      the <var>&lt;resolution></var> type and ‘<code
      class=css>dpi</code>’, ‘<code class=css>dpcm</code>’, ‘<code
      class=css>dppx</code>’ units</a>
    </ul>

   <li><a href="#defined-elsewhere"><span class=secno>7. </span> Data Types
    Defined Elsewhere</a>
    <ul class=toc>
     <li><a href="#colors"><span class=secno>7.1. </span> Colors: the
      ‘<code class=css>&lt;color></code>’ type</a>

     <li><a href="#images"><span class=secno>7.2. </span> Images: the
      ‘<code class=css>&lt;image></code>’ type</a>

     <li><a href="#position"><span class=secno>7.3. </span> 2D Positioning:
      the ‘<code class=css>&lt;position></code>’ type</a>
    </ul>

   <li><a href="#functional-notation"><span class=secno>8. </span> Functional
    Notations</a>
    <ul class=toc>
     <li><a href="#calc-notation"><span class=secno>8.1. </span> Mathematical
      Expressions: ‘<code class=css>calc()</code>’</a>
      <ul class=toc>
       <li><a href="#calc-syntax"><span class=secno>8.1.1. </span> Syntax</a>

       <li><a href="#calc-type-checking"><span class=secno>8.1.2. </span>
        Type Checking</a>

       <li><a href="#calc-computed-value"><span class=secno>8.1.3. </span>
        Computed Value</a>

       <li><a href="#calc-range"><span class=secno>8.1.4. </span> Range
        Checking</a>
      </ul>

     <li><a href="#toggle-notation"><span class=secno>8.2. </span> Toggling
      Between Values: ‘<code class=css>toggle()</code>’</a>

     <li><a href="#attr-notation"><span class=secno>8.3. </span> Attribute
      References: ‘<code class=css>attr()</code>’</a>
    </ul>

   <li><a href="#iana"><span class=secno>9. </span> Appendix A: IANA
    Considerations</a>
    <ul class=toc>
     <li><a href="#about-invalid"><span class=secno>9.1. </span> Registration
      for the <code>about:invalid</code> URL scheme</a>
    </ul>

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

   <li class=no-num><a href="#conformance"> Conformance</a>
    <ul class=toc>
     <li class=no-num><a href="#conventions"> Document conventions</a>

     <li class=no-num><a href="#conformance-classes"> Conformance classes</a>

     <li class=no-num><a href="#partial"> Partial implementations</a>

     <li class=no-num><a href="#experimental"> Experimental
      implementations</a>

     <li class=no-num><a href="#testing"> Non-experimental
      implementations</a>

     <li class=no-num><a href="#cr-exit-criteria"> CR exit criteria</a>
    </ul>

   <li class=no-num><a href="#changes">Changes</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>
  </ul>
  <!--end-toc-->

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

  <p>The value definition field of each CSS property can contain keywords,
   data types (which appear between ‘<code class=css>&lt;</code>’ and
   ‘<code class=css>></code>’), and information on how they can be
   combined. Generic data types (<a
   href="#length-value"><var>&lt;length></var></a> being the most widely
   used) that can be used by many properties are described in this
   specification, while more specific data types (e.g.,
   <var>&lt;spacing-limit></var>) are described in the corresponding modules.

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

   href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections <a
   href="http://www.w3.org/TR/CSS21/about.html#value-defs">1.4.2.1</a>, <a
   href="http://www.w3.org/TR/CSS21/syndata.html#values">4.3</a>, and <a
   href="http://www.w3.org/TR/CSS21/aural.html#aural-intro">A.2</a>.

  <h2 id=value-defs><span class=secno>2. </span> Value Definition Syntax</h2>

  <p>The syntax described here is used to define the set of valid values for
   CSS properties. A property value can have one or more components.

