csswg-drafts/css-fonts/Fonts.html at ed5c925e83831fcf2e8ae97d6ed41c9c0d884127 · 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><meta content="text/html;charset=utf-8" http-equiv=Content-Type>

  <title>CSS Fonts Module Level 3</title>
  <!--
      FIXME when publishing: copy the current default.css and link to
      "default.css" rather than "../default.css"
   -->
  <link href="../default.css" rel=stylesheet type="text/css">
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
  <!-- I'm just experimenting here, don't get your bee in a bonnet -->
  <script src="http://use.typekit.com/xon2bky.js"
  type="text/javascript"></script>
  <script type="text/javascript">try{Typekit.load();}catch(e){}</script>

  <style type="text/css">

  body, th, td, h1, h2, h3, h4, h5, h6 {
    font-family: "myriad-pro", sans-serif !important;
  }

  body {
    padding: 2em 70px 2em 70px;
  }

  p + p, p.mtb {
    margin-top: 0.8em;
    text-indent: 0px;
  }

  #bolderlighter {
    width: 40%;
  }

  #bolderlighter th {
    text-align: center;
  }

  #fontformats td, #eventhandlers td, #fontformats th, #eventhandlers th {
    padding-right: 2em;
    text-align: left;
  }

  dd {
    margin-bottom: 1em;
  }

  #authors dd {
    margin-bottom: 0;
  }

  #fontstylematchingalg {
    list-style-type: lower-alpha;
  }

  #fontmatchingalg ul, #fontmatchingalg ol {
    margin-top: 0.8em;
  }

  #fontmatchingalg li + li {
    margin-top: 0.8em;
  }

  div.example {
    padding: 1em;
    margin-top: 1em;
  }

  div.example + div.example {
    margin-top: 2em;
  }

  div.figure {
    page-break-inside: avoid;
  }

  pre.prod { white-space: pre-wrap; margin: 1em 0 1em 2em }

  div.featex {
    width: 700px;
  }

  div.featex img {
    margin: auto;
    display: block;
  }

  span.tag {
    font-family: monospace;
    font-size: 120%;
  }

  ol ol {
    list-style-type: lower-alpha;
  }

  .idl-code {
    font-weight: bold;
    color: #c50;
  }
  </style>
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
  type="text/css">
  <script defer=defer
  src="http://test.csswg.org/harness/annotate.js#CSS3-FONTS_DEV"
  type="text/javascript"></script>

 <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>CSS Fonts Module Level 3</h1>

   <h2 class="no-num no-toc" id=editors-draft-11-june-2013>Editor's Draft 11
    June 2013</h2>

   <dl id=authors>
    <dt>This version:

    <dd><a
     href="http://dev.w3.org/csswg/css-fonts/">http://dev.w3.org/csswg/css-fonts/</a>
     <!-- <dd><a href="http://www.w3.org/TR/2013/ED-css3-fonts-20130611/">http://www.w3.org/TR/2013/ED-css3-fonts-20130611/</a> -->


    <dt>Latest version:

    <dd><a
     href="http://www.w3.org/TR/css3-fonts/">http://www.w3.org/TR/css3-fonts/</a>

    <dt>Latest editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css-fonts/">http://dev.w3.org/csswg/css-fonts/</a>
     (<a
     href="https://dvcs.w3.org/hg/csswg/log/tip/css-fonts/Fonts.html">change
     log</a>)

    <dt>Previous version:

    <dd><a
     href="http://www.w3.org/TR/2013/WD-css3-fonts-20130212/">http://www.w3.org/TR/2013/WD-css3-fonts-20130212/</a>

    <dt>Issues List:

    <dd><a href="http://www.w3.org/Style/CSS/Tracker/products/18">CSS3 Fonts
     issues in Tracker</a>

    <dd><a
     href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;product=CSS&amp;component=Fonts&amp;bug_status=NEW&amp;bug_status=ASSIGNED&amp;bug_status=REOPENED">CSS3
     Fonts issues in Bugzilla</a>

    <dt>Discussion:

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

    <dt>Editor:

