csswg-drafts/css3-writing-modes/Overview.html at 163a8b6aa66cfd08ed4db3f4135b43a81c1fc5a1 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">

<html lang=en>
 <head><meta content="text/html; charset=utf-8" http-equiv=Content-Type>

  <title>CSS Writing Modes Module Level 3</title>
  <link href=default.css rel=stylesheet type="text/css">

  <style type="text/css">
    object { vertical-align: middle; }
    .sidebar { float: right; background: #eee; border: solid gray; margin: 1em; }
    .sidebar .figure { margin: 1em; }
    .sidebar object { margin: 0 auto; display: block; }
    .figurepair { display: table; margin: 1em auto; }
    .figurepair .figure { display: table-cell; }
     h2, .example { clear: both; }
    .figure img,
    .figure object,
    .example img,
    dd img { max-width: 100%; display: block; margin: 1em auto; }
    div.figure table {
    	margin:auto;
    }
  </style>
  <link href="http://www.w3.org/StyleSheets/TR/W3C-WD.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>CSS Writing Modes Module Level 3</h1>

   <h2 class="no-num no-toc" id=longstatus-date>W3C Working Draft 30 August
    2011</h2>

   <dl>
    <dt>This version:</dt>
    <!--
    <dd><a href="http://dev.w3.org/csswg/css3-writing-modes/">http://dev.w3.org/csswg/css3-writing-modes/</a>
-->

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-writing-modes-20110830">http://www.w3.org/TR/2011/WD-css3-writing-modes-20110830/</a>


    <dt>Latest version:

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

    <dt>Latest editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css3-writing-modes/">http://dev.w3.org/csswg/css3-writing-modes/</a>

    <dt>Previous version:

    <dd><a
     href="http://www.w3.org/TR/2011/WD-css3-writing-modes-20110531/">http://www.w3.org/TR/2011/WD-css3-writing-modes-20110531/</a>

    <dt>Editors:

    <dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
     (Mozilla)

    <dd><a href="mailto:kojiishi@gluesoft.co.jp">Koji Ishii</a> (Invited
     Expert)

    <dd><a href="mailto:murakami@antenna.co.jp">Shinyu Murakami</a> (<a
     href="http://www.antenna.co.jp/">Antenna House</a>)

    <dt>Previous Editors:

    <dd><a href="mailto:paulnel@microsoft.com">Paul Nelson</a> (<a
     href="http://www.microsoft.com/">Microsoft</a>)

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

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

  <p>CSS Writing Modes Level 3 defines CSS features to support for various
   international writing modes, such as left-to-right (e.g. Latin or Indic),
   right-to-left (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and
   Arabic) and vertical (e.g. Asian scripts).

  <p>Inherently bottom-to-top scripts are not handled in this version. See <a
   href="#UTN22" rel=biblioentry>[UTN22]<!--{{UTN22}}--></a> for an
   explanation of relevant issues.

  <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="http://www.w3.org/TR/">W3C technical reports
   index at http://www.w3.org/TR/.</a></em>

  <p>Publication as a Working Draft 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>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a href="mailto:www-style@w3.org">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-writing-modes&#8221; in the subject, preferably like
   this: &#8220;[<!---->css3-writing-modes<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

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

  <p>This document was produced by a group operating under the <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
   2004 W3C Patent Policy</a>. W3C maintains a <a
   href="http://www.w3.org/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
   6 of the W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>The following features are at-risk and may be dropped during CR:

  <ul>
   <li>The &lsquo;<a href="#use-glyph-orientation"><code
    class=css>use-glyph-orientation</code></a>&rsquo; of &lsquo;<a
    href="#text-orientation0"><code
    class=property>text-orientation</code></a>&rsquo;

   <li>The &lsquo;<code class=css>ascii-digits</code>&rsquo;, &lsquo;<code
    class=css>digits</code>&rsquo;, &lsquo;<code
    class=css>alpha</code>&rsquo;, &lsquo;<code
    class=css>latin</code>&rsquo;, and &lsquo;<code
    class=css>alphanumeric</code>&rsquo; values of &lsquo;<a
    href="#text-combine-horizontal0"><code
    class=property>text-combine-horizontal</code></a>&rsquo;.