  <h3 id=component-types><span class=secno>2.1. </span> Component value types</h3>

  <p>Component value types are designated in several ways:

  <ol>
   <li><a href="#keywords">keyword</a> values (such as ‘<code
    class=css>auto</code>’, ‘<code class=css>disc</code>’, etc.), which
    appear literally, without quotes (e.g. <code>auto</code>)

   <li>basic data types, which appear between ‘<code
    class=css>&lt;</code>’ and ‘<code class=css>></code>’ (e.g., <a
    href="#length-value"><var>&lt;length></var></a>, <a
    href="#percentage-value"><var>&lt;percentage></var></a>, etc.).

   <li>types that have the same range of values as a property bearing the
    same name (e.g., <var>&lt;‘<code
    class=property>border-width</code>’></var> <var>&lt;‘<code
    class=property>background-attachment</code>’></var>, etc.). In this
    case, the type name is the property name (complete with quotes) between
    the brackets. Such a type does <em>not</em> include <a
    href="#common-keywords">CSS-wide keywords</a> such as ‘<a
    href="#inherit"><code class=css>inherit</code></a>’.

   <li>non-terminals that do not share the same name as a property. In this
    case, the non-terminal name appears between ‘<code
    class=css>&lt;</code>’ and ‘<code class=css>></code>’, as in
    <var>&lt;spacing-limit></var>. Notice the distinction between
    <var>&lt;border-width></var> and <var>&lt;‘<code
    class=property>border-width</code>’></var>: the latter is defined as
    the value of the ‘<code class=property>border-width</code>’ property,
    the former requires an explicit expansion elsewhere. The definition of a
    non-terminal is typically located near its first appearance in the
    specification.
  </ol>

  <p> Some property value definitions also include the slash (/), the comma
   (,), and/or parentheses as literals. These represent their corresponding
   tokens. Other non-keyword literal characters that may appear in a
   component value, such as “+”, must be written enclosed in single
   quotes.

  <p> All CSS properties also accept the <a href="#common-keywords">CSS-wide
   keyword values</a> as the sole component of their property value. For
   readability these are not listed explicitly in the property value syntax
   definitions. For example, the full value definition of <a
   href="http://www.w3.org/TR/css3-background/#border-color">‘<code
   class=property>border-color</code>’</a> is ‘<code
   class=css>&lt;color>{1,4} | inherit | initial | unset</code>’ (even
   though it is listed as ‘<code class=css>&lt;color>{1,4}</code>’).

  <p class=note> Note: This implies that, in general, combining these
   keywords with other component values in the same declaration results in an
   invalid declaration. For example, ‘<code class=css>background:
   url(corner.png) no-repeat, inherit;</code>’ is invalid.

  <h3 id=component-combinators><span class=secno>2.2. </span> Component value
   combinators</h3>

  <p>Component values can be arranged into property values as follows:

  <ul>
   <li>Several juxtaposed words mean that all of them must occur, in the
    given order.

   <li>A double ampersand (&&) separates two or more components, all of which
    must occur, in any order.

   <li>A double bar (||) separates two or more options: one or more of them
    must occur, in any order.

   <li>A bar (|) separates two or more alternatives: exactly one of them must
    occur.

   <li>Brackets ([ ]) are for grouping.
  </ul>

  <p>Juxtaposition is stronger than the double ampersand, the double
   ampersand is stronger than the double bar, and the double bar is stronger
   than the bar. Thus, the following lines are equivalent:

  <pre>
<!---->  a b   |   c ||   d &&   e f
<!---->[ a b ] | [ c || [ d && [ e f ]]]</pre>

  <h3 id=component-multipliers><span class=secno>2.3. </span> Component value
   multipliers</h3>

  <p>Every type, keyword, or bracketed group may be followed by one of the
   following modifiers:

  <ul>
   <li>An asterisk (*) indicates that the preceding type, word, or group
    occurs zero or more times.

   <li>A plus (+) indicates that the preceding type, word, or group occurs
    one or more times.

   <li>A question mark (?) indicates that the preceding type, word, or group
    is optional (occurs zero or one times).

   <li>A single number in curly braces ({<var>A</var>}) indicates that the
    preceding type, word, or group occurs <var>A</var> times.

