drafts: css3-values/Overview.html@324c6ef7c447 (annotated)

css3-values/Overview.html@324c6ef7c447 (annotated)

css3-values/Overview.html

Wed, 10 Aug 2011 23:45:29 +0000

author: Elika Etemad <fantasai.cvs@inkedblade.net>
date: Wed, 10 Aug 2011 23:45:29 +0000
changeset 3292: 324c6ef7c447
parent 3291: 8e1babb965e4
child 3293: c38cec6b116e
permissions: -rw-r--r--

Generic Datatype Reorganization Part V: Strings and URLs
Also, create a cleaner split between this module and syntax.

 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
 "http://www.w3.org/TR/html4/strict.dtd">
 <html lang=en>
  <head><meta content="text/html; charset=iso-8859-1" http-equiv=Content-Type>
   <title>CSS3 Values and Units</title>
   <style type="text/css">
     .issue { color: red }
     table { border-collapse: collapse; border: 1px solid black; margin: 1em 0 }
     th, td { border: 1px solid black; padding: 0.2em; text-align: left }
     tt.declaration { white-space: nowrap }
     .del { text-decoration: line-through}
     .say { font-style: italic }
     .value { font: inherit; white-space: pre-wrap; margin: 0; padding: 0; }
     #propvalues td { text-align: right; }
     #propvalues td + td { text-align: left; }
     p { text-indent: 0 !important; margin: 1em 0 !important; }
   </style>
   <link href="../default.css" rel=stylesheet type="text/css">
   <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
   type="text/css">
  <body>
   <div class=head> <!--begin-logo-->
    <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 id=css3-template>CSS3 Values and Units</h1>
    <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 10 August 2011</h2>
    <dl>
     <dt>This version:
     <dd><a
      href="http://www.w3.org/TR/2011/ED-css3-values-20110810/">http://www.w3.org/TR/2011/ED-css3-values-20110810/</a>
     <dt>Latest version:
     <dd><a
      href="http://www.w3.org/TR/css3-values">http://www.w3.org/TR/css3-values</a>
     <dt>Previous version:
     <dd><a
      href="http://www.w3.org/TR/2005/WD-css3-values-20050726">http://www.w3.org/TR/2005/WD-css3-values-20050726</a>
     <dt>Editors:
     <dd>H&aring;kon Wium Lie, Opera Software &lt;howcome@opera.com&gt;
     <dd>Tab Atkins, Google
     <dd>Elika J. Etemad, Invited Expert
    </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>This CSS3 module describes the various values and units that CSS
    properties accept. Also, it describes how values are computed from
    "specified" through "computed" and "used" into "actual" values. The main
    purpose of this module is to define common values and units in one
    specification which can be referred to by other modules. As such, it does
    not make sense to claim conformance with this module alone.
   <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=%5Bcss3-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
    &#8220;css3-values&#8221; in the subject, preferably like this:
    &#8220;[<!---->css3-values<!---->] <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>All features described in this specification that also exist in CSS 2.1
    <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> are intended
    to be backwards compatible. In case of conflict between this draft and
    CSS&nbsp;2.1 <a href="#CSS21"
    rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, CSS&nbsp;2.1 probably
    represents the intention of the CSS WG better than this draft (other than
    on values and units that are new to CSS3).
   <p>This is a draft of a <a href="http://www.w3.org/TR/css3-roadmap/">module
    of CSS level 3</a>. It will probably be bundled with some other modules
    before it becomes a <a href="http://www.w3.org/TR/#About">W3C
    Recommendation</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="#syntax-and-terminology"><span class=secno>3. </span>Syntax
     and terminology</a>
     <ul class=toc>
      <li><a href="#functional-notation"><span class=secno>3.1.
       </span>Functional notation</a>
     </ul>
    <li><a href="#textual-values"><span class=secno>4. </span> Textual Data
     Types</a>
     <ul class=toc>
      <li><a href="#keywords"><span class=secno>4.1. </span> Pre-defined
       Keywords</a>
       <ul class=toc>
        <li><a href="#initial-inherit"><span class=secno>4.1.1. </span> The
         &lsquo;<code class=css>initial</code>&rsquo; and &lsquo;<code
         class=css>inherit</code>&rsquo; keywords</a>
       </ul>
      <li><a href="#identifiers"><span class=secno>4.2. </span> User-defined
       Identifiers: the &lsquo;<code
       class=css>&lt;identifier&gt;</code>&rsquo; type</a>
      <li><a href="#strings"><span class=secno>4.3. </span> Quoted Strings:
       the &lsquo;<code class=css>&lt;string&gt;</code>&rsquo; type</a>
      <li><a href="#urls"><span class=secno>4.4. </span> Resource Locators:
       the &lsquo;<code class=css>&lt;url&gt;</code>&rsquo; type</a>
     </ul>
    <li><a href="#numeric-types"><span class=secno>5. </span> Numeric Data
     Types</a>
     <ul class=toc>
      <li><a href="#integers"><span class=secno>5.1. </span> Integers: the
       &lsquo;<code class=css>&lt;integer&gt;</code>&rsquo; type</a>
      <li><a href="#numbers"><span class=secno>5.2. </span> Numbers: the
       &lsquo;<code class=css>&lt;number&gt;</code>&rsquo; type</a>
      <li><a href="#percentages"><span class=secno>5.3. </span> Percentages:
       the &lsquo;<code class=css>&lt;percentage&gt;</code>&rsquo; type</a>
     </ul>
    <li><a href="#lengths"><span class=secno>6. </span> Distance Units: the
     &lsquo;<code class=css>&lt;length&gt;</code>&rsquo; type</a>
     <ul class=toc>
      <li><a href="#relative-lengths"><span class=secno>6.1. </span> Relative
       lengths</a>
       <ul class=toc>
        <li><a href="#font-relative-lengths"><span class=secno>6.1.1. </span>
         Font-relative lengths: the &lsquo;<code class=css>em</code>&rsquo;,
         &lsquo;<code class=css>ex</code>&rsquo;, &lsquo;<code
         class=css>ch</code>&rsquo;, &lsquo;<code class=css>rem</code>&rsquo;
         units</a>
        <li><a href="#viewport-relative-lengths"><span class=secno>6.1.2.
         </span> Viewport-relative lengths: the &lsquo;<code
         class=css>vw</code>&rsquo;, &lsquo;<code class=css>vh</code>&rsquo;,
         &lsquo;<code class=css>vm</code>&rsquo; units</a>
       </ul>
      <li><a href="#absolute-lengths"><span class=secno>6.2. </span> Absolute
       lengths: the &lsquo;<code class=css>cm</code>&rsquo;, &lsquo;<code
       class=css>mm</code>&rsquo;, &lsquo;<code class=css>in</code>&rsquo;,
       &lsquo;<code class=css>pt</code>&rsquo;, &lsquo;<code
       class=css>pc</code>&rsquo;, &lsquo;<code class=css>px</code>&rsquo;
       units </a>
     </ul>
    <li><a href="#other-units"><span class=secno>7. </span> Other Units</a>
     <ul class=toc>
      <li><a href="#angles"><span class=secno>7.1. </span> Angles: the
       &lsquo;<code class=css>&lt;angle&gt;</code>&rsquo; type and
       &lsquo;<code class=css>deg</code>&rsquo;, &lsquo;<code
       class=css>grad</code>&rsquo;, &lsquo;<code class=css>rad</code>&rsquo;,
       &lsquo;<code class=css>turn</code>&rsquo; units</a>
      <li><a href="#time"><span class=secno>7.2. </span> Times: the
       &lsquo;<code class=css>&lt;time&gt;</code>&rsquo; type and &lsquo;<code
       class=css>s</code>&rsquo;, &lsquo;<code class=css>ms</code>&rsquo;
       units</a>
      <li><a href="#frequencies-the-ltfrequencygt-type-and-h"><span
       class=secno>7.3. </span>Frequencies: the &lsquo;<code
       class=css>&lt;frequency&gt;</code>&rsquo; type and &lsquo;<code
       class=css>Hz</code>&rsquo;, &lsquo;<code class=css>kHz</code>&rsquo;
       units</a>
       <ul class=toc>
        <li><a href="#the-calc-min-and-max-functions"><span class=secno>7.3.1.
         </span>The &lsquo;<code class=property>calc</code>&rsquo;,
         &lsquo;<code class=property>min</code>&rsquo; and &lsquo;<code
         class=property>max</code>&rsquo; functions</a>
       </ul>
      <li><a href="#ltcolorgt"><span class=secno>7.4. </span>&lt;color&gt;</a>
      <li><a href="#ltattrgt"><span class=secno>7.5. </span>&lt;attr&gt;</a>
      <li><a href="#ltimagegt"><span class=secno>7.6. </span>&lt;image&gt;</a>
      <li><a href="#ltfractiongt"><span class=secno>7.7.