    <dd><a href="mailto:jdaggett@mozilla.com">John Daggett (Mozilla)</a>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> © 2013 <a href="http://www.w3.org/"><abbr
    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
    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 how font properties are specified and how
   font resources are loaded dynamically. The contents of this specification
   are a consolidation of content previously divided into <a
   href="http://www.w3.org/TR/2002/WD-css3-fonts-20020802/">CSS3 Fonts</a>
   and <a href="http://www.w3.org/TR/2002/WD-css3-webfonts-20020802/">CSS3
   Web Fonts</a> modules. The description of font load events was moved into
   the <a href="http://dev.w3.org/csswg/css3-font-load-events/">CSS3 Font
   Load Events</a> module.

  <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-fonts%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
   “css3-fonts” in the subject, preferably like this:
   “[<!---->css3-fonts<!---->] <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-->

  <h3 class="no-num no-toc" id=atrisk>Features at risk</h3>

  <p>The following features are at risk and may be removed when exiting CR:

  <ul>
   <li>fallback handling for text decoration in superscript/subscript variant
    glyphs (‘<a href="#propdef-font-variant-position"><code
    class=property>font-variant-position</code></a>’)

   <li>‘<a href="#propdef-font-language-override"><code
    class=property>font-language-override</code></a>’ property
  </ul>

  <p>This draft is a <strong>Last Call Working Draft</strong>. Please send
   comments to the <a
   href="http://lists.w3.org/Archives/Public/www-style/">www-style mailing
   list</a> as described above. The <strong>deadline for comments</strong> is
   six weeks after the date of publication in the header.

  <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>

   <li><a href="#typography-background"><span class=secno>2 </span>Typography
    Background</a>

   <li><a href="#basic-font-props"><span class=secno>3 </span>Basic Font
    Properties</a>
    <ul class=toc>
     <li><a href="#font-family-prop"><span class=secno>3.1 </span>Font
      family: the font-family property</a>
      <ul class=toc>
       <li><a href="#generic-font-families"><span class=secno>3.1.1
        </span>Generic font families</a>
      </ul>

     <li><a href="#font-weight-prop"><span class=secno>3.2 </span>Font
      weight: the font-weight property</a>

     <li><a href="#font-stretch-prop"><span class=secno>3.3 </span>Font
      width: the font-stretch property</a>

     <li><a href="#font-style-prop"><span class=secno>3.4 </span>Font style:
      the font-style property</a>

     <li><a href="#font-size-prop"><span class=secno>3.5 </span>Font size:
      the font-size property</a>

     <li><a href="#font-size-adjust-prop"><span class=secno>3.6
      </span>Relative sizing: the font-size-adjust property</a>

     <li><a href="#font-prop"><span class=secno>3.7 </span>Shorthand font
      property: the font property</a>

     <li><a href="#font-synthesis-prop"><span class=secno>3.8
      </span>Controlling synthetic faces: the font-synthesis property</a>
    </ul>

   <li><a href="#font-resources"><span class=secno>4 </span>Font
    Resources</a>
    <ul class=toc>
     <li><a href="#font-face-rule"><span class=secno>4.1 </span>The
      <code>@font-face</code> rule</a>

     <li><a href="#font-family-desc"><span class=secno>4.2 </span>Font
      family: the font-family descriptor</a>

     <li><a href="#src-desc"><span class=secno>4.3 </span>Font reference: the
      src descriptor</a>

     <li><a href="#font-prop-desc"><span class=secno>4.4 </span>Font property
      descriptors: the font-style, font-weight, font-stretch descriptors</a>

     <li><a href="#unicode-range-desc"><span class=secno>4.5 </span>Character
      range: the unicode-range descriptor</a>

     <li><a href="#composite-fonts"><span class=secno>4.6 </span>Using
      character ranges to define composite fonts</a>

     <li><a href="#font-rend-desc"><span class=secno>4.7 </span>Font
      features: the font-variant and font-feature-settings descriptors</a>

     <li><a href="#font-face-loading"><span class=secno>4.8 </span>Font
      loading guidelines</a>

