csswg-drafts/css-fonts-3/Overview.html at 53e43f58fd6af5b09034785c9e3afb1713ad57af · w3c/csswg-drafts · GitHub

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<!DOCTYPE html>
<html lang=en>
 <head>
  <meta content="text/html;charset=utf-8" http-equiv=Content-Type>
  <title>CSS Fonts Module Level 3</title>

  <link href="https://www.w3.org/TR/2018/CR-css-fonts-3-20180626/" rel=canonical>
   <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

  <link href="default.css" rel=stylesheet type="text/css">

  <style type="text/css">

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

  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;
  }

  #fonts ul.indexlist ul, #fonts ul.indexlist dl { font-size: inherit; }

  </style>

  <link rel="stylesheet" type="text/css" href="https://www.w3.org/StyleSheets/TR/2016/W3C-CR" />

  <script defer=defer
   src="https://test.csswg.org/harness/annotate.js#CSS3-FONTS_DEV"
   type="text/javascript"></script>


  <div class=head> <!--begin-logo-->
   <p><a href="https://www.w3.org/"><img alt=W3C height=48
    src="https://www.w3.org/StyleSheets/TR/2016/logos/W3C" width=72></a>
    <!--end-logo-->

   <h1>CSS Fonts Module Level 3</h1>

   <h2 class="no-num no-toc">W3C Candidate Recommendation 26 June 2018</h2>

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

    <dd><a
     href="https://drafts.csswg.org/css-fonts/">https://drafts.csswg.org/css-fonts/</a>

     <!-- <dd><a href="https://www.w3.org/TR/2018/CR-css-fonts-3-20180626/">https://www.w3.org/TR/2018/CR-css-fonts-3-20180626/</a> -->

    <dt>Latest version:

    <dd><a
     href="https://www.w3.org/TR/css-fonts-3/">https://www.w3.org/TR/css-fonts-3/</a>

    <dt>Latest editor's draft:
    <dd><a
     href="https://drafts.csswg.org/css-fonts/">https://drafts.csswg.org/css-fonts/</a>

    <dt>Previous versions:

    <dd><a
     href="https://www.w3.org/TR/2018/CR-css-fonts-3-20180315/">https://www.w3.org/TR/2018/CR-css-fonts-3-20180315/</a>

    <dd><a
     href="https://www.w3.org/TR/2013/CR-css-fonts-3-20131003/">https://www.w3.org/TR/2013/CR-css-fonts-3-20131003/</a>

    <dt>Issues List:

    <dd><a
     href="https://github.com/w3c/csswg-drafts/labels/css-fonts-3">css-fonts-3
     issues in github</a>

    <dt>Discussion:

    <dd> <a href="https://github.com/w3c/csswg-drafts/labels/css-fonts-3">on
     GitHub</a> (preferred), or <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="https://lists.w3.org/Archives/Public/www-style/"
     rel=discussion>archives</a>)

    <dt>Test Suite:

    <dd><a
     href="https://test.csswg.org/harness/results/css-fonts-3_dev/grouped/">https://test.csswg.org/harness/results/css-fonts-3_dev/grouped/</a>

    <dt>Editors:

    <dd><a href="https://twitter.com/nattokirai">John Daggett (Invited
     Expert)</a>

    <dd><a href="mailto:mmaxfield@apple.com">Myles C. Maxfield (Apple
     Inc.)</a>

    <dd><a href="http://svgees.us/">Chris Lilley (W3C)</a>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> © 2018 <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>). 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="https://www.w3.org/TR/2002/WD-css3-fonts-20020802/">CSS3 Fonts</a>
   and <a href="https://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="https://drafts.csswg.org/css-font-loading-3/">CSS Font
   Loading</a> module.

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

  <p><em>This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the <a href="https://www.w3.org/TR/">W3C technical reports index</a> at https://www.w3.org/TR/.</em></p>

  <p>This CR is expected to be the final publication before requesting Proposed Recommendation.
    Features previously at risk have been moved to Fonts 4.
    The CR exit criteria have been met. The CSS WG expects to request Proposed Recommendation
    immediately after the CR exclusion period ends.
  </p>

  <p>Publication as a Candidate Recommendation does not imply endorsement by
   the W3C Membership. This is a draft document and may be updated, replaced
   or obsoleted by other documents at any time. It is inappropriate to cite
   this document as other than work in progress.