       </span>&lt;fraction&gt;</a>
       <ul class=toc>
        <li><a href="#the-fr-unit"><span class=secno>7.7.1. </span>The
         &lsquo;<code class=property>fr</code>&rsquo; unit</a>
       </ul>
      <li><a href="#ltgridgt"><span class=secno>7.8. </span>&lt;grid&gt;</a>
       <ul class=toc>
        <li><a href="#the-gr-unit"><span class=secno>7.8.1. </span>The
         &lsquo;<code class=property>gr</code>&rsquo; unit</a>
       </ul>
     </ul>
    <li><a href="#specified-computed-used-and-actual-value"><span
     class=secno>8. </span>Specified, computed, used, and actual values</a>
     <ul class=toc>
      <li><a href="#finding-the-specified-value"><span class=secno>8.1.
       </span>Finding the specified value</a>
      <li><a href="#finding-the-computed-value"><span class=secno>8.2.
       </span>Finding the computed value</a>
      <li><a href="#finding-the-used-value"><span class=secno>8.3.
       </span>Finding the used value</a>
      <li><a href="#finding-the-actual-value"><span class=secno>8.4.
       </span>Finding the actual value</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>
   </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 &lsquo;<code class=css><</code>&rsquo;
    and &lsquo;<code class=css>></code>&rsquo;), and information on how they
    can be combined. Generic data types (<a
    href="#length-value"><code>&lt;length&gt;</code></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.,
    <code>&lt;border-width&gt;</code>) are described in the corresponding
    modules. </code>
   <h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>
   <p>This module replaces and extends the data type definitions in <a
    href="#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="#keyword">keyword</a> values (such as &lsquo;<code
     class=css>auto</code>&rsquo;, &lsquo;<code class=css>disc</code>&rsquo;,
     etc.), which appear literally, without quotes (e.g. <code>auto</code>)
    <li>basic data types, which appear between &lsquo;<code
     class=css>&lt;</code>&rsquo; and &lsquo;<code
     class=css>&gt;</code>&rsquo; (e.g., <a
     href="#length-value"><code>&lt;length&gt;</code></a>, <a
     href="#percentage-value"><code>&lt;percentage&gt;</code></a>, etc.).
    <li>types that have the same range of values as a property bearing the
     same name (e.g., <code>&lt;'border-width'&gt;</code>
     <code>&lt;'background-attachment'&gt;</code>, 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 the value &lsquo;<code
     class=property>inherit</code>&rsquo;.
    <li>non-terminals that do not share the same name as a property. In this
     case, the non-terminal name appears between &lsquo;<code
     class=css>&lt;</code>&rsquo; and &lsquo;<code
     class=css>&gt;</code>&rsquo;, as in <code>&lt;spacing&gt;</code>. Notice
     the distinction between <code>&lt;border-width&gt;</code> and
     <code>&lt;'border-width'&gt;</code>: the latter is defined as the value
     of the &lsquo;<code class=property>border-width</code>&rsquo; property,
     the former requires an explicit expansion elsewhere. The definition of a
     non-terminal is located near its first appearance in the specification.
   </ol>
   <p>Some property value definitions also include the slash (/) and/or the
    comma (,) as literals. These represent their corresponding tokens.
   <p>All CSS properties also accept the keyword values &lsquo;<code
    class=css>inherit</code>&rsquo; and &lsquo;<code
    class=css>initial</code>&rsquo; as their property value, but for
    readability these are not listed explicitly in the property value syntax
    definitions. These keywords cannot be combined with other component values
    in same declaration; such a declaration is invalid. For example,
    &lsquo;<code class=css>background: url(corner.png) no-repeat,
    inherit;</code>&rsquo; 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 ([&nbsp;]) 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.
    <li>A 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.
    <li>A hash mark (#) indicates that the preceding type, word, or group
     occurs one or more times, separated by comma tokens.
   </ul>
   <h3 id=component-whitespace><span class=secno>2.4. </span> Component values
    and white space</h3>
   <p>Component values are specified in terms of tokens, as described in <a
    href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">Chapter 4</a>
    of <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. As the
    grammar allows spaces between tokens in the components of the
    <code>value</code> production, spaces may appear between tokens in
    property values.
   <p class=note>Note: In many cases, spaces will in fact be <em>required</em>
    between tokens in order to distinguish them from each other. For example,
    the value &lsquo;<code class=css>1em2em</code>&rsquo; would be parsed as a
    single <code>DIMEN</code> token with the number &lsquo;<code
    class=css>1</code>&rsquo; and the identifier &lsquo;<code
    class=css>em2em</code>&rsquo;, which is an invalid unit. In this case, a
    space would be required before the &lsquo;<code class=css>2</code>&rsquo;
    to get this parsed as the two lengths &lsquo;<code
    class=css>1em</code>&rsquo; and &lsquo;<code class=css>2em</code>&rsquo;.
   <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>&lsquo;<code class=property>orphans</code>&rsquo;
       <td>&lt;integer&gt;
       <td>&lsquo;<code class=css>3</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>text-align</code>&rsquo;
       <td>left | right | center | justify
       <td>&lsquo;<code class=css>center</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>padding-top</code>&rsquo;
       <td>&lt;length&gt; | &lt;percentage&gt;
       <td>&lsquo;<code class=css>5%</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>outline-color</code>&rsquo;
       <td>&lt;color&gt; | invert
       <td>&lsquo;<code class=css>#fefefe</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>text-decoration</code>&rsquo;
       <td>none | underline || overline || line-through || blink
       <td>&lsquo;<code class=css>overline underline</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>font-family</code>&rsquo;
       <td>&lt;family-name&gt;#
       <td>&lsquo;<code class=css>"Gill Sans", Futura,
        sans-serif</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>border-width</code>&rsquo;
       <td>[ &lt;length&gt; | thick | medium | thin ]{1,4}
       <td>&lsquo;<code class=css>2px medium 4px</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>text-shadow</code>&rsquo;
       <td>[ inset? && [ &lt;length&gt;{2,4} && &lt;color&gt;? ] ]# | none
       <td>&lsquo;<code class=css>3px 3px rgba(50%, 50%, 50%, 50%),
        lemonchiffon 0 0 4px inset</code>&rsquo;
      <tr>
       <td>&lsquo;<code class=property>voice-pitch</code>&rsquo;
       <td>
        <pre class=value>&lt;frequency> && absolute |
 <!--				      -->[[x-low | low | medium | high | x-high] ||
 <!--				      --> [&lt;frequency> | &lt;semitones> | &lt;percentage>]]</pre>
       <td>&lsquo;<code class=css>-2st x-low</code>&rsquo;
    </table>
   </div>
   <h2 id=syntax-and-terminology><span class=secno>3. </span>Syntax and
    terminology</h2>
   <p>The generic data types described in the next sections use some common
    syntactic building blocks and terms that are described in this section.
   <h3 id=functional-notation><span class=secno>3.1. </span>Functional
    notation</h3>
   <p>Some values use a <dfn id=functional-notation0>functional notation</dfn>
    to type values and to and lump values together. The syntax starts with the
    name of the function followed by a left parenthesis followed by optional
    whitespace followed by the argument(s) to the functions followed by
    optional whitespace followed by a right parenthesis. If a function takes
    more than one argument, the arguments are separated by a comma
    (&lsquo;<code class=css>,</code>&rsquo;) with optional whitespace before
    and after the comma.
   <div class=example> background: url(http://www.example.org/image); color:
    rgb(100, 200, 50 );</div>
   <p>Some properties accept space- or comma-separated lists of values. A
    value that is composed of several values with spaces or commas between
    them, is called a <dfn id=compound-value>compound value</dfn>. A value
    that is not a compound value is a <dfn id=simple-value>simple value</dfn>.
   <h2 id=textual-values><span class=secno>4. </span> Textual Data Types</h2>
   <p>An <dfn id=identifier>identifier</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="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> Identifiers
    cannot be quoted; otherwise they would be interpreted as a string.
   <h3 id=keywords><span class=secno>4.1. </span> Pre-defined Keywords</h3>
   <p>In the value definition fields, keywords with a pre-defined meaning
    appear literally. Keywords are CSS <i>identifiers</i> 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 &lsquo;<code
     class=property>border-collapse</code>&rsquo; property:</p>
    <pre>Value:&nbsp;collapse | separate</pre>
    <p>And here is an example of its use:</p>
    <pre>table { border-collapse: separate }</pre>
   </div>
   <h4 id=initial-inherit><span class=secno>4.1.1. </span> The &lsquo;<code
    class=css>initial</code>&rsquo; and &lsquo;<code
    class=css>inherit</code>&rsquo; keywords</h4>
   <p>As defined <a href="#component-types">above</a>, all properties accept
    the &lsquo;<code class=css>initial</code>&rsquo; and &lsquo;<code
    class=css>inherit</code>&rsquo; keywords, which represent value concepts
    common to all CSS properties.
   <p>The <dfn id=inherit>&lsquo;<code class=css>inherit</code>&rsquo;</dfn>
    keyword is <a
    href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">defined</a>
    in <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
   <p>The <dfn id=initial>&lsquo;<code class=css>initial</code>&rsquo;</dfn>
    keyword represents the value that is designated as the property's initial
    value. <a href="#CSS3CASCADE"
    rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a>
   <p class=issue>Should these keywords affect the specified or computed
    value? See <a
    href="http://lists.w3.org/Archives/Public/www-style/2011Jan/0075.html">various</a>
    <a
    href="http://lists.w3.org/Archives/Public/www-style/2011May/0402.html">issues</a>.