   <li>The &lsquo;<a href="#text-combine-mode0"><code
    class=property>text-combine-mode</code></a>&rsquo; property
  </ul>

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

  <ul class=toc>
   <li><a href="#text-flow"><span class=secno>1. </span> Introduction to
    Writing Modes</a>
    <ul class=toc>
     <li><a href="#placement"><span class=secno>1.1. </span> Module
      Interactions</a>

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

   <li><a href="#text-direction"><span class=secno>2. </span> Inline
    Direction and Bidirectionality</a>
    <ul class=toc>
     <li><a href="#direction"><span class=secno>2.1. </span> Specifying
      Directionality: the &lsquo;<code class=property>direction</code>&rsquo;
      property</a>

     <li><a href="#unicode-bidi"><span class=secno>2.2. </span> Embeddings
      and Overrides: the &lsquo;<code
      class=property>unicode-bidi</code>&rsquo; property</a>

     <li><a href="#bidi-example"><span class=secno>2.3. </span> Example of
      Bidirectional Text</a>

     <li><a href="#bidi-box-model"><span class=secno>2.4. </span> Box model
      for inline elements in bidirectional context</a>
    </ul>

   <li><a href="#vertical-intro"><span class=secno>3. </span> Introduction to
    Vertical Text</a>
    <ul class=toc>
     <li><a href="#writing-mode"><span class=secno>3.1. </span> Block Flow
      Direction: the &lsquo;<code class=property>writing-mode</code>&rsquo;
      property</a>
      <ul class=toc>
       <li><a href="#svg-writing-mode"><span class=secno>3.1.1. </span>
        SVG1.1 &lsquo;<code class=property>writing-mode</code>&rsquo;
        Values</a>
      </ul>
    </ul>

   <li><a href="#inline-alignment"><span class=secno>4. </span> Inline-level
    Alignment</a>
    <ul class=toc>
     <li><a href="#intro-baselines"><span class=secno>4.1. </span>
      Introduction to Baselines</a>

     <li><a href="#text-baselines"><span class=secno>4.2. </span> Text
      Baselines</a>

     <li><a href="#replaced-baselines"><span class=secno>4.3. </span> Atomic
      Inline Baselines</a>

     <li><a href="#baseline-alignment"><span class=secno>4.4. </span>
      Baseline Alignment</a>
    </ul>

   <li><a href="#intro-text-layout"><span class=secno>5. </span> Introduction
    to Vertical Text Layout</a>
    <ul class=toc>
     <li><a href="#text-orientation"><span class=secno>5.1. </span> Orienting
      Text: the &lsquo;<code class=property>text-orientation</code>&rsquo;
      property</a>
    </ul>

   <li><a href="#abstract-box"><span class=secno>6. </span> Abstract Box
    Terminology</a>
    <ul class=toc>
     <li><a href="#abstract-dimensions"><span class=secno>6.1. </span>
      Logical Dimensions</a>

     <li><a href="#abstract-directions"><span class=secno>6.2. </span>
      Abstract and Physical Directions</a>

     <li><a href="#line-directions"><span class=secno>6.3. </span>
      Line-relative Directions</a>

     <li><a href="#logical-directions"><span class=secno>6.4. </span>
      Flow-relative Directions</a>

     <li><a href="#logical-to-physical"><span class=secno>6.5. </span>
      Abstract-to-Physical Mappings</a>
    </ul>

   <li><a href="#abstract-layout"><span class=secno>7. </span> Abstract Box
    Layout</a>
    <ul class=toc>
     <li><a href="#vertical-layout"><span class=secno>7.1. </span> Principles
      of Layout in Vertical Writing Modes</a>

     <li><a href="#dimension-mapping"><span class=secno>7.2. </span>
      Dimensional Mapping</a>