   <li>A comma-separated pair of numbers in curly braces
    ({<var>A</var>,<var>B</var>}) indicates that the preceding type, word, or
    group occurs at least <var>A</var> and at most <var>B</var> times. The
    <var>B</var> may be omitted ({<var>A</var>,}) to indicate that there must
    be at least <var>A</var> repetitions, with no upper bound on the number
    of repetitions.

   <li>A hash mark (#) indicates that the preceding type, word, or group
    occurs one or more times, separated by comma tokens. It may optionally be
    followed by the curly brace forms, above, to indicate precisely how many
    times the repetition occurs, like ‘<code
    class=css>&lt;length>#{1,4}</code>’.
  </ul>

  <p>For repeated component values (indicated by ‘<code
   class=css>*</code>’, ‘<code class=css>+</code>’, or ‘<code
   class=css>#</code>’), UAs must support at least 20 repetitions of the
   component. If a property value contains more than the supported number of
   repetitions, the declaration must be ignored as if it were invalid.

  <h3 id=component-whitespace><span class=secno>2.4. </span> Component values
   and white space</h3>

  <p>Unless otherwise specified, white space and/or comments may appear
   before, after, and/or between components in a property value or
   subcomponents in a <a href="#functional-notation">functional notation</a>
   that is defined using these component <a
   href="#component-combinators">combinators</a> and <a
   href="#component-multipliers">multipliers</a>.

  <p class=note>Note: In many cases, spaces will in fact be <em>required</em>
   between components in order to distinguish them from each other. For
   example, the value ‘<code class=css>1em2em</code>’ would be parsed as
   a single <code>DIMEN</code> token with the number ‘<code
   class=css>1</code>’ and the identifier ‘<code
   class=css>em2em</code>’, which is an invalid unit. In this case, a space
   would be required before the ‘<code class=css>2</code>’ to get this
   parsed as the two lengths ‘<code class=css>1em</code>’ and ‘<code
   class=css>2em</code>’.

  <h3 id=value-examples><span class=secno>2.5. </span> Property value
   examples</h3>

  <p>Below are some examples of properties with their corresponding value
   definition fields

  <div class=example>
   <table class=data id=propvalues>
    <thead>
     <tr>
      <th>Property

      <th>Value definition field

      <th>Example value

    <tbody>
     <tr>
      <td>‘<code class=property>orphans</code>’

      <td>&lt;integer>

      <td>‘<code class=css>3</code>’

     <tr>
      <td>‘<code class=property>text-align</code>’

      <td>left | right | center | justify

      <td>‘<code class=css>center</code>’

     <tr>
      <td>‘<code class=property>padding-top</code>’

      <td>&lt;length> | &lt;percentage>

      <td>‘<code class=css>5%</code>’

     <tr>
      <td>‘<code class=property>outline-color</code>’

      <td>&lt;color> | invert

      <td>‘<code class=css>#fefefe</code>’

     <tr>
      <td>‘<code class=property>text-decoration</code>’

      <td>none | underline || overline || line-through || blink

      <td>‘<code class=css>overline underline</code>’

     <tr>
      <td>‘<code class=property>font-family</code>’

      <td>[ &lt;family-name> | &lt;generic-family> ]#

      <td>‘<code class=css>"Gill Sans", Futura, sans-serif</code>’

     <tr>
      <td>‘<code class=property>border-width</code>’

      <td>[ &lt;length> | thick | medium | thin ]{1,4}

      <td>‘<code class=css>2px medium 4px</code>’

     <tr>
      <td>‘<code class=property>text-shadow</code>’

      <td>[ inset? && [ &lt;length>{2,4} && &lt;color>? ] ]# | none

      <td>‘<code class=css>3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0
       0 4px inset</code>’

     <tr>
      <td>‘<code class=property>voice-pitch</code>’

      <td>
       <pre class=value>&lt;frequency> && absolute |
<!--				      -->[[x-low | low | medium | high | x-high] ||
<!--				      --> [&lt;frequency> | &lt;semitones> | &lt;percentage>]]</pre>

      <td>‘<code class=css>-2st x-low</code>’
   </table>
  </div>

  <h2 id=textual-values><span class=secno>3. </span> Textual Data Types</h2>

  <p>An <dfn id=identifier>identifier</dfn> (or <dfn
   id=ltident>&lt;ident></dfn>) is a sequence of characters conforming to the
   <code>IDENT</code> token in the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
   <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
   Identifiers cannot be quoted; otherwise they would be interpreted as a
   string.