   <p class=issue>Would it be useful to have a &lsquo;<code
    class=property>default</code>&rsquo; value, defined to be equivalent to <a
    class=css href="#inherit">&lsquo;<code
    class=property>inherit</code>&rsquo;</a> for properties that are inherited
    by default and equivalent to <a class=css href="#initial">&lsquo;<code
    class=property>initial</code>&rsquo;</a> for properties that are not
    inherited by default? This might be easier for authors to use than <a
    class=css href="#initial">&lsquo;<code
    class=property>initial</code>&rsquo;</a> and <a class=css
    href="#inherit">&lsquo;<code class=property>inherit</code>&rsquo;</a>
    since it wouldn't require thinking about whether a property is inherited
    by default or not (which isn't obvious for some properties, such as
    text-decoration and visibility).
   <h3 id=identifiers><span class=secno>4.2. </span> User-defined Identifiers:
    the &lsquo;<a href="#identifier-value"><code
    class=css>&lt;identifier&gt;</code></a>&rsquo; type</h3>
   <p>Some properties accept arbitrary user-defined identifiers as a component
    value. This generic data type is denoted by <dfn
    id=identifier-value><code>&lt;identifier&gt;</code></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. &lsquo;<code class=css>example</code>&rsquo; and &lsquo;<code
    class=css>EXAMPLE</code>&rsquo; are two different, unrelated user-defined
    identifiers).
   <h3 id=strings><span class=secno>4.3. </span> Quoted Strings: the &lsquo;<a
    href="#string-value"><code class=css>&lt;string&gt;</code></a>&rsquo; type</h3>
   <p>Strings are denoted by <dfn
    id=string-value><code>&lt;string&gt;</code></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="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
   <div class=example>
    <p>Double quotes cannot occur inside double quotes, unless escaped (as
     &lsquo;<code class=css>\"</code>&rsquo; or as &lsquo;<code
     class=css>\22</code>&rsquo;). Analogously for single quotes
     ("\&lsquo;<code class=css>" or "\27"). </code>
    <pre>
 <!-- -->content: "this is a &rsquo;string'.";
 <!-- -->content: "this is a \"string\".";
 <!-- -->content: &lsquo;<code class=css>this is a "string".</code>&rsquo;;
 <!-- -->content: &lsquo;<code class=css>this is a \</code>&rsquo;string\&lsquo;<code class=css>.</code>&rsquo;;</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):</p>
    <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>4.4. </span> Resource Locators: the &lsquo;<a
    href="#url-value"><code class=css>&lt;url&gt;</code></a>&rsquo; 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><code>&lt;url&gt;</code></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="#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"><code>&lt;string&gt;</code></a> rather than by <a
    href="#url-value"><code>&lt;URL&gt;</code></a>. An example of this is the
    <a href="http://www.w3.org/TR/CSS21/cascade.html#at-import">&lsquo;<code
    class=css>@import</code>&rsquo; rule</a>.
   <p>Parentheses, whitespace characters, single quotes (&apos;) 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.
    &lsquo;<code class=css>url(open\(parens)</code>&rsquo;, &lsquo;<code
    class=css>url(close\)parens)</code>&rsquo;. Depending on the type of URL,
    it might also be possible to write these characters as URI-escapes (where
    <code>(</code> = <code>%28</code>, <code>)</code> = <code>%29</code>,
    etc.) as described in <a href="#URI"
    rel=biblioentry>[URI]<!--{{URI}}--></a>. Alternatively a URL containing
    such characters may be represented as a quoted <a
    href="#string">string</a> within the &lsquo;<a href="#url"><code
    class=css>url()</code></a>&rsquo; 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 [[RFC3986]]) are resolved to full URIs using
    a base URI. RFC&nbsp;3986, section&nbsp;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.
   <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:</p>
    <pre>http://www.example.org/style/basic.css</pre>
    <p>The background of the source document's <code>&lt;body&gt;</code> 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 <code>&lt;body&lt;</code>.
   </div>
   <h2 id=numeric-types><span class=secno>5. </span> Numeric Data Types</h2>
   <h3 id=integers><span class=secno>5.1. </span> Integers: the &lsquo;<a
    href="#integer-value"><code class=css>&lt;integer&gt;</code></a>&rsquo;
    type</h3>
   <p>Integer values are denoted by <dfn
    id=integer-value><code>&lt;integer&gt;</code></dfn>. An <dfn
    id=integer>integer</dfn> is one or more decimal digits &lsquo;<code
    class=css>0</code>&rsquo; through &lsquo;<code class=css>9</code>&rsquo;
    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>.
    Integers may be immediately preceded by &lsquo;<code
    class=css>-</code>&rsquo; or &lsquo;<code class=css>+</code>&rsquo; to
    indicate the sign.
   <p>Properties may restrict the integer 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>.
   <h3 id=numbers><span class=secno>5.2. </span> Numbers: the &lsquo;<a
    href="#number-value"><code class=css>&lt;number&gt;</code></a>&rsquo; type</h3>
   <p>Number values are denoted by <dfn
    id=number-value><code>&lt;number&gt;</code></dfn>. A <dfn
    id=number>number</dfn> is either an <a href="#integer"><i>integer</i></a>,
    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>.
    Like integers, numbers may also be immediately preceded by &lsquo;<code
    class=css>-</code>&rsquo; or &lsquo;<code class=css>+</code>&rsquo; to
    indicate the sign.
   <p>Properties may restrict the number 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>.
   <h3 id=percentages><span class=secno>5.3. </span> Percentages: the
    &lsquo;<a href="#percentage-value"><code
    class=css>&lt;percentage&gt;</code></a>&rsquo; type</h3>
   <p>A percentage value is denoted by <dfn
    id=percentage-value><code>&lt;percentage&gt;</code></dfn>, consists of a
    <a href="#number-value"><i>&lt;number&gt;</i></a> immediately followed by
    a percent sign &lsquo;<code class=css>%</code>&rsquo;. It corresponds to
    the <code>PERCENTAGE</code> 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 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.
   <p>Properties may restrict the percentage 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>.
   <h2 id=lengths><span class=secno>6. </span> Distance Units: the &lsquo;<a
    href="#length-value"><code class=css>&lt;length&gt;</code></a>&rsquo; type</h2>
   <p>Lengths refer to distance measurements and are denoted by <dfn
    id=length-value><code>&lt;length&gt;</code></dfn> in the property
    definitions. A length is a <a href="#dimension"><i>dimension</i></a>. A
    zero length may be represented instead as the <a
    href="#number-value"><code>&lt;number&gt;</code></a> &lsquo;<code
    class=css>0</code>&rsquo;. (In other words, for zero lengths the unit
    identifier is optional.)
   <p> A <dfn id=dimension>dimension</dfn> is a <a
    href="#number"><i>number</i></a> immediately followed by a unit
    identifier. It corresponds to the <a
    href="#dimension"><code>DIMENSION</code></a> token in the <a
    href="http://www.w3.org/TR/CSS21/syndata.html#tokenization">grammar</a>.
    <a href="#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 <a href="#used-value">used</a> length cannot be
    supported, user agents must approximate it in the <a
    href="#actual-value">actual</a> value.
   <p>There are two types of length units: relative and absolute.
   <h3 id=relative-lengths><span class=secno>6.1. </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>&lsquo;<a href="#em-unit"><code class=css>em</code></a>&rsquo;
      <td>font size of the element
     <tr>
      <td>&lsquo;<a href="#ex-unit"><code class=css>ex</code></a>&rsquo;
      <td><i>x-height</i> of the element's font
     <tr>
      <td>&lsquo;<a href="#ch-unit"><code class=css>ch</code></a>&rsquo;
      <td>width of the "0" glyph in the element's font
     <tr>
      <td>&lsquo;<a href="#rem-unit"><code class=css>rem</code></a>&rsquo;
      <td>font size of the root element
     <tr>
      <td>&lsquo;<a href="#vw-unit"><code class=css>vw</code></a>&rsquo;
      <td>viewport's width
     <tr>
      <td>&lsquo;<a href="#vh-unit"><code class=css>vh</code></a>&rsquo;
      <td>viewport's height
     <tr>
      <td>&lsquo;<a href="#vm-unit"><code class=css>vm</code></a>&rsquo;
      <td>minimum of the viewport's height and width
   </table>
   <p>Child elements do not inherit the relative values as specified for their
    parent; they inherit the <a href="#computed-value">computed values</a>.
   <h4 id=font-relative-lengths><span class=secno>6.1.1. </span> Font-relative
    lengths: the &lsquo;<a href="#em-unit"><code
    class=css>em</code></a>&rsquo;, &lsquo;<a href="#ex-unit"><code
    class=css>ex</code></a>&rsquo;, &lsquo;<a href="#ch-unit"><code
    class=css>ch</code></a>&rsquo;, &lsquo;<a href="#rem-unit"><code
    class=css>rem</code></a>&rsquo; units</h4>
   <p>Aside from &lsquo;<a href="#rem-unit"><code
    class=css>rem</code></a>&rsquo; (which refers to the font-size of the root
    element), the font-relative lengths refer to the computed font metrics of
    the element on which they are used. The exception is when they occur in
    the value of the &lsquo;<code class=property>font-size</code>&rsquo;
    property itself, in which case they refer to the font metrics of the
    parent element (or the font metrics corresponding to the initial values of
    the &lsquo;<code class=property>font</code>&rsquo; property, if the
    element has no parent).