     <li><a href="#same-origin-restriction"><span class=secno>4.9
      </span>Same-origin restriction for fonts</a>
      <ul class=toc>
       <li><a href="#default-same-origin-restriction"><span class=secno>4.9.1
        </span>Default same-origin restriction</a>

       <li><a href="#allowing-cross-origin-font-loading"><span
        class=secno>4.9.2 </span>Allowing cross-origin font loading</a>
      </ul>
    </ul>

   <li><a href="#font-matching-algorithm"><span class=secno>5 </span>Font
    Matching Algorithm</a>
    <ul class=toc>
     <li><a href="#font-family-casing"><span class=secno>5.1 </span>Case
      sensitivity of font family names</a>

     <li><a href="#font-style-matching"><span class=secno>5.2 </span>Matching
      font styles</a>

     <li><a href="#cluster-matching"><span class=secno>5.3 </span>Cluster
      matching</a>

     <li><a href="#char-handling-issues"><span class=secno>5.4
      </span>Character handling issues</a>

     <li><a href="#font-matching-changes"><span class=secno>5.5 </span>Font
      matching changes since CSS 2.1</a>

     <li><a href="#font-matching-examples"><span class=secno>5.6 </span>Font
      matching examples</a>
    </ul>

   <li><a href="#font-rend-props"><span class=secno>6 </span>Font Feature
    Properties</a>
    <ul class=toc>
     <li><a href="#glyph-selection-positioning"><span class=secno>6.1
      </span>Glyph selection and positioning</a>

     <li><a href="#language-specific-support"><span class=secno>6.2
      </span>Language-specific display</a>

     <li><a href="#font-kerning-prop"><span class=secno>6.3 </span>Kerning:
      the font-kerning property</a>

     <li><a href="#font-variant-ligatures-prop"><span class=secno>6.4
      </span>Ligatures: the font-variant-ligatures property</a>

     <li><a href="#font-variant-position-prop"><span class=secno>6.5
      </span>Subscript and superscript forms: the font-variant-position
      property</a>

     <li><a href="#font-variant-caps-prop"><span class=secno>6.6
      </span>Capitalization: the font-variant-caps property</a>

     <li><a href="#font-variant-numeric-prop"><span class=secno>6.7
      </span>Numerical formatting: the font-variant-numeric property</a>

     <li><a href="#font-variant-alternates-prop"><span class=secno>6.8
      </span>Alternates and swashes: the font-variant-alternates property</a>


     <li><a href="#font-feature-values"><span class=secno>6.9 </span>Defining
      font specific alternates: the <code>@font-feature-values</code>
      rule</a>
      <ul class=toc>
       <li><a href="#basic-syntax"><span class=secno>6.9.1 </span>Basic
        syntax</a>

       <li><a href="#multi-valued-feature-value-definitions"><span
        class=secno>6.9.2 </span>Multi-valued feature value definitions</a>
      </ul>

     <li><a href="#font-variant-east-asian-prop"><span class=secno>6.10
      </span>East Asian text rendering: the font-variant-east-asian
      property</a>

     <li><a href="#font-variant-prop"><span class=secno>6.11 </span>Overall
      shorthand for font rendering: the font-variant property</a>

     <li><a href="#font-feature-settings-prop"><span class=secno>6.12
      </span>Low-level font feature settings control: the
      font-feature-settings property</a>

     <li><a href="#font-language-override-prop"><span class=secno>6.13
      </span>Font language override: the font-language-override property</a>
    </ul>

   <li><a href="#font-feature-resolution"><span class=secno>7 </span>Font
    Feature Resolution </a>
    <ul class=toc>
     <li><a href="#default-features"><span class=secno>7.1 </span>Default
      features</a>

     <li><a href="#feature-precedence"><span class=secno>7.2 </span>Feature
      precedence</a>

     <li><a href="#feature-precedence-examples"><span class=secno>7.3
      </span>Feature precedence examples</a>
    </ul>

   <li><a href="#object-model"><span class=secno>8 </span>Object Model</a>
    <ul class=toc>
     <li><a href="#om-fontface"><span class=secno>8.1 </span>The
      <code>CSSFontFaceRule</code> interface</a>