     <li><a href="#orthogonal-flows"><span class=secno>7.3. </span>
      Orthogonal Flows</a>
      <ul class=toc>
       <li><a href="#orthogonal-auto"><span class=secno>7.3.1. </span>
        Auto-sizing in Orthogonal Flows</a>

       <li><a href="#orthogonal-multicol"><span class=secno>7.3.2. </span>
        Multi-column Layout in Orthogonal Flows</a>

       <li><a href="#orthogonal-pagination"><span class=secno>7.3.3. </span>
        Paginating Orthogonal Flows</a>
      </ul>

     <li><a href="#logical-direction-layout"><span class=secno>7.4. </span>
      Flow-Relative Mappings</a>

     <li><a href="#line-mappings"><span class=secno>7.5. </span>
      Line-Relative Mappings</a>

     <li><a href="#physical-only"><span class=secno>7.6. </span> Purely
      Physical Mappings</a>

     <li><a href="#caption-side"><span class=secno>7.7. </span> Table Caption
      Mappings: the &lsquo;<code class=property>caption-side</code>&rsquo;
      keywords</a>
    </ul>

   <li><a href="#page-direction"><span class=secno>8. </span> Page Flow: the
    page progression direction</a>

   <li><a href="#text-combine"><span class=secno>9. </span> Glyph
    Composition</a>
    <ul class=toc>
     <li><a href="#text-combine-horizontal"><span class=secno>9.1. </span>
      Horizonal-in-Vertical Composition: the &lsquo;<code
      class=property>text-combine-horizontal</code>&rsquo; property</a>

     <li><a href="#text-combine-mode"><span class=secno>9.2. </span>
      Horizonal-in-Vertical Glyph Scaling: the &lsquo;<code
      class=property>text-combine-mode</code>&rsquo; property</a>
    </ul>

   <li class=no-num><a href="#changes">Changes</a>
    <ul class=toc>
     <li class=no-num><a href="#recent-changes"> Changes from the May 2011
      CSS Writing Modes Module Level 3 <abbr title="Working
      Draft">WD</abbr></a>
    </ul>

   <li><a href="#conformance"><span class=secno>10. </span> Conformance</a>
    <ul class=toc>
     <li><a href="#conventions"><span class=secno>10.1. </span> Document
      Conventions</a>

     <li><a href="#conformance-classes"><span class=secno>10.2. </span>
      Conformance Classes</a>

     <li><a href="#partial"><span class=secno>10.3. </span> Partial
      Implementations</a>

     <li><a href="#experimental"><span class=secno>10.4. </span> Experimental
      Implementations</a>

     <li><a href="#testing"><span class=secno>10.5. </span>Non-Experimental
      Implementations</a>

     <li><a href="#cr-exit-criteria"><span class=secno>10.6. </span> CR Exit
      Criteria</a>
    </ul>

   <li class=no-num><a href="#acknowledgements"> Acknowledgements</a>

   <li class=no-num><a href="#character-properties">Appendix A. Characters
    and Properties</a>

   <li class=no-num><a href="#script-orientations">Appendix B:
    Bi-orientational Transformations</a>

   <li class=no-num><a href="#vertical-typesetting-details">Appendix C:
    Vertical Typesetting Synthesis</a>

   <li class=no-num><a href="#intrinsic-sizing">Appendix D: Intrinsic
    Dimensions</a>
    <ul class=toc>
     <li class=no-num><a href="#multicol-intrinsic"> Intrinsic Sizes in
      Multi-column Layout</a>
    </ul>

   <li class=no-num><a href="#bidi-html"> Appendix E: Bidi Rules for HTML</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="#property-index"> Property Index</a>
  </ul>
  <!--end-toc-->

  <h2 id=text-flow><span class=secno>1. </span> Introduction to Writing Modes</h2>

  <p>CSS Writing Modes Level 3 defines CSS features to support for various
   international writing modes, such as left-to-right (e.g. Latin or Indic),
   right-to-left (e.g. Hebrew or Arabic), bidirectional (e.g. mixed Latin and
   Arabic) and vertical (e.g. Asian scripts).