   <dl>
    <dt><dfn id=em-unit title=em>em unit</dfn>
    <dd>
     <p>Equal to the computed value of the &lsquo;<code
      class=property>font-size</code>&rsquo; property of the element on which
      it is used.
     <div class=example>
      <p>The rule:</p>
      <pre>h1 { line-height: 1.2em }</pre>
      <p>means that the line height of <code>h1</code> elements will be 20%
       greater than the font size of <code>h1</code> element. On the other
       hand:
      <pre>h1 { font-size: 1.2em }</pre>
      <p>means that the font size of <code>h1</code> elements will be 20%
       greater than the font size inherited by <code>h1</code> elements.</p>
     </div>
    <dt><dfn id=ex-unit title=ex>ex unit</dfn>
    <dd>
     <p>Equal to the font's x-height. The x-height is so called because it is
      often equal to the height of the lowercase "x". However, an &lsquo;<a
      href="#ex-unit"><code class=css>ex</code></a>&rsquo; is defined even for
      fonts that do not contain an "x".
     <p>The x-height of a font can be found in different ways. Some fonts
      contain reliable metrics for the x-height. If reliable font metrics are
      not available, UAs may determine the x-height from the height of a
      lowercase glyph. One possible heuristic is to look at how far the glyph
      for the lowercase "o" extends below the baseline, and subtract that
      value from the top of its bounding box. In the cases where it is
      impossible or impractical to determine the x-height, a value of 0.5em
      must be assumed.
    <dt><dfn id=ch-unit title=ch>ch unit</dfn>
    <dd>
     <p>Equal to the advance measure of the "0" (ZERO, U+0030) glyph found in
      the font used to render it.
    <dt><dfn id=rem-unit title=rem>rem unit</dfn>
    <dd>
     <p>Equal to the computed value of &lsquo;<code
      class=property>font-size</code>&rsquo; on the root element.
     <p>When specified on the &lsquo;<code
      class=property>font-size</code>&rsquo; property of the root element, the
      &lsquo;<a href="#rem-unit"><code class=css>rem</code></a>&rsquo; units
      refer to the property's <em>initial value</em>.</p>
   </dl>
   <h4 id=viewport-relative-lengths><span class=secno>6.1.2. </span>
    Viewport-relative lengths: the &lsquo;<a href="#vw-unit"><code
    class=css>vw</code></a>&rsquo;, &lsquo;<a href="#vh-unit"><code
    class=css>vh</code></a>&rsquo;, &lsquo;<a href="#vm-unit"><code
    class=css>vm</code></a>&rsquo; units</h4>
   <p>The viewport-relative lengths are relative to the size of the initial
    containing block. When the height or width of the viewport is changed,
    they are scaled proportionally.
   <dl>
    <dt><dfn id=vw-unit title=vw>vw unit</dfn>
    <dd>Equal to 1/100th of the width of the initial containing block.
     <div class=example>
      <p>In the example below, if the width of the viewport is 200mm, the font
       size of <code>h1</code> elements will be 16mm (i.e.
       (8&times;200mm)/100).
      <pre>h1 { font-size: 8vw }</pre>
     </div>
    <dt><dfn id=vh-unit title=vh>vh unit</dfn>
    <dd>Equal to 1/100th of the height of the initial containing block.
    <dt><dfn id=vm-unit title=vm>vm unit</dfn>
    <dd>Equal to the smaller of &lsquo;<a href="#vw-unit"><code
     class=css>vw</code></a>&rsquo; or &lsquo;<a href="#vh-unit"><code
     class=css>vh</code></a>&rsquo;.
     <p class=issue>Do we need this now that we have the min() function?
   </dl>
   <h3 id=absolute-lengths><span class=secno>6.2. </span> Absolute lengths:
    the &lsquo;<code class=css>cm</code>&rsquo;, &lsquo;<code
    class=css>mm</code>&rsquo;, &lsquo;<code class=css>in</code>&rsquo;,
    &lsquo;<code class=css>pt</code>&rsquo;, &lsquo;<code
    class=css>pc</code>&rsquo;, &lsquo;<code class=css>px</code>&rsquo; units</h3>
   <p>The <dfn id=absolute-length-units>absolute length units</dfn> are fixed
    in relation to each other and anchored to some physical measurement. They
    are mainly useful when the output environment is known. The absolute units
    consist of the physical units (in, cm, mm, pt, pc) and the px unit:
   <table class=data>
    <thead>
     <tr>
      <th>unit
      <th>definition
    <tbody>
     <tr>
      <td>&lsquo;<code class=css>cm</code>&rsquo;
      <td>centimeters
     <tr>
      <td>&lsquo;<code class=css>mm</code>&rsquo;
      <td>millimeters
     <tr>
      <td>&lsquo;<code class=css>in</code>&rsquo;
      <td>inches; 1in is equal to 2.54cm
     <tr>
      <td>&lsquo;<code class=css>px</code>&rsquo;
      <td>pixels; 1px is equal to 1/96th of 1in
     <tr>
      <td>&lsquo;<code class=css>pt</code>&rsquo;
      <td>points; 1pt is equal to 1/72nd of 1in
     <tr>
      <td>&lsquo;<code class=css>pc</code>&rsquo;
      <td>picas; 1pc is equal to 12pt
   </table>
   <pre class=example>
 h1 { margin: 0.5in }      /* inches  */
 h2 { line-height: 3cm }   /* centimeters */
 h3 { word-spacing: 4mm }  /* millimeters */
 h4 { font-size: 12pt }    /* points */
 h4 { font-size: 1pc }     /* picas */
 p  { font-size: 12px }    /* px */</pre>
   <p>For a CSS device, these dimensions are either anchored (i) by relating
    the physical units to their physical measurements, or (ii) by relating the
    pixel unit to the <a href="#reference-pixel"><i>reference pixel</i></a>.
    For print media and similar high-resolution devices, the anchor unit
    should be one of the standard physical units (inches, centimeters, etc).
    For lower-resolution devices, and devices with unusual viewing distances,
    it is recommended instead that the anchor unit be the pixel unit. For such
    devices it is recommended that the pixel unit refer to the whole number of
    device pixels that best approximates the reference pixel.
   <p class=note>Note that if the anchor unit is the pixel unit, the physical
    units might not match their physical measurements. Alternatively if the
    anchor unit is a physical unit, the pixel unit might not map to a whole
    number of device pixels.
   <p class=note>Note that this definition of the pixel unit and the physical
    units differs from previous versions of CSS. In particular, in previous
    versions of CSS the pixel unit and the physical units were not related by
    a fixed ratio: the physical units were always tied to their physical
    measurements while the pixel unit would vary to most closely match the
    reference pixel. (This change was made because too much existing content
    relies on the assumption of 96dpi, and breaking that assumption breaks the
    content.)
   <p>The <dfn id=reference-pixel>reference pixel</dfn> is the visual angle of
    one pixel on a device with a pixel density of 96dpi and a distance from
    the reader of an arm's length. For a nominal arm's length of 28 inches,
    the visual angle is therefore about 0.0213 degrees. For reading at arm's
    length, 1px thus corresponds to about 0.26&nbsp;mm (1/96&nbsp;inch).
   <p>The image below illustrates the effect of viewing distance on the size
    of a reference pixel: a reading distance of 71&nbsp;cm (28&nbsp;inches)
    results in a reference pixel of 0.26&nbsp;mm, while a reading distance of
 .5&nbsp;m (12&nbsp;feet) results in a reference pixel of 1.3&nbsp;mm.
   <div class=figure>
    <p><img alt="This diagram illustrates how the definition of a pixel
     depends on the users distance from the viewing surface (paper or screen).
     The image depicts the user looking at two planes, one 28 inches (71 cm)
     from the user, the second 140 inches (3.5 m) from the user. An expanding
     cone is projected from the user's eye onto each plane. Where the cone
     strikes the first plane, the projected pixel is 0.26 mm high. Where the
     cone strikes the second plane, the projected pixel is 1.4 mm high."
     src=pixel1.png>
    <p class=caption>Showing that pixels must become larger if the viewing
     distance increases
   </div>
   <p>This second image illustrates the effect of a device's resolution on the
    pixel unit: an area of 1px by 1px is covered by a single dot in a
    low-resolution device (e.g. a typical computer display), while the same
    area is covered by 16 dots in a higher resolution device (such as a
    printer).