     <li><a href="#om-fontfeaturevalues"><span class=secno>8.2 </span>The
      <code>CSSFontFeatureValuesRule</code> interface</a>
    </ul>

   <li class=no-num><a href="#platform-props-to-css">Appendix A: Mapping
    platform font properties to CSS properties</a>

   <li class=no-num><a href="#ch-ch-ch-changes">Changes</a>
    <ul class=toc>
     <li class=no-num><a href="#recent-changes"> Changes from the February
      2013 CSS3 Fonts Working Draft</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>
    </ul>

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

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

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

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

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

  <p>A font provides a resource containing the visual representation of
   characters. At the simplest level it contains information that maps
   character codes to shapes (called glyphs) that represent these characters.
   Fonts sharing a common design style are commonly grouped into font
   families classified by a set of standard font properties. Within a family,
   the shape displayed for a given character can vary by stroke weight, slant
   or relative width, among others. An individual font face is described by a
   unique combination of these properties. For a given range of text, CSS
   font properties are used to select a font family and a specific font face
   within that family to be used when rendering that text. As a simple
   example, to use the bold form of Helvetica one could use:

  <pre>body {
    font-family: Helvetica;
    font-weight: bold;
}</pre>

  <p>Font resources may be installed locally on the system on which a user
   agent is running or downloadable. For local font resources descriptive
   information can be obtained directly from the font resource. For
   downloadable font resources (sometimes referred to as web fonts), the
   descriptive information is included with the reference to the font
   resource.

  <p>Families of fonts typically don't contain a single face for each
   possible variation of font properties. The CSS font selection mechanism
   describes how to match a given set of CSS font properties to a single font
   face.

  <h2 id=typography-background><span class=secno>2 </span>Typography
   Background</h2>

  <p><em>This section is non-normative.</em>

  <p> Typographic traditions vary across the globe, so there is no unique way
   to classify all fonts across languages and cultures. For even common Latin
   letters, wide variations are possible:

  <div class=figure><img alt="variations in glyphs for a single character"
   src=aaaaaa.png>
   <p class=caption>One character, many glyph variations
  </div>

  <p>Differences in the anatomy of letterforms is one way to distinguish
   fonts. For Latin fonts, flourishes at the ends of a character's main
   strokes, or serifs, can distinguish a font from those without. Similar
   comparisons exist in non-Latin fonts between fonts with tapered strokes
   and those using primarily uniform strokes:

  <div class=figure><img alt="serif vs. non-serifs" src=serifvssansserif.png>
   <p class=caption>Letterforms with and without serifs
  </div>

  <div class=figure><img alt="serif vs. non-serifs for japanese"
   src=minchovsgothic.png>
   <p class=caption>Similar groupings for Japanese typefaces
  </div>

  <p>Fonts contain letterforms and the data needed to map characters to these
   letterforms. Often this may be a simple one-to-one mapping, but more
   complex mappings are also possible. The use of combining diacritic marks
   creates many variations for an underlying letterform:

  <div class=figure><img alt="diacritic marks" src=aaaaaa-diacritics.png>
   <p class=caption>Variations with diacritic marks
  </div>

  <p>A sequence of characters can be represented by a single glyph known as a
   ligature:

  <div class=figure><img alt="example of a fi ligature"
   src=final-ligature.png>
   <p class=caption>Ligature example
  </div>

  <p>Visual transformations based on textual context are often stylistic
   option in European languages. They are required to correctly render
   languages like Arabic, the lam and alef characters below <em>must</em> be
   combined when they exist in sequence:

  <div class=figure><img alt="lam alef ligature" src=lamaleflig.png>
   <p class=caption>Required Arabic ligature
  </div>

  <p>The relative complexity of these shaping transformations requires
   additional data within the font.