  <h3 id=keywords><span class=secno>3.1. </span> Pre-defined Keywords</h3>

  <p>In the value definition fields, keywords with a pre-defined meaning
   appear literally. Keywords are CSS <a
   href="#identifier"><i>identifiers</i></a> and are interpreted
   case-insensitively within the ASCII range (i.e., [a-z] and [A-Z] are
   equivalent).

  <div class=example>
   <p>For example, here is the value definition for the ‘<code
    class=property>border-collapse</code>’ property:

   <pre>Value: collapse | separate</pre>

   <p>And here is an example of its use:

   <pre>table { border-collapse: separate }</pre>
  </div>

  <h4 id=common-keywords><span class=secno>3.1.1. </span> CSS-wide keywords:
   ‘<a href="#initial"><code class=css>initial</code></a>’ and ‘<a
   href="#inherit"><code class=css>inherit</code></a>’</h4>

  <p>As defined <a href="#component-types">above</a>, all properties accept
   the <dfn id=css-wide-keywords>CSS-wide keywords</dfn>, which represent
   value computations common to all CSS properties.

  <p>The <dfn id=inherit>‘<code class=css>inherit</code>’</dfn> keyword
   is <a
   href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">defined</a>
   in <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.

  <p>The <dfn id=initial>‘<code class=css>initial</code>’</dfn> keyword
   represents the specified value that is designated as the property's
   initial value. <a href="#ref-CSS3CASCADE"
   rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a>

  <p>Other CSS specifications can define additional CSS-wide keywords. For
   example, <a href="#ref-CSS3CASCADE"
   rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a> adds an ‘<code
   class=css>unset</code>’ keyword to this set.
   <!-- Make it easier to add CSS-wide keywords by defining a grammar production. -->

  <h3 id=custom-idents><span class=secno>3.2. </span> Author-defined
   Identifiers: the ‘<a href="#identifier-value"><code
   class=css>&lt;custom-ident></code></a>’ type</h3>

  <p> Some properties accept arbitrary author-defined identifiers as a
   component value. This generic data type is denoted by <dfn
   id=identifier-value><var>&lt;custom-ident></var></dfn>, and represents any
   valid CSS <a href="#identifier"><i>identifier</i></a> that does not
   otherwise appear as a pre-defined keyword in that property's value
   definition. Such identifiers are fully case-sensitive, even in the ASCII
   range (e.g. ‘<code class=css>example</code>’ and ‘<code
   class=css>EXAMPLE</code>’ are two different, unrelated user-defined
   identifiers).

  <p> The <a href="#common-keywords">CSS-wide keywords</a> are not valid <a
   href="#identifier-value"><var>&lt;custom-ident>s</var></a>. The ‘<code
   class=css>default</code>’ keyword is reserved and is also not a valid <a
   href="#identifier-value"><var>&lt;custom-ident>s</var></a>.

  <p> Reserved keywords are reserved in all ASCII case permutations.

  <h3 id=strings><span class=secno>3.3. </span> Quoted Strings: the ‘<a
   href="#string-value"><code class=css>&lt;string></code></a>’ type</h3>

  <p>Strings are denoted by <dfn id=string-value><var>&lt;string></var></dfn>
   and consist of a sequence of characters delimited by double quotes or
   single quotes. They correspond to the <code>STRING</code> token in the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
   <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>

  <div class=example>
   <p>Double quotes cannot occur inside double quotes, unless escaped (as
    <code>"\""</code> or as <code>"\22"</code>). Analogously for single
    quotes (<code>'\''</code> or <code>'\27'</code>).