   <div class=figure>
    <p><img alt="This diagram illustrates the relationship between the
     reference pixel and device pixels (called &#34;dots&#34; below). The
     image depicts a high resolution (large dot density) laser printer output
     on the left and a low resolution monitor screen on the right. For the
     laser printer, one square reference pixel is implemented by 16 dots. For
     the monitor screen, one square reference pixel is implemented by a single
     dot." src=pixel2.png>
    <p class=caption>Showing that more device pixels (dots) are needed to
     cover a 1px by 1px area on a high-resolution device than on a low-res one
   </div>
   <h2 id=other-units><span class=secno>7. </span> Other Units</h2>
   <h3 id=angles><span class=secno>7.1. </span> Angles: the &lsquo;<a
    href="#angle-value"><code class=css>&lt;angle&gt;</code></a>&rsquo; type
    and &lsquo;<a href="#deg"><code class=css>deg</code></a>&rsquo;, &lsquo;<a
    href="#grad"><code class=css>grad</code></a>&rsquo;, &lsquo;<a
    href="#rad"><code class=css>rad</code></a>&rsquo;, &lsquo;<a
    href="#turn"><code class=css>turn</code></a>&rsquo; units</h3>
   <p>Angle values are <i>dimensions</i> denoted by <dfn
    id=angle-value>&lt;angle&gt;</dfn>. The angle unit identifiers are:
   <dl>
    <dt><dfn id=deg title=deg>deg</dfn>
    <dd>Degrees. There are 360 degrees in a full circle.
    <dt><dfn id=grad title=grad>grad</dfn>
    <dd>Gradians. There are 400 gradians in a full circle.
    <dt><dfn id=rad title=rad>rad</dfn>
    <dd>Radians. There are 2&pi; radians in a full circle. <span
     class=issue>Who is going to use this anyway?</span>
    <dt><dfn id=turn title=turn>turn</dfn>
    <dd>Turns. There is 1 turn in a full circle.
   </dl>
   <p>For example, a right angle is &lsquo;<code class=css>90deg</code>&rsquo;
    or &lsquo;<code class=css>100grad</code>&rsquo; or &lsquo;<code
    class=css>0.25turn</code>&rsquo; or approximately &lsquo;<code
    class=css>1.570796326794897rad</code>&rsquo;.
   <h3 id=time><span class=secno>7.2. </span> Times: the &lsquo;<a
    href="#time-value"><code class=css>&lt;time&gt;</code></a>&rsquo; type and
    &lsquo;<a href="#s"><code class=css>s</code></a>&rsquo;, &lsquo;<a
    href="#ms"><code class=css>ms</code></a>&rsquo; units</h3>
   <p>Time values are <i>dimensions</i> denoted by <dfn
    id=time-value>&lt;time&gt;</dfn>. The time unit identifiers are:
   <dl>
    <dt><dfn id=s title=s>s</dfn>
    <dd>Seconds.
    <dt><dfn id=ms title=ms>ms</dfn>
    <dd>Milliseconds. There are 1000 milliseconds in a second.
   </dl>
   <p>Properties may restrict the time 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>.
   <h3 id=frequencies-the-ltfrequencygt-type-and-h><span class=secno>7.3.
    </span>Frequencies: the &lsquo;<a href="#frequency-value"><code
    class=css>&lt;frequency&gt;</code></a>&rsquo; type and &lsquo;<a
    href="#hz"><code class=css>Hz</code></a>&rsquo;, &lsquo;<a
    href="#khz"><code class=css>kHz</code></a>&rsquo; units</h3>
   <p>Frequency values are <i>dimensions</i> denoted by <dfn
    id=frequency-value>&lt;frequency&gt;</dfn>. The frequency unit identifiers
    are:
   <dl>
    <dt><dfn id=hz title=Hz>Hz</dfn>
    <dd>Hertz. It represents the number of occurrences per second.
    <dt><dfn id=khz title=kHz>kHz</dfn>
    <dd>KiloHertz. A kiloHertz is 1000 Hertz.
     <p>For example, when representing sound pitches, 200Hz (or 200hz) is a
      bass sound, and 6kHz (or 6khz) is a treble sound.</p>
     <h4 id=the-calc-min-and-max-functions><span class=secno>7.3.1. </span>The
      &lsquo;<a href="#calc"><code class=property>calc</code></a>&rsquo;,
      &lsquo;<a href="#min"><code class=property>min</code></a>&rsquo; and
      &lsquo;<a href="#max"><code class=property>max</code></a>&rsquo;
      functions</h4>
     <p>The <dfn id=calc>calc()</dfn>, <dfn id=min>min()</dfn>, and <dfn
      id=max>max()</dfn> functions can be used wherever length, frequency,
      angle, time, or number values are allowed.
     <div class=example>
      <pre>
 section {
   float: left;
   margin: 1em; border: solid 1px;
   width: calc(100%/3 - 2*1em - 2*1px);
 }
 </pre>
     </div>
     <div class=example>
      <pre>
 p {
   margin: calc(1rem - 2px) calc(1rem - 1px);
 }
 </pre>
     </div>
     <div class=example>
      <pre>
 p { font-size: min(10px, 3em) }
 blockquote { font-size: max(30px, 3em) }
 </pre>
     </div>
     <div class=example>
      <pre>
 .box { width: min(10% + 20px, 300px) }
 </pre>
     </div>
     <p>The expression language of these functions is described by the grammar
      and prose below.
     <pre>
 S       : calc | min | max;
 calc    : "calc(" S* sum ")" S*;
 min     : "min(" S* sum [ "," S* sum ]* ")" S*;
 max     : "max(" S* sum [ "," S* sum ]* ")" S*;
 sum     : product [ [ "+" | "-" ] S* product ]*;
 product : unit [ [ "*" | "/" | "mod" ] S* unit ]*;
 unit    : ["+"|"-"]? [ NUMBER S* | DIMENSION S* | PERCENTAGE S* |
           min | max | "(" S* sum ")" S* ];
 </pre>
     <p>The context of the expression imposes a target type, which is one of
      length, frequency, angle, time, or number. NUMBER tokens are of type
      number. DIMENSION tokens have types of their units (&lsquo;<code
      class=property>cm</code>&rsquo; is length, &lsquo;<a href="#deg"><code
      class=property>deg</code></a>&rsquo; is angle etc.); any DIMENSION whose
      type does not match the target type is not allowed and must be a parse
      error. If percentages are accepted in that context and convertible to
      the target type, a PERCENTAGE token in the expression has the target
      type; otherwise percentages are not allowed and must be a parse error.
     <p>To make expressions simpler, operators have restrictions on the types
      they accept. At each operator, the types of the left and right side have
      to be checked for these restrictions. If compatible, they return roughly
      as follows (the following ignores precedence rules on the operators for
      simplicity):
     <ol>
      <li>At ",", "+", "-":<br>
       check: both sides have the same type<br>
       return: that type
      <li>At "*":<br>
       check: at least one side is "number" <br>
       return: the type of the other side
      <li>At "/":<br>
       check: right side is "number"<br>
       return: the type of the left side
     </ol>
     <p>Division by zero is a parse error.
     <p>The value resulting from an expression must be clamped to the range
      allowed in the target context.
     <div class=example> These two are equivalentequivalent to &lsquo;<code
      class=css>width: 0px</code>&rsquo; since widths smaller than 0px are not
      allowed.
      <pre>
 width: calc(5px - 10px);
 width: 0px;
 </pre>
     </div>
     <p>Given the complexities of &lsquo;<code
      class=property>width</code>&rsquo; and &lsquo;<code
      class=property>height</code>&rsquo; on table cells and table elements,
      calc() expressions for &lsquo;<code class=property>width</code>&rsquo;
      and &lsquo;<code class=property>height</code>&rsquo; on table columns,
      table column groups, table rows, table row groups, and table cells in
      both auto and fixed layout tables are treated as if &lsquo;<code
      class=property>auto</code>&rsquo; had been specified.
     <h3 id=ltcolorgt><span class=secno>7.4. </span>&lt;color&gt;</h3>
     <p><em>This section is not normative. The CSS3 Color module <a
      href="#CSS3COLOR" rel=biblioentry>[CSS3COLOR]<!--{{!CSS3COLOR}}--></a>
      defines the CSS3 color values.</em>
     <p>Color values are denoted by <dfn id=ltcolorgt0>&lt;color&gt;</dfn> in
      the value definitions.
     <p>A <dfn id=color>color</dfn> value can either be a keyword, a numerical
      specification in a functional notation, or a numerical RGB specification
      in a hexadecimal notation. The hexadecimal notation is special shorthand
      format that allows compact color descriptions.
     <div class=example>
      <pre>
 em { color: #F00 }
 span.issue { color: red }
 * { color: hsl(120, 75%, 75%) }
 </pre>
     </div>
     <h3 id=ltattrgt><span class=secno>7.5. </span>&lt;attr&gt;</h3>
     <p class=issue>Describe the feature fully here, not just a delta from CSS
 .
     <p class=issue>When attr is set on a pseudo-element, it should apply to
      the originating element
     <p>In CSS2.1 <a href="#CSS21"
      rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, the &lsquo;<code
      class=css>attr()</code>&rsquo; expression always returns a string. In
      CSS3, the &lsquo;<code class=css>attr()</code>&rsquo; expression can
      return many different types. The new syntax for the attr() expression
      is:
     <pre>
 'attr(' ident [ ',' &lt;type> [ ',' &lt;value> ]? ]? ')'
 </pre>
     <p>The first argument represents the attribute name. The value of the
      attribute with that name on the element whose computed values are being
      computed is used as the value of the expression, according to the rules
      given below.