  <p>Sets of font faces with various stylistic variations are often grouped
   together into font families. In the simplest case a regular face is
   supplemented with bold and italic faces, but much more extensive groupings
   are possible. Variations in the thickness of letterform strokes, the <dfn
   id=weight>weight</dfn>, and the overall proportions of the letterform, the
   <dfn id=width>width</dfn>, are most common. In the example below, each
   letter uses a different font face within the Univers font family. The
   width used increases from top to bottom and the weight increases from left
   to right:

  <div class=figure><img alt="various width and weight variations within a
   single family" src=weightwidthvariations.png>
   <p class=caption>Weight and width variations within a single font family
  </div>

  <p>Creating fonts that support multiple scripts is a difficult task;
   designers need to understand the cultural traditions surrounding the use
   of type in different scripts and come up with letterforms that somehow
   share a common theme. Many languages often share a common script and each
   of these languages may have noticeable stylistic differences. For example,
   the Arabic script is shared by Persian and Urdu and Cyrillic is used with
   many languages, not just Russian.

  <p>The <a href="#character-map"><em>character map</em></a> of a font
   defines the mapping of characters to glyphs for that font. If a document
   contains characters not supported by the <a href="#character-map"><em
   title="character map">character maps</em></a> of the fonts contained in a
   font family list, a user agent may use a <a
   href="#system-font-fallback"><em>system font fallback</em></a> procedure
   to locate an appropriate font that does. If no appropriate font can be
   found, some form of "missing glyph" character will be rendered by the user
   agent. System fallback can occur when the specified list of font families
   does not include a font that supports a given character.

  <p>Although the <a href="#character-map"><em>character map</em></a> of a
   font maps a given character to a glyph for that character, modern font
   technologies such as OpenType and AAT (Apple Advanced Typography) provide
   ways of mapping a character to different glyphs based upon feature
   settings. Fonts in these formats allow these features to be embedded in
   the font itself and controlled by applications. Common typographic
   features which can be specified this way include ligatures, swashes,
   contextual alternates, proportional and tabular figures, and automatic
   fractions, to list just a few. For a visual overview of OpenType features,
   see the <a href="#OPENTYPE-FONT-GUIDE"
   rel=biblioentry>[OPENTYPE-FONT-GUIDE]<!--{{OPENTYPE-FONT-GUIDE}}--></a>.

  <h2 id=basic-font-props><span class=secno>3 </span>Basic Font Properties</h2>

  <p>The particular font face used to render a character is determined by the
   font family and other font properties that apply to a given element. This
   structure allows settings to be varied independent of each other.</p>
  <!-- prop: font-family -->

  <h3 id=font-family-prop><span class=secno>3.1 </span>Font family: the <a
   href="#propdef-font-family">font-family</a> property</h3>

  <table class=propdef id=namefont-familyvalue-ltfamily-namegt-ltg>
   <tbody>
    <tr>
     <td>Name:

     <td><dfn id=propdef-font-family
      title="font-family!!property">font-family</dfn>

    <tr>
     <td>Value:

     <td>[ <a href="#family-name-value"><var>&lt;family-name&gt;</var></a> |
      <a href="#generic-family-value"><var>&lt;generic-family&gt;</var></a> ]
      #

    <tr>
     <td>Initial:

     <td>depends on user agent

    <tr>
     <td>Applies to:

     <td>all elements

    <tr>
     <td>Inherited:

     <td>yes

    <tr>
     <td>Percentages:

     <td>N/A

    <tr>
     <td>Media:

     <td>visual

    <tr>
     <td>Computed value:

     <td>as specified

    <tr>
     <td>Animatable:

     <td>no
  </table>

  <p>This property specifies a prioritized list of font family names or
   generic family names. A font family defines a set of faces that vary in
   weight, width or slope. CSS uses the combination of a family name with
   other style attributes to select an individual face. Using this selection
   mechanism, rather than selecting a face via the style name as is often
   done in design applications, allows some degree of regularity in textual
   display when fallback occurs.

  <p class=note>Designers should note that the CSS definition of font
   attributes used for selection are explicitly not intended to define a font
   taxonomy. A type designer's idea of a family may often extend to a set of
   faces that vary along axes other than just the standard axes of weight,
   width and slope. A family may extend to include both a set of serif faces
   and a set of sans-serif faces or vary along axes that are unique to that
   family. The CSS font selection mechanism merely provides a way to
   determine the “closest” substitute when substitution is necessary.