   <pre>
<!-- -->content: "this is a 'string'.";
<!-- -->content: "this is a \"string\".";
<!-- -->content: 'this is a "string".';
<!-- -->content: 'this is a \'string\'.';</pre>
  </div>

  <p>It is possible to break strings over several lines, for aesthetic or
   other reasons, but in such a case the newline itself has to be escaped
   with a backslash (\). The newline is subsequently removed from the string.
   For instance, the following two selectors are exactly the same:

  <div class=example>
   <p style="display:none">Example(s):

   <pre>
<!-- -->a[title="a not s\
<!-- -->o very long title"] {/*...*/}
<!-- -->a[title="a not so very long title"] {/*...*/}</pre>
  </div>

  <p>Since a string cannot directly represent a newline, to include a newline
   in a string, use the escape "\A". (Hexadecimal A is the line feed
   character in Unicode (U+000A), but represents the generic notion of
   "newline" in CSS.)

  <h3 id=urls><span class=secno>3.4. </span> Resource Locators: the ‘<a
   href="#url-value"><code class=css>&lt;url></code></a>’ type</h3>

  <p>A <dfn id=url>URL</dfn> is a pointer to a resource and is a <a
   href="http://www.w3.org/TR/CSS21/syndata.html#uri">specially-parsed</a> <a
   href="#functional-notation">functional notation</a> denoted by <dfn
   id=url-value><var>&lt;url></var></dfn>. It corresponds to the
   <code>URI</code> token in the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
   <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>

  <div class=example>
   <p>Below is an example of a URL being used as a background image:

   <pre>body { background: url("http://www.example.com/pinkish.gif") }</pre>

   <p>The same example can be written without quotes:

   <pre>body { background: url(http://www.example.com/pinkish.gif) }</pre>
  </div>

  <p class=note>Note that in some CSS syntactic contexts (as defined by that
   context), a URL can be represented as a <a
   href="#string-value"><var>&lt;string></var></a> rather than by <a
   href="#url-value"><var>&lt;URL></var></a>. An example of this is the <a
   href="http://www.w3.org/TR/CSS21/cascade.html#at-import">‘<code
   class=css>@import</code>’ rule</a>.

  <p>Parentheses, whitespace characters, single quotes (') and double quotes
   (") appearing in a URL must be escaped with a backslash so that the
   resulting value is a valid <a href="#url"><code>URL</code></a> token, e.g.
   ‘<code class=css>url(open\(parens)</code>’, ‘<code
   class=css>url(close\)parens)</code>’. Depending on the type of URL, it
   might also be possible to write these characters as URI-escapes (e.g.
   ‘<code class=css>url(open%28parens)</code>’ or ‘<code
   class=css>url(close%29parens)</code>’) as described in <a
   href="#ref-URI" rel=biblioentry>[URI]<!--{{URI}}--></a>. Alternatively a
   URL containing such characters may be represented as a quoted <a
   href="#strings">string</a> within the ‘<code class=css>url()</code>’
   notation.

  <p>In order to create modular style sheets that are not dependent on the
   absolute location of a resource, authors should use relative URIs.
   Relative URIs (as defined in <a href="#ref-URI"
   rel=biblioentry>[URI]<!--{{!URI}}--></a>) are resolved to full URIs using
   a base URI. RFC 3986, section 3, defines the normative algorithm for
   this process. For CSS style sheets, the base URI is that of the style
   sheet, not that of the source document.

  <p>When a <a href="#url-value"><i>&lt;url></i></a> appears in the computed
   value of a property, it is resolved to an absolute URL, as described in
   the preceding paragraph. The computed value of a URI that the UA cannot
   resolve to an absolute URI is the specified value.

  <div class=example>
   <p>For example, suppose the following rule:

   <pre>body { background: url("tile.png") }</pre>

   <p>is located in a style sheet designated by the URL:

   <pre>http://www.example.org/style/basic.css</pre>

   <p>The background of the source document's <var>&lt;body></var> will be
    tiled with whatever image is described by the resource designated by the
    URL:

   <pre>http://www.example.org/style/tile.png</pre>

   <p>The same image will be used regardless of the URL of the source
    document containing the <var>&lt;body></var>.
  </div>

  <h2 id=numeric-types><span class=secno>4. </span> Numeric Data Types</h2>

  <p> Properties may restrict numeric values to some range. If the value is
   outside the allowed range, the declaration is invalid and must be <a
   href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.