  <p>A <dfn id=writing-mode0>writing mode</dfn> in CSS is determined by the
   &lsquo;<a href="#writing-mode1"><code
   class=property>writing-mode</code></a>&rsquo;, &lsquo;<a
   href="#direction0"><code class=property>direction</code></a>&rsquo;, and
   &lsquo;<a href="#text-orientation0"><code
   class=property>text-orientation</code></a>&rsquo; properties. It is
   defined primarily in terms of its <a
   href="#inline-base-direction"><i>inline base direction</i></a> and <a
   href="#block-flow-direction"><i>block flow direction</i></a>:

  <div class=sidebar>
   <div class="figure right"> <a href="diagrams/text-flow-vectors-tb.svg"
    type="image/svg+xml"> <img alt="Latin-based writing mode" class=landscape
    src="diagrams/text-flow-vectors-tb.png"></a>
    <p class=caption>Latin-based writing mode
   </div>

   <div class="figure left"> <a
    href="diagrams/text-flow-vectors-lr-reverse.svg" type="image/svg+xml">
    <img alt="Mongolian-based writing mode" class=landscape
    src="diagrams/text-flow-vectors-lr-reverse.png"></a>
    <p class=caption>Mongolian-based writing mode
   </div>

   <div class="figure right"> <a href="diagrams/text-flow-vectors-tb.svg"
    type="image/svg+xml"> <img alt="Han-based writing mode" class=landscape
    src="diagrams/text-flow-vectors-tb.png"></a> <a
    href="diagrams/text-flow-vectors-rl.svg" type="image/svg+xml"> <img
    alt="Han-based writing mode" class=landscape
    src="diagrams/text-flow-vectors-rl.png"></a>
    <p class=caption>Han-based writing mode
   </div>
  </div>

  <p>The <dfn id=inline-base-direction>inline base direction</dfn> is the
   primary direction in which content is ordered on a line and defines on
   which sides the "start" and "end" of a line are. The &lsquo;<a
   href="#direction0"><code class=property>direction</code></a>&rsquo;
   property specifies the inline base direction of an element and, together
   with the &lsquo;<a href="#unicode-bidi0"><code
   class=property>unicode-bidi</code></a>&rsquo; property and the inherent
   directionality of any text content, determines the ordering of
   inline-level content within a line.

  <p>The <dfn id=block-flow-direction>block flow direction</dfn> is the
   direction in which block-level boxes stack and the direction in which line
   boxes stack within a block container. The &lsquo;<a
   href="#writing-mode1"><code class=property>writing-mode</code></a>&rsquo;
   property determines the block flow direction.

  <p>A <dfn id=horizontal-writing-mode>horizontal writing mode</dfn> is one
   with horizontal lines of text, i.e. a downward or upward block flow. A
   <dfn id=vertical-writing-mode>vertical writing mode</dfn> is one with
   vertical lines of text, i.e. a leftward or rightward block flow.

  <p class=note>These terms should not be confused with <dfn
   id=vertical-block-flow>vertical block flow</dfn> (which is a downward or
   upward block flow) and <dfn id=horizontal-block-flow>horizontal block
   flow</dfn> (which is leftward or rightward block flow). To avoid
   confusion, CSS specifications avoid this latter set of terms.

  <p>Writing systems typically have one or two native writing modes. Some
   examples are:

  <ul>
   <li>Latin-based systems are typically written using a left-to-right inline
    direction with a downward (top-to-bottom) block flow direction.

   <li>Arabic-based systems are typically written using a right-to-left
    inline direction with a downward (top-to-bottom) block flow direction.

   <li>Mongolian-based systems are typically written using a top-to-bottom
    inline direction with a rightward (left-to-right) block flow direction.

   <li>Han-based systems are commonly written using a left-to-right inline
    direction with a downward (top-to-bottom) block flow direction,
    <strong>or</strong> a top-to-bottom inline direction with a leftward
    (right-to-left) block flow direction. Many magazines and newspapers will
    mix these two writing modes on the same page.
  </ul>

  <p>The &lsquo;<a href="#text-orientation0"><code
   class=property>text-orientation</code></a>&rsquo; component of the writing
   mode determines the <a href="#line-orientation"><i>line
   orientation</i></a> and the <i>typesetting mode</i>, and controls details
   of text layout such as the <i>glyph orientation</i>.