  <p>Unlike other CSS properties, component values are a comma-separated list
   indicating alternatives. A user agent iterates through the list of family
   names until it matches an available font that contains a glyph for the
   character to be rendered. This allows for differences in available fonts
   across platforms and for differences in the range of characters supported
   by individual fonts.

  <p>A font family name only specifies a name given to a set of font faces,
   it does not specify an individual face. For example, given the
   availability of the fonts below, Futura would match but Futura Medium
   would not:

  <div class=figure><img alt="family and face names"
   src=familyvsfacename.png>
   <p class=caption>Family and individual face names
  </div>

  <p>Consider the example below:

  <div class=example>
   <pre>body {
    font-family: Helvetica, Verdana, sans-serif;
}</pre>

   <p>If Helvetica is available it will be used when rendering. If neither
    Helvetica nor Verdana is present, then the user-agent-defined sans serif
    font will be used.</p>
  </div>

  <p>There are two types of font family names:

  <dl>
   <dt><dfn id=family-name-value><var>&lt;family-name&gt;</var></dfn>

   <dd>The name of a font family of choice such as Helvetica or Verdana in
    the previous example.

   <dt><dfn id=generic-family-value><var>&lt;generic-family&gt;</var></dfn>

   <dd> The following generic family keywords are defined: ‘<a
    href="#serif"><code class=property>serif</code></a>’, ‘<a
    href="#sans-serif"><code class=property>sans-serif</code></a>’, ‘<a
    href="#cursive"><code class=property>cursive</code></a>’, ‘<a
    href="#fantasy"><code class=property>fantasy</code></a>’, and ‘<a
    href="#monospace"><code class=property>monospace</code></a>’. These
    keywords can be used as a general fallback mechanism when an author's
    desired font choices are not available. As keywords, they must not be
    quoted. Authors are encouraged to append a generic font family as a last
    alternative for improved robustness.
  </dl>

  <p>Font family names other than generic families must either be given
   quoted as <a
   href="//www.w3.org/TR/CSS21/syndata.html#strings">strings,</a> or unquoted
   as a sequence of one or more <a
   href="//www.w3.org/TR/CSS21/syndata.html#value-def-identifier">identifiers.</a>
   This means most punctuation characters and digits at the start of each
   token must be escaped in unquoted font family names.

  <p>To illustrate this, the following declarations are invalid:

  <pre>
font-family: Red/Black, sans-serif;
font-family: "Lucida" Grande, sans-serif;
font-family: Ahem!, sans-serif;
font-family: test@foo, sans-serif;
font-family: #POUND, sans-serif;
font-family: Hawaii 5-0, sans-serif;
</pre>

  <p>If a sequence of identifiers is given as a font family name, the
   computed value is the name converted to a string by joining all the
   identifiers in the sequence by single spaces.

  <p>To avoid mistakes in escaping, it is recommended to quote font family
   names that contain white space, digits, or punctuation characters other
   than hyphens:

  <pre>
body { font-family: "New Century Schoolbook", serif }

&lt;BODY STYLE="font-family: '21st Century', fantasy">
</pre>

  <p>Font family <em>names</em> that happen to be the same as a keyword value
   (‘<code class=property>inherit</code>’, ‘<a href="#serif"><code
   class=property>serif</code></a>’, ‘<a href="#sans-serif"><code
   class=property>sans-serif</code></a>’, ‘<a href="#monospace"><code
   class=property>monospace</code></a>’, ‘<a href="#fantasy"><code
   class=property>fantasy</code></a>’, and ‘<a href="#cursive"><code
   class=property>cursive</code></a>’) must be quoted to prevent confusion
   with the keywords with the same names. The keywords ‘<code
   class=property>initial</code>’ and ‘<code
   class=property>default</code>’ are reserved for future use and must also
   be quoted when used as font names. UAs must not consider these keywords as
   matching the <a
   href="#family-name-value"><var>&lt;family-name&gt;</var></a> type.