     <p>The first argument accepts an optional namespace prefix to identify
      the namespace of the attribute. The namespace prefix and the attribute
      name is separated by &lsquo;<code class=css>|</code>&rsquo;, with no
      whitespace before or after the separator <a href="#CSS3NAMESPACE"
      rel=biblioentry>[CSS3NAMESPACE]<!--{{CSS3NAMESPACE}}--></a>.
     <p>The second argument (which is optional but must be present if the
      third argument is present) is a &lt;type> and tells the UA how to
      interpret the attribute value. It may be one of the values from the list
      below.
     <p>The third argument (which is optional) is a CSS value which must be
      valid where the attr() expression is placed. If it is not valid, then
      the whole attr() expression is invalid.
     <p>If the attribute named by the first argument is missing, cannot be
      parsed, or is invalid for the property, then the value returned by
      attr() will be the third argument, or, if the third argument is absent,
      will be the value given as the default for the relevant type in the list
      below.
     <dl>
      <dt>string
      <dd>The attribute value will be interpreted as the contents of a CSS
       string. The default is the empty string.
      <dt>color
      <dd>The attribute value will be interpreted as a CSS &lt;color> value.
       The default is UA dependent but must be the same as the initial value
       of the &lsquo;<a href="#color"><code
       class=property>color</code></a>&rsquo; property.
      <dt>url
      <dd>The attribute value will be interpreted as the URI part of a
       &lsquo;<a href="#url"><code class=css>url()</code></a>&rsquo;
       expression. The default is a UA-dependent URI defined to point to a
       non-existent document with a generic error condition. (i.e. it
       shouldn't be an FTP URI that causes a DNS error, or an HTTP URI that
       results in a 404, it should be a nondescript error condition.)
      <dt>integer
      <dd>The attribute value will be interpreted as a CSS integer. The
       default is 0. The default should also be used if the property in
       question only accepts integers within a certain range and the attribute
       is out of range.
      <dt>number
      <dd>The attribute value will be interpreted as a CSS number. The default
       is 0.0. The default should also be used if the property in question
       only accepts numbers within a certain range and the attribute is out of
       range.
      <dt>length, angle, time, frequency
      <dd>The attribute value will be interpreted as a CSS length, angle, time
       or frequency (respectively), and the unit identifier (if any) will
       appear in the attribute value. The default is 0. The default should
       also be used if the property in question only accepts values within a
       certain range (e.g. positive lengths or angles from 0 to 90deg) and the
       attribute is out of range (e.g. a negative length or 180deg).
      <dt>em, ex, px, gr, rem, vw, vh, vm, mm, cm, in, pt, pc, deg, grad, rad,
       ms, s, Hz, kHz, %
      <dd>The attribute value will be interpreted as a float, with the given
       type suffixed as a unit. The default is 0 in the relevant units.
     </dl>
     <p class=issue>Should there also be a "keyword" type to, e.g., support
      &lsquo;<code class=css>float: attr(align)</code>&rsquo;</p>
     <p>If the &lt;type> is missing, &lsquo;<code
      class=property>string</code>&rsquo; is implied.
     <p class=issue>Ideally, it shouldn't be necessary to specify the type if
      it is obvious. For example, this should be valid: "<tt>background-image:
      attr(href);</tt>". This could be described as: <q>If the property only
      accepts one type of value (aside from &lsquo;<code
      class=property>inherit</code>&rsquo; and &lsquo;<code
      class=property>initial</code>&rsquo;), that type is implied</q>.
     <p>The attr() form is only valid if the type given (or implied, if it is
      missing) is valid for the property. For example, all of the following
      are invalid and would cause a parse-time error (and thus cause the
      relevant declaration, in this case all of them, to be ignored):
     <div class="illegal example">
      <p style="display:none">Illegal Examples:
      <pre>
       content: attr(title, color); /* 'content' doesn't accept colors */
       content: attr(end-of-quote, string, inherit) close-quote; /* the
       'inherit' value is not allowed there, since the result would be
       'inherit close-quote', which is invalid. */
       margin: attr(vertical, length) attr(horizontal, deg); /* deg
       units are not valid at that point */
 <!--
       font: attr(weight, integer) attr(size, length)/attr(height,
       integer) attr(family, string); /* invalid because
       &lt;'font-weight'>s are not integers, but identifiers. */
 -->
       color: attr(color); /* 'color' doesn't accept strings */
 </pre>
     </div>
     <p>The attr() expression cannot return everything, for example it cannot
      do counters, named strings, quotes, or values such as &lsquo;<code
      class=property>auto</code>&rsquo;, &lsquo;<code
      class=property>nowrap</code>&rsquo;, or &lsquo;<code
      class=property>baseline</code>&rsquo;. This is intentional, as the
      intent of the &lsquo;<code class=css>attr()</code>&rsquo; expression is
      not to make it possible to describe a presentational language's
      formatting using CSS, but to enable CSS to take semantic data into
      account.
     <p>Note that the default value need not be of the type given. For
      instance, if the type required of the attribute by the author is
      &lsquo;<code class=property>px</code>&rsquo;, the default could still be
      &lsquo;<code class=css>5em</code>&rsquo;.
     <div class=example>
      <p>Examples:
      <pre>
       &lt;stock>
         &lt;wood length="12"/>
         &lt;wood length="5"/>
         &lt;metal length="19"/>
         &lt;wood length="4"/>
       &lt;/stock>
       stock::before {
         display: block;
         content: "To scale, the lengths of materials in stock are:";
       }
       stock > * {
         display: block;
         width: attr(length, em); /* default 0 */
         height: 1em;
         border: solid thin;
         margin: 0.5em;
       }
       wood {
         background: orange url(wood.png);
       }
       metal {
         background: silver url(metal.png);
       }
       /* this also uses a possible extension to the 'content' property
       to handle replaced content and alternatives to unavailable,
       corrupted or unsupported content */
       img {
          content: replaced attr(src, url), attr(alt, string, none);
          height: attr(height, px, auto);
          width: attr(width, px, auto);
       }
 </pre>
     </div>
     <p>The attr() expression cannot currently fall back onto another
      attribute. Future versions of CSS may extend attr() in this direction.
     <p class=issue>Should &lsquo;<code class=css>attr()</code>&rsquo; be
      allowed on any property, in any source language? For example, do we
      expect UAs to honor this rule for HTML documents?: <tt>P[COLOR] { color:
      attr(COLOR, color) }</tt>.
     <h3 id=ltimagegt><span class=secno>7.6. </span>&lt;image&gt;</h3>
     <p>TBD. <!--
 <h4>The 'counter' function</h4>
 <p><dfn title="&lt;counter&gt;, definition of">Counters</dfn> are denoted by
 identifiers (see the <span class="property">'counter-increment'</span> and
 <span class="property">'counter-reset'</span> properties). To refer to the
 value of a counter, the notation <dfn>'counter(&lt;identifier&gt;)'</dfn> or
 <dfn>'counter(&lt;identifier&gt;, &lt;list-style-type&gt;)'</dfn> is used.
 The default style is 'decimal'.</p>
 <p>To refer to a sequence of nested counters of the same name, the notation
 is 'counters(&lt;identifier&gt;, &lt;string&gt;)' or
 'counters(&lt;identifier&gt;, &lt;string&gt;, &lt;list-style-type&gt;)'. See
 "Nested counters and scope" [add ref] in the chapter on generated content
 [add ref].</p>
 <p>In CSS2.1, the values of counters can only be referred to from the <span
 class="property">'content'</span> property. Note that 'none' is a possible
 &lt;list-style-type&gt;: 'counter(x, none)' yields an empty string.</p>
 <div class="example">
 <p style="display:none">Example(s):</p>
 <p>Here is a style sheet that numbers paragraphs (P) for each chapter (H1).
 The paragraphs are numbered with roman numerals, followed by a period and a
 space:</p>
 <pre>
 p { counter-increment: par-num }
 h1 { counter-reset: par-num }
 p:before { content: counter(par-num, upper-roman) ". " }
 </pre>
 </div>
 -->
     <h3 id=ltfractiongt><span class=secno>7.7. </span>&lt;fraction&gt;</h3>
     <p>Some properties accept a series of length values that, in sum, should
      add up to a certain length. To take up any remaining space, fractions
      can be used.
     <h4 id=the-fr-unit><span class=secno>7.7.1. </span>The &lsquo;<code
      class=property>fr</code>&rsquo; unit</h4>
     <p>The <dfn id=fr title="fr (unit)">fr</dfn> unit is used to distribute
      any remaining space in a series of length values. If multiple fractions
      are specified, they take up space proportionally to their numeric value.
     <div class=example>
      <pre>
 border-parts: 10px 1fr 10px;
 border-parts: 10px 1fr 10px 1fr 10px;
 border-parts: 10px 2fr 10px 2fr 10px;
 </pre>
     </div>
     <p>The &lsquo;<code class=css>fr</code>&rsquo; unit can only be used in
      combination with regular length units.
     <h3 id=ltgridgt><span class=secno>7.8. </span>&lt;grid&gt;</h3>
     <p>A grid is a set of invisible vertical and horizontal lines that can be
      used to align content. In CSS3, a grid lines can be established
      implicitly or explicitly <a href="#CSS3COL"
      rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a> <a href="#CSS3GRID"
      rel=biblioentry>[CSS3GRID]<!--{{!CSS3GRID}}--></a>. In any case, the
      distance between grid lines can be referred to by the &lsquo;<code
      class=css>gr</code>&rsquo; unit.