  <p class=note>See Unicode Technical Note #22 <a href="#UTN22"
   rel=biblioentry>[UTN22]<!--{{UTN22}}--></a> (<a
   href="http://fantasai.inkedblade.net/style/discuss/vertical-text/paper">HTML
   version</a>) for a more in-depth introduction to writing modes and
   vertical text.

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

  <p>This module replaces and extends the &lsquo;<a
   href="#unicode-bidi0"><code class=property>unicode-bidi</code></a>&rsquo;
   and &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; features defined in <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> sections 8.6
   and 9.10.

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

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

  <p>In addition to the property-specific values listed in their definitions,
   all properties defined in this specification also accept the <a
   href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a>
   keyword as their property value. For readability it has not been repeated
   explicitly.

  <h2 id=text-direction><span class=secno>2. </span> Inline Direction and
   Bidirectionality</h2>

  <p>While the characters in most scripts are written from left to right,
   certain scripts are written from right to left. In some documents, in
   particular those written with the Arabic or Hebrew script, and in some
   mixed-language contexts, text in a single (visually displayed) block may
   appear with mixed directionality. This phenomenon is called <span
   class=index-def id=bidirectionality title="bidirectionality (bidi)"><dfn
   id=bidirectionality0>bidirectionality</dfn></span>, or "bidi" for short.

  <div class=figure>
   <p><img alt="An example of bidirectional text is a Latin name in an Arabic
    sentence. The sentence overall is typeset right-to-left, but the letters
    in the Latin word in the middle are typeset left-to-right."
    src="diagrams/bidi.png">

   <p class=caption>Bidirectionality</p>
  </div>

  <p>The Unicode standard (<a
   href="http://www.unicode.org/reports/tr9/">Unicode Standard Annex #9</a>)
   defines a complex algorithm for determining the proper ordering of
   bidirectional text. The algorithm consists of an implicit part based on
   character properties, as well as explicit controls for embeddings and
   overrides. CSS relies on this algorithm to achieve proper bidirectional
   rendering. The &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; and &lsquo;<a
   href="#unicode-bidi0"><code class=property>unicode-bidi</code></a>&rsquo;
   properties allow authors to specify how the elements and attributes of a
   document language map to this algorithm.

  <p>User agents that support bidirectional text must apply the Unicode
   bidirectional algorithm to every sequence of inline-level boxes
   uninterrupted by a forced (<a
   href="http://www.unicode.org/reports/tr9/#Bidirectional_Character_Types">bidi
   class B</a>) paragraph break or block boundary. This sequence forms the
   <dfn id=paragraph>paragraph</dfn> unit in the bidirectional algorithm.

  <p>Except when the &lsquo;<a href="#plaintext"><code
   class=css>plaintext</code></a>&rsquo; value of &lsquo;<a
   href="#unicode-bidi0"><code class=property>unicode-bidi</code></a>&rsquo;
   is in effect, the paragraph embedding level is set according to the value
   of the &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; property of the paragraph's
   element rather than by the heuristic given in steps P2 and P3 of the
   Unicode algorithm. The paragraph's element is usually the containing
   block, but in the case of a paragraph contained by bidi <a
   href="#isolate">isolation</a> it is the isolating inline element instead.

  <p>Because the base directionality of a text depends on the structure and
   semantics of the document, the &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; and &lsquo;<a
   href="#unicode-bidi0"><code class=property>unicode-bidi</code></a>&rsquo;
   properties should in most cases be used only to map bidi information in
   the markup to its corresponding CSS styles. If a document language
   provides markup features to control bidi, authors and users should use
   those features and not specify CSS rules to override them.

  <p>The HTML 4 specification (<a href="#HTML401"
   rel=biblioentry>[HTML401]<!--{{HTML401}}--></a>, section 8.2) defines
   bidirectionality behavior for HTML elements. The HTML 4 specification also
   