  <p>The precise way a set of fonts are grouped into font families varies
   depending upon the platform font management API's. The Windows GDI API
   only allows four faces to be grouped into a family while the DirectWrite
   API and API's on OSX and other platforms support font families with a
   variety of weights, widths and slopes (see <a
   href="#platform-props-to-css">Appendix A</a> for more details).

  <p>Some font formats allow fonts to carry multiple localizations of the
   family name. User agents must recognize and correctly match all of these
   names independent of the underlying platform localization, system API used
   or document encoding:

  <div class=figure><img alt="examples of localized family names"
   src=localizedfamilynames.png>
   <p class=caption>Localized family names
  </div>

  <h4 id=generic-font-families><span class=secno>3.1.1 </span>Generic font
   families</h4>

  <p>All five generic font families are defined to exist in all CSS
   implementations (they need not necessarily map to five distinct actual
   fonts). User agents should provide reasonable default choices for the
   generic font families, which express the characteristics of each family as
   well as possible within the limits allowed by the underlying technology.
   User agents are encouraged to allow users to select alternative choices
   for the generic fonts.

  <h5 class="no-num no-toc"> <span class=index-def id=serif0 title="serif,
   definition of"><a name=serif-def><dfn id=serif>serif</dfn></a></span></h5>

  <p>Serif fonts represent the formal text style for a script. This often
   means but is not limited to glyphs that have finishing strokes, flared or
   tapering ends, or have actual serifed endings (including slab serifs).
   Serif fonts are typically proportionately-spaced. They often display a
   greater variation between thick and thin strokes than fonts from the ‘<a
   href="#sans-serif"><code class=property>sans-serif</code></a>’ generic
   font family. CSS uses the term ‘<a href="#serif"><code
   class=property>serif</code></a>’ to apply to a font for any script,
   although other names may be more familiar for particular scripts, such as
   Mincho (Japanese), Sung, Song or Kai (Chinese), Batang (Korean). For
   Arabic, the Naskh style would correspond to ‘<a href="#serif"><code
   class=property>serif</code></a>’ more due to its typographic role rather
   than its actual design style. Any font that is so described may be used to
   represent the generic ‘<a href="#serif"><code
   class=property>serif</code></a>’ family.

  <div class=figure><img alt="sample serif fonts" src=serifexamples.png>
   <p class=caption>Sample serif fonts
  </div>

  <h5 class="no-num no-toc"> <span class=index-def id=sans-serif0
   title="sans-serif, definition of"> <a name=sans-serif-def><dfn
   id=sans-serif>sans-serif</dfn></a></span></h5>

  <p>Glyphs in sans-serif fonts, as the term is used in CSS, are generally
   low contrast (vertical and horizontal stems have the close to the same
   thickness) and have stroke endings that are plain -- without any flaring,
   cross stroke, or other ornamentation. Sans-serif fonts are typically
   proportionately-spaced. They often have little variation between thick and
   thin strokes, compared to fonts from the ‘<a href="#serif"><code
   class=property>serif</code></a>’ family. CSS uses the term ‘<a
   href="#sans-serif"><code class=property>sans-serif</code></a>’ to apply
   to a font for any script, although other names may be more familiar for
   particular scripts, such as Gothic (Japanese), Hei (Chinese), or Gulim
   (Korean). Any font that is so described may be used to represent the
   generic ‘<a href="#sans-serif"><code
   class=property>sans-serif</code></a>’ family.

  <div class=figure><img alt="sample sans-serif fonts"
   src=sansserifexamples.png>
   <p class=caption>Sample sans-serif fonts
  </div>

  <h5 class="no-num no-toc"> <span class=index-def id=cursive0
   title="cursive, definition of"> <a name=cursive-def><dfn
   id=cursive>cursive</dfn></a></span></h5>

  <p>Glyphs in cursive fonts generally use a more informal script style, and
   the result looks more like handwritten pen or brush writing than printed
   letterwork. CSS uses the term ‘<a href="#cursive"><code
   class=property>cursive</code></a>’ to apply to a font for any script,
   although other names such as Chancery, Brush, Swing and Script are also
   used in font names.