  <p> CSS theoretically supports infinite precision and infinite ranges for
   all value types; however in reality implementations have finite capacity.
   UAs should support reasonably useful ranges and precisions. <!--
		The recommended minimum ranges and precision,
		and the required rounding and clamping rules,
		are given in <a href="#required-ranges">Appendix A</a>.
-->

  <h3 id=integers><span class=secno>4.1. </span> Integers: the ‘<a
   href="#integer-value"><code class=css>&lt;integer></code></a>’ type</h3>

  <p>Integer values are denoted by <dfn
   id=integer-value><var>&lt;integer></var></dfn>. An <dfn
   id=integer>integer</dfn> is one or more decimal digits ‘<code
   class=css>0</code>’ through ‘<code class=css>9</code>’ and
   corresponds to a subset of the <a href="#number"><code>NUMBER</code></a>
   token in the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
   The first digit of an integer may be immediately preceded by ‘<code
   class=css>-</code>’ or ‘<code class=css>+</code>’ to indicate the
   integer's sign.

  <h3 id=numbers><span class=secno>4.2. </span> Numbers: the ‘<a
   href="#number-value"><code class=css>&lt;number></code></a>’ type</h3>

  <p>Number values are denoted by <dfn
   id=number-value><var>&lt;number></var></dfn>. A <dfn
   id=number>number</dfn> is either an &lt;integer> or zero or more decimal
   digits followed by a dot (.) followed by one or more decimal digits. It
   corresponds to the <a href="#number"><code>NUMBER</code></a> token in the
   <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
   As with integers, the first character of a number may be immediately
   preceded by ‘<code class=css>-</code>’ or ‘<code
   class=css>+</code>’ to indicate the number's sign.

  <h3 id=percentages><span class=secno>4.3. </span> Percentages: the ‘<a
   href="#percentage-value"><code class=css>&lt;percentage></code></a>’
   type</h3>

  <p>A <dfn id=percentage>percentage</dfn> value is denoted by <dfn
   id=percentage-value><var>&lt;percentage></var></dfn>, and consists of a <a
   href="#number-value"><i>&lt;number></i></a> immediately followed by a
   percent sign ‘<code class=css>%</code>’. It corresponds to the <a
   href="#percentage"><code>PERCENTAGE</code></a> token in the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.

  <p>Percentage values are always relative to another value, for example a
   length. Each property that allows percentages also defines the value to
   which the percentage refers. The value may be that of another property for
   the same element, a 
2D6A
property for an ancestor element, or a value of the
   formatting context (e.g., the width of a <i>containing block</i>). When a
   percentage value is set for a property of the <i>root</i> element and the
   percentage is defined as referring to the inherited value of some
   property, the resultant value is the percentage times the <i>initial
   value</i> of that property.

  <h3 id=dimensions><span class=secno>4.4. </span> Numbers with Units: the
   ‘<a href="#ltdimension"><code class=css>&lt;dimension></code></a>’
   type</h3>

  <p> A <dfn data-export=data-export id=dimension>dimension</dfn> value is
   denoted by <dfn data-dfn-type=type id=ltdimension>&lt;dimension></dfn> and
   consists of a <a href="#number-value"><i>&lt;number></i></a> representing
   the value immediately followed by an <a
   href="#ltident"><i>&lt;ident></i></a> representing the unit. It
   corresponds to the <i>&lt;dimension-token</i> syntax production.

  <p> The <a href="#ltdimension"><i>&lt;dimension></i></a> type is only used
   when writing very generic grammars; in most cases, a more specific type
   based on the unit is used instead, such as <a
   href="#length-value"><i>&lt;length></i></a> or <a
   href="#time-value"><i>&lt;time></i></a>.