     <h4 id=the-gr-unit><span class=secno>7.8.1. </span>The &lsquo;<code
      class=property>gr</code>&rsquo; unit</h4>
     <p>The <dfn id=gr title="gr (unit)">gr</dfn> unit is used to position
      elements in relation to grid lines.
     <div class=example>
      <pre>
 img {
   float: top left multicol;
   float-offset: 2gr;
   width: 1gr }
 }
 </pre>
     </div>
     <p>Grid lines can be laid out in uneven patterns. Therefore, the
      &lsquo;<code class=css>gr</code>&rsquo; unit is not linear.
     <div class=example>
      <p>For example, "2gr" is not necessarily twice as long as "1gr".
     </div>
     <!--
 <h3>Special cases</h3>
 <p>Two common types of values fall outside the types
 described above: the value for font families and the hexadecimal color
 notation.
 <h4>Font families</h4>
 <p>The <span class="property">'font'</span>
 and <span class="property">'font-family'</span> properties accept a
 comma-separated list of font families. Font families can either be the
 name of a certain font, or it can be one of
 five <em class="index">generic font families</em>: 'serif',
 'sans-serif', 'cursive','fantasy', and 'monospace'. Font family names
 are like strings, except that the quotes around them may be omitted.
 If quoting is omitted, any white space characters before and after the
 font name are ignored and any sequence of white space characters
 inside the font name is converted to a single space. Font family names
 that happen to be the same as a keyword value (e.g. 'initial',
 'inherit', 'default', 'serif', 'sans-serif', 'monospace', 'fantasy',
 and 'cursive') must be quoted to prevent confusion with the keywords
 with the same names. UAs must not consider these keywords as matching
 the ''&lt;family-name&gt;'' type.
 <p>Generic font family names are keywords and must not be quoted.
 <div class="example">
 <pre>body { font-family: "Helvetica", "Univers", "Arial", sans-serif }</pre>
 </div>
 -->
     <h2 id=specified-computed-used-and-actual-value><span class=secno>8.
      </span>Specified, computed, used, and actual values</h2>
     <p>The final value of a CSS3 property for a given element is the result
      of a four-step calculation. First, cascading and inheritance yields the
      <a href="#specified-value"><em>specified value</em></a> <a
      href="#CSS3CASCADE"
      rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a>. Second,
      relative values are computed into absolute values as far as possible
      without formatting the document, thereby yielding the <a
      href="#computed-value"><em>computed value</em></a>. The computed value
      is transformed into the <a href="#used-value"><em>used value</em></a> in
      the formatting process. Finally, the computed value is transformed to
      the <a href="#actual-value"><em>actual value</em></a> based on
      constraints in the user agent.</p>
     <h3 id=finding-the-specified-value><span class=secno>8.1. </span>Finding
      the <dfn id=specified-value>specified value</dfn></h3>
     <p>The <a href="#specified-value"><em>specified value</em></a> is the
      output of the cascading and inheritance process <a href="#CSS3CASCADE"
      rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a>.
     <h3 id=finding-the-computed-value><span class=secno>8.2. </span>Finding
      the <dfn id=computed-value>computed value</dfn></h3>
     <p>Specified values may be absolute (i.e., they are not specified
      relative to another value, as in &lsquo;<code
      class=property>red</code>&rsquo; or &lsquo;<code
      class=css>2mm</code>&rsquo;) or relative (i.e., they are specified
      relative to another value, as in &lsquo;<code
      class=property>auto</code>&rsquo;, &lsquo;<code
      class=css>2em</code>&rsquo;). For absolute values, no processing is
      needed to find the computed value.</p>
     <p>For relative values, on the other hand, computation is necessary to
      find the computed values: percentages must be multiplied by a reference
      value (each property defines which value that is), values with relative
      units (em, ex, px) must be made absolute by multiplying with the
      appropriate font or pixel size, &lsquo;<code
      class=property>auto</code>&rsquo; values must be computed by the
      formulas given with each property, certain keywords (e.g., &lsquo;<code
      class=property>smaller</code>&rsquo;, &lsquo;<code
      class=property>bolder</code>&rsquo;) must be replaced according to their
      definitions. See example (f), (g) and (h) in the table below.</p>
     <p>Also, relative URIs are computed into absolute URIs at this stage. The
      computed value of invalid and absolute URIs is the same as the specified
      value.
     <h3 id=finding-the-used-value><span class=secno>8.3. </span>Finding the
      <dfn id=used-value>used value</dfn></h3>
     <p>Computed values are processed as far as possible without formatting
      the document. Some values, however, can only be determined when the
      document is being laid out. For example, if the width of an element is
      set to be a certain percentage of its containing block, the width cannot
      be determined until the width of the containing block has been
      determined. The used value is the result of taking the computed value
      and resolving any remaining dependencies into an absolute value.
     <h3 id=finding-the-actual-value><span class=secno>8.4. </span>Finding the
      <dfn id=actual-value>actual value</dfn></h3>
     <p>A used value is in principle ready to be used, but a user agent may
      not be able to make use of the value in a given environment. For
      example, a user agent may only be able to render borders with integer
      pixel widths and may therefore have to approximate the computed width.
      Also, the font size of an element may need adjustment based on the
      availability of fonts or the value of the &lsquo;<code
      class=property>font-size-adjust</code>&rsquo; property. The actual value
      is the computed value after adjustments have been made.
     <p>By probing the actual values of elements, much can be learned about
      how the document is laid out. However, not all information is recorded
      in the actual values. For example, the actual value of the &lsquo;<code
      class=property>page-break-after</code>&rsquo; property does not reflect
      whether there is a page break or not after the element. Similarly, the
      actual value of &lsquo;<code class=property>orphans</code>&rsquo; does
      not reflect how many orphan lines there is in a certain element. See
      examples (j) and (k) in the table below.
     <table>
      <tbody>
       <tr>
        <th>Example
        <th>Winning declaration
        <th>Property</th>
        <!--      <th>Cascaded value</th>  -->
        <th>Specified value
        <th>Computed value
        <th>Used value
        <th>Actual value
       <tr>
        <td>a
        <td><tt class=declaration>text-align: left</tt>
        <td>text-align</td>
        <!--      <td>left</td>-->
        <td>left
        <td>left
        <td>left
        <td>left
       <tr>
        <td>b
        <td><tt class=declaration>border-width: inherit</tt>
        <td>border-top-width, border-right-width, border-bottom-width,
         border-left-width</td>
        <!--      <td>inherit</td> -->
        <td class=say>4.2px
        <td>4.2px
        <td>4.2px
        <td>4px
       <tr>
        <td>c
        <td>(no winning declaration)
        <td>width</td>
        <!--      <td>(no specified value)</td>-->
        <td>auto (initial&nbsp;value)
        <td>auto
        <td>120px
        <td>120px
       <tr>
        <td>d
        <td><tt class=declaration>list-style-position: inherit</tt>
        <td>list-style-position</td>
        <!--      <td>inherit</td> -->
        <td class=say>inside
        <td>inside
        <td>inside
        <td>inside
       <tr>
        <td>e
        <td><tt class=declaration>list-style-position: initial</tt>
        <td>list-style-position</td>
        <!--      <td>initial</td>-->
        <td>outside (initial&nbsp;value)
        <td>outside
        <td>outside
        <td>outside
       <tr>
        <td>f
        <td><tt class=declaration>font-size: 1.2em</tt>
        <td>font-size</td>
        <!--      <td>1.2em</td>-->
        <td>1.2em
        <td class=say>14.1px
        <td>14.1px
        <td>14px
       <tr>
        <td>g
        <td><tt class=declaration>width: 80%</tt>
        <td>width</td>
        <!--      <td>80%</td>-->
        <td>80%
        <td>80%
        <td class=say>354.2px
        <td>354px
       <tr>
        <td>h
        <td><tt class=declaration>width: auto</tt>
        <td>width</td>
        <!--      <td>auto</td>-->