  <div class=figure><img alt="sample cursive fonts" src=cursiveexamples.png>
   <p class=caption>Sample cursive fonts
  </div>

  <h5 class="no-num no-toc"> <span class=index-def id=fantasy0
   title="fantasy, definition of"> <a name=fantasy-def><dfn
   id=fantasy>fantasy</dfn></a></span></h5>

  <p>Fantasy fonts are primarily decorative or expressive fonts that contain
   playful representations of characters. These do not include Pi or Picture
   fonts which do not represent actual characters.

  <div class=figure><img alt="sample fantasy fonts" src=fantasyexamples.png>
   <p class=caption>Sample fantasy fonts
  </div>

  <h5 class="no-num no-toc"> <span class=index-def id=monospace0
   title="monospace, definition of"> <a name=monospace-def><dfn
   id=monospace>monospace</dfn></a></span></h5>

  <p>The sole criterion of a monospace font is that all glyphs have the same
   fixed width. This is often used to render samples of computer code.

  <div class=figure><img alt="sample monospace fonts"
   src=monospaceexamples.png>
   <p class=caption>Sample monospace fonts
  </div>
  <!-- prop: font-weight -->

  <h3 id=font-weight-prop><span class=secno>3.2 </span>Font weight: the <a
   href="#propdef-font-weight">font-weight</a> property</h3>

  <table class=propdef id=namefont-weightvaluenormal-bold-bolder-l>
   <tbody>
    <tr>
     <td>Name:

     <td><dfn id=propdef-font-weight
      title="font-weight!!property">font-weight</dfn>

    <tr>
     <td>Value:

     <td><a href="#font-weight-normal-value"
      title="normal!!font-weight">normal</a> | <a href="#bold">bold</a> | <a
      href="#bolder">bolder</a> | <a href="#lighter">lighter</a> | <a
      href="#font-weight-numeric-values" title="100...900 weight
      values">100</a> | <a href="#font-weight-numeric-values"
      title="100...900 weight values">200</a> | <a
      href="#font-weight-numeric-values" title="100...900 weight
      values">300</a> | <a href="#font-weight-numeric-values"
      title="100...900 weight values">400</a> | <a
      href="#font-weight-numeric-values" title="100...900 weight
      values">500</a> | <a href="#font-weight-numeric-values"
      title="100...900 weight values">600</a> | <a
      href="#font-weight-numeric-values" title="100...900 weight
      values">700</a> | <a href="#font-weight-numeric-values"
      title="100...900 weight values">800</a> | <a
      href="#font-weight-numeric-values" title="100...900 weight
      values">900</a>

    <tr>
     <td>Initial:

     <td>normal

    <tr>
     <td>Applies to:

     <td>all elements

    <tr>
     <td>Inherited:

     <td>yes

    <tr>
     <td>Percentages:

     <td>N/A

    <tr>
     <td>Media:

     <td>visual

    <tr>
     <td>Computed value:

     <td>numeric weight value (see description)

    <tr>
     <td>Animatable:

     <td>as <a
      href="http://dev.w3.org/csswg/css3-transitions/#animtype-font-weight">font
      weight</a>
  </table>

  <p>The <a href="#propdef-font-weight"
   title="font-weight!!property">‘<code
   class=property>font-weight</code>’</a> property specifies the weight of
   glyphs in the font, their degree of blackness or stroke thickness.

  <p>Values have the following meanings:

  <dl>
   <dt><dfn id=font-weight-numeric-values title="100...900 weight values">100
    to 900</dfn>

   <dd>These values form an ordered sequence, where each number indicates a
    weight that is at least as dark as its predecessor. These roughly
    correspond to the commonly used weight names below:
  </dl>

  <ul>
   <li>100 - Thin

   <li>200 - Extra Light (Ultra Light)

   <li>300 - Light

   <li>400 - Normal

   <li>500 - Medium

   <li>600 - Semi Bold (Demi Bold)

   <li>700 - Bold