  <h2 id=lengths><span class=secno>5. </span> Distance Units: the ‘<a
   href="#length-value"><code class=css>&lt;length></code></a>’ type</h2>

  <p>Lengths refer to distance measurements and are denoted by <dfn
   id=length-value><var>&lt;length></var></dfn> in the property definitions.
   A length is a <a href="#dimension0"><i>dimension</i></a>. However, for
   zero lengths the unit identifier is optional (i.e. can be syntactically
   represented as the <a href="#number-value"><var>&lt;number></var></a>
   ‘<code class=css>0</code>’).

  <p>A <dfn id=dimension0>dimension</dfn> is a <a
   href="#number"><i>number</i></a> immediately followed by a unit
   identifier. It corresponds to the <a
   href="#dimension0"><code>DIMENSION</code></a> token in the <a
   href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
   <a href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> Like
   keywords, unit identifiers are case-insensitive within the ASCII range.

  <p>Properties may restrict the length value to some range. If the value is
   outside the allowed range, the declaration is invalid and must be <a
   href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.

  <p>While some properties allow negative length values, this may complicate
   the formatting and there may be implementation-specific limits. If a
   negative length value is allowed but cannot be supported, it must be
   converted to the nearest value that can be supported.

  <p>In cases where the <!--FIXME: href--><a>used</a> length cannot be
   supported, user agents must approximate it in the
   <!--FIXME: href--><a>actual</a> value.

  <p>There are two types of length units: relative and absolute.

  <h3 id=relative-lengths><span class=secno>5.1. </span> Relative lengths</h3>

  <p><dfn id=relative-length-units title="relative length">Relative length
   units</dfn> specify a length relative to another length. Style sheets that
   use relative units can more easily scale from one output environment to
   another.

  <p>The relative units are:

  <table class=data>
   <caption>Informative Summary of Relative Units</caption>

   <thead>
    <tr>
     <th>unit

     <th>relative to

   <tbody>
    <tr>
     <td>‘<a href="#em-unit"><code class=css>em</code></a>’

     <td>font size of the element

    <tr>
     <td>‘<a href="#ex-unit"><code class=css>ex</code></a>’

     <td><i>x-height</i> of the element's font

    <tr>
     <td>‘<a href="#ch-unit"><code class=css>ch</code></a>’

     <td>width of the "0" (ZERO, U+0030) glyph in the element's font

    <tr>
     <td>‘<a href="#rem-unit"><code class=css>rem</code></a>’

     <td>font size of the root element

    <tr>
     <td>‘<a href="#vw-unit"><code class=css>vw</code></a>’

     <td>1% of viewport's width

    <tr>
     <td>‘<a href="#vh-unit"><code class=css>vh</code></a>’

     <td>1% viewport's height

    <tr>
     <td>‘<a href="#vmin-unit"><code class=css>vmin</code></a>’

     <td>1% of viewport's smaller dimension

    <tr>
     <td>‘<a href="#vmax-unit"><code class=css>vmax</code></a>’

     <td>1% of viewport's larger dimension
  </table>

  <p>Child elements do not inherit the relative values as specified for their
   parent; they inherit the <!--FIXME: href--><a>computed values</a>.

  <h4 id=font-relative-lengths><span class=secno>5.1.1. </span> Font-relative
   lengths: the ‘<a href="#em-unit"><code class=css>em</code></a>’, ‘<a
   href="#ex-unit"><code class=css>ex</code></a>’, ‘<a
   href="#ch-unit"><code class=css>ch</code></a>’, ‘<a
   href="#rem-unit"><code class=css>rem</code></a>’ units</h4>

  <p>Aside from ‘<a href="#rem-unit"><code class=css>rem</code></a>’
   (which refers to the font-size of the root element), the font-relative
   lengths refer to the font metrics of the element on which they are used.
   The exception is when they occur in the value of the ‘<code
   class=property>font-size</code>’ property itself, in which case they
   refer to the computed font metrics of the parent element (or the computed
   font metrics corresponding to the initial values of the ‘<code
   class=property>font</code>’ property, if the element has no parent).