        <td>auto
        <td>auto
        <td class=say>134px
        <td>134px
       <tr>
        <td>i
        <td><tt class=declaration>height: auto</tt>
        <td>height</td>
        <!--      <td>auto</td>-->
        <td>auto
        <td>auto
        <td class=say>176px
        <td>176px
       <tr>
        <td>j
        <td>(no winning declaration)
        <td>page-break-after</td>
        <!--      <td>(no specified value)</td>-->
        <td>auto (initial&nbsp;value)
        <td>auto
        <td>auto
        <td>auto
       <tr>
        <td>k
        <td><tt class=declaration>orphans: 3</tt>
        <td>orphans</td>
        <!--      <td>3</td>-->
        <td>3
        <td>3
        <td>3
        <td>3
     </table>
     <h2 class=no-num id=acknowledgments>Acknowledgments</h2>
     <p>Comments and suggestions from Giovanni Campagna, Christoph P&auml;per,
      Keith Rarick, Alex Mogilevsky, Ian Hickson, David Baron, Edward
      Welbourne, Boris Zbarsky, Bj&ouml;rn H&ouml;hrmann and Michael Day
      improved this module. <!--
 Ian's proposal:
   http://lists.w3.org/Archives/Member/w3c-css-wg/2002OctDec/0141.html
 David's proposal
   http://lists.w3.org/Archives/Member/w3c-css-wg/2002OctDec/0191.html
 -->
     <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=CSS3CASCADE>[CSS3CASCADE]
      <dd>H&#229;kon Wium Lie. <a
       href="http://www.w3.org/TR/2005/WD-css3-cascade-20051215"><cite>CSS3
       module: Cascading and inheritance.</cite></a> 15 December 2005. W3C
       Working Draft. (Work in progress.) URL: <a
       href="http://www.w3.org/TR/2005/WD-css3-cascade-20051215">http://www.w3.org/TR/2005/WD-css3-cascade-20051215</a>
       </dd>
      <!---->
      <dt id=CSS3COL>[CSS3COL]
      <dd>H&#229;kon Wium Lie. <a
       href="http://www.w3.org/TR/2011/CR-css3-multicol-20110412"><cite>CSS
       Multi-column Layout Module.</cite></a> 12 April 2011. W3C Candidate
       Recommendation. (Work in progress.) URL: <a
       href="http://www.w3.org/TR/2011/CR-css3-multicol-20110412">http://www.w3.org/TR/2011/CR-css3-multicol-20110412</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>
      <!---->
      <dt id=CSS3GRID>[CSS3GRID]
      <dd>Alex Mogilevsky; Markus Mielke. <a
       href="http://www.w3.org/TR/2007/WD-css3-grid-20070905"><cite>CSS Grid
       Positioning Module Level&#160;3.</cite></a> 5 September 2007. W3C
       Working Draft. (Work in progress.) URL: <a
       href="http://www.w3.org/TR/2007/WD-css3-grid-20070905">http://www.w3.org/TR/2007/WD-css3-grid-20070905</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=CSS3CASCADE>[CSS3CASCADE]
      <dd>H&#229;kon Wium Lie. <a
       href="http://www.w3.org/TR/2005/WD-css3-cascade-20051215"><cite>CSS3
       module: Cascading and inheritance.</cite></a> 15 December 2005. W3C
       Working Draft. (Work in progress.) URL: <a
       href="http://www.w3.org/TR/2005/WD-css3-cascade-20051215">http://www.w3.org/TR/2005/WD-css3-cascade-20051215</a>
       </dd>
      <!---->
      <dt id=CSS3NAMESPACE>[CSS3NAMESPACE]
      <dd>Elika J. Etemad; Anne van Kesteren. <a
       href="http://www.w3.org/TR/2008/CR-css3-namespace-20080523/"><cite>CSS
       Namespaces Module.</cite></a> 23 May 2008. W3C Candidate
       Recommendation. (Work in progress.) URL: <a
       href="http://www.w3.org/TR/2008/CR-css3-namespace-20080523/">http://www.w3.org/TR/2008/CR-css3-namespace-20080523/</a>
       </dd>
      <!---->
      <dt id=URI>[URI]
      <dd>T. Berners-Lee; R. Fielding; L. Masinter. <a
       href="http://www.ietf.org/rfc/rfc3986.txt"><cite>Uniform Resource
       Identifiers (URI): generic syntax.</cite></a> January 2005. Internet
       RFC 3986. URL: <a
       href="http://www.ietf.org/rfc/rfc3986.txt">http://www.ietf.org/rfc/rfc3986.txt</a>
       </dd>
      <!---->
     </dl>
     <!--end-informative-->
     <h2 class=no-num id=index>Index</h2>
     <!--begin-index-->
     <ul class=indexlist>
      <li>absolute length units, <a href="#absolute-length-units"
       title="absolute length units"><strong>6.2.</strong></a>
      <li>actual value, <a href="#actual-value" title="actual
       value"><strong>8.4.</strong></a>
      <li>&lt;angle&gt;, <a href="#angle-value"
       title="&lt;angle&gt;"><strong>7.1.</strong></a>
      <li>calc(), <a href="#calc" title="calc()"><strong>7.3.1.</strong></a>
      <li>ch, <a href="#ch-unit" title=ch><strong>6.1.1.</strong></a>
      <li>color, <a href="#color" title=color><strong>7.4.</strong></a>
      <li>&lt;color&gt;, <a href="#ltcolorgt0"
       title="&lt;color&gt;"><strong>7.4.</strong></a>
      <li>compound value, <a href="#compound-value" title="compound
       value"><strong>3.1.</strong></a>
      <li>computed value, <a href="#computed-value" title="computed
       value"><strong>8.2.</strong></a>
      <li>deg, <a href="#deg" title=deg><strong>7.1.</strong></a>
      <li>dimension, <a href="#dimension"
       title=dimension><strong>6.</strong></a>
      <li>em, <a href="#em-unit" title=em><strong>6.1.1.</strong></a>
      <li>ex, <a href="#ex-unit" title=ex><strong>6.1.1.</strong></a>
      <li>&lt;frequency&gt;, <a href="#frequency-value"
       title="&lt;frequency&gt;"><strong>7.3.</strong></a>
      <li>fr (unit), <a href="#fr" title="fr
       (unit)"><strong>7.7.1.</strong></a>
      <li>functional notation, <a href="#functional-notation0"
       title="functional notation"><strong>3.1.</strong></a>
      <li>grad, <a href="#grad" title=grad><strong>7.1.</strong></a>
      <li>gr (unit), <a href="#gr" title="gr
       (unit)"><strong>7.8.1.</strong></a>
      <li>Hz, <a href="#hz" title=Hz><strong>7.3.</strong></a>
      <li>identifier, <a href="#identifier"
       title=identifier><strong>4.</strong></a>
      <li><a href="#identifier-value"><code>&lt;identifier&gt;</code></a>, <a
       href="#identifier-value"
       title="&lt;identifier&gt;"><strong>4.2.</strong></a>
      <li>&lsquo;<code class=css>inherit</code>&rsquo;, <a href="#inherit"
       title="''inherit''"><strong>4.1.1.</strong></a>
      <li>&lsquo;<code class=css>initial</code>&rsquo;, <a href="#initial"
       title="''initial''"><strong>4.1.1.</strong></a>
      <li>integer, <a href="#integer" title=integer><strong>5.1.</strong></a>
      <li><a href="#integer-value"><code>&lt;integer&gt;</code></a>, <a
       href="#integer-value" title="&lt;integer&gt;"><strong>5.1.</strong></a>
      <li>kHz, <a href="#khz" title=kHz><strong>7.3.</strong></a>
      <li><a href="#length-value"><code>&lt;length&gt;</code></a>, <a
       href="#length-value" title="&lt;length&gt;"><strong>6.</strong></a>
      <li>max(), <a href="#max" title="max()"><strong>7.3.1.</strong></a>
      <li>min(), <a href="#min" title="min()"><strong>7.3.1.</strong></a>
      <li>ms, <a href="#ms" title=ms><strong>7.2.</strong></a>
      <li>number, <a href="#number" title=number><strong>5.2.</strong></a>
      <li><a href="#number-value"><code>&lt;number&gt;</code></a>, <a
       href="#number-value" title="&lt;number&gt;"><strong>5.2.</strong></a>
      <li><a href="#percentage-value"><code>&lt;percentage&gt;</code></a>, <a
       href="#percentage-value"
       title="&lt;percentage&gt;"><strong>5.3.</strong></a>
      <li>rad, <a href="#rad" title=rad><strong>7.1.</strong></a>
      <li>reference pixel, <a href="#reference-pixel" title="reference
       pixel"><strong>6.2.</strong></a>
      <li>relative length, <a href="#relative-length-units" title="relative
       length"><strong>6.1.</strong></a>
      <li>rem, <a href="#rem-unit" title=rem><strong>6.1.1.</strong></a>
      <li>s, <a href="#s" title=s><strong>7.2.</strong></a>
      <li>simple value, <a href="#simple-value" title="simple
       value"><strong>3.1.</strong></a>
      <li>specified value, <a href="#specified-value" title="specified
       value"><strong>8.1.</strong></a>
      <li><a href="#string-value"><code>&lt;string&gt;</code></a>, <a
       href="#string-value" title="&lt;string&gt;"><strong>4.3.</strong></a>
      <li>&lt;time&gt;, <a href="#time-value"
       title="&lt;time&gt;"><strong>7.2.</strong></a>
      <li>turn, <a href="#turn" title=turn><strong>7.1.</strong></a>
      <li>URL, <a href="#url" title=URL><strong>4.4.</strong></a>
      <li><a href="#url-value"><code>&lt;url&gt;</code></a>, <a
       href="#url-value" title="&lt;url&gt;"><strong>4.4.</strong></a>
      <li>used value, <a href="#used-value" title="used
       value"><strong>8.3.</strong></a>
      <li>vh, <a href="#vh-unit" title=vh><strong>6.1.2.</strong></a>
      <li>vm, <a href="#vm-unit" title=vm><strong>6.1.2.</strong></a>
      <li>vw, <a href="#vw-unit" title=vw><strong>6.1.2.</strong></a>
     </ul>
     <!--end-index-->
   </dl>

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