  <p>This document was produced by the <a
   href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> as a <a
   href="http://www.w3.org/Consortium/Process/tr#RecsCR">Candidate
   Recommendation.</a>
   This document is intended to become a W3C Recommendation.
	This document will remain a Candidate Recommendation at least until <time class="status-deadline" datetime="2018-07-24">24 July 2018</time> in order
	to ensure the opportunity for wide review. </p>

  <p>A Candidate Recommendation is a document that has been widely reviewed
   and is ready for implementation. W3C encourages everybody to implement
   this specification and return comments to the (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss-fonts-3%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
   (see <a href="http://www.w3.org/Mail/Request">instructions</a>). When
   sending e-mail, please put the text “css-fonts-3” in the subject,
   preferably like this: “[<!---->css-fonts-3<!---->] <em>…summary of
   comment…</em>”

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

   <p>This document is governed by the <a id="w3c_process_revision" href="https://www.w3.org/2018/Process-20180201/">1 February 2018 W3C Process Document</a>. </p>

  <!--end-status-->

  <p>See the section <a href="#cr-exit-criteria">CR Exit Criteria</a> for
   details on advancing this specification to W3C Recommendation. A
   <a href="https://test.csswg.org/harness/suite/css-fonts-3_dev/">test suite</a> and
   <a
   href="https://test.csswg.org/harness/results/css-fonts-3_dev/grouped/">implementation report</a>
   are available.

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

  <p>None.</p>
  <!-- at-risk features removed in prepartion for Proposed Recommendation-->

  <nav id=toc>
  <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-feature-settings descriptor</a>

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

     <li><a href="#font-fetching-requirements"><span class=secno>4.9.
      </span>Font fetching requirements</a>
    </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-east-asian-prop"><span class=secno>6.8.
      </span>East Asian text rendering: the font-variant-east-asian
      property</a>

     <li><a href="#font-variant-prop"><span class=secno>6.9. </span>Overall
      shorthand for font rendering: the font-variant property</a>

     <li><a href="#font-feature-settings-prop"><span class=secno>6.10.
      </span>Low-level font feature settings control: the
      font-feature-settings 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>
    </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 March 15
      2018 CSS Fonts 3 Candidate Recommendation</a>

     <li class=no-num><a href="#less-recent-changes"> Changes from the
      October 2013 CSS3 Fonts Candidate Recommendation</a>
    </ul>

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

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

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

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

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

     <li class=no-num><a href="#testing"> Non-Experimental
      Implementations</a>

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

   <li class=no-num><a href="#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--> </nav>
  <h2 id=introduction><span class=secno>1. </span>Introduction</h2>

  <p>A font provides a resource containing the visual representation of
   characters <a href="#ref-CHARMOD"
    >[CHARMOD]<!--{{CHARMOD}}--></a><a href="#ref-UNICODE"
    >[UNICODE]<!--{{UNICODE}}--></a>. 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 <a href="#ref-ARABIC-TYPO"
    >[ARABIC-TYPO]<!--{{ARABIC-TYPO}}--></a>, 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, when used for Persian and Urdu, exhibits significant
   and systematic differences in letterforms, as does Cyrillic when used with
   languages such as Serbian and 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 <a href="#ref-OPENTYPE"
    >[OPENTYPE]<!--{{OPENTYPE}}--></a> and AAT (Apple Advanced
   Typography) <a href="#ref-AAT-FEATURES"
    >[AAT-FEATURES]<!--{{AAT-FEATURES}}--></a> 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="#ref-OPENTYPE-FONT-GUIDE"
    >[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>
   <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>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 ot
F824
her 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.
  </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="https://www.w3.org/TR/CSS21/syndata.html#strings">strings,</a> or
   unquoted as a sequence of one or more <a
   href="https://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 keyword value
   (‘<code class=property>inherit</code>’, ‘<a href="#serif"><code
   class=property>serif</code></a>’, etc.) must be quoted to prevent
   confusion with the keywords with the same names. UAs must not consider
   these keywords as matching the <a
   href="#family-name-value"><var>&lt;family-name&gt;</var></a> type. This
   applies to any keyword across all of CSS.

  <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>

  <p>The details of localized font family name matching and the corresponding
   issues of case sensitivity are described below in the <a
   href="#font-family-casing">font matching</a> section.

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

  <p>All five generic font families must always result in at least one
   matched font face, for all CSS implementations. However, the generics may
   be composite faces (with different typefaces based on such things as the
   Unicode range of the character, the language of the containing element,
   user preferences and system settings, among others). They are also not
   guaranteed to always be different from each other.

  <p>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 or Song (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
   decorative or expressive 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>
   <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>Computed value:

     <td>numeric weight value (see description)

    <tr>
     <td>Animatable:

     <td>as <a
      href="https://drafts.csswg.org/css-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

   <li>800 - Extra Bold (Ultra Bold)

   <li>900 - Black (Heavy)
  </ul>

  <dl>
   <dt><dfn id=font-weight-normal-value
    title="normal!!font-weight">normal</dfn>

   <dd>Same as ‘<code class=css>400</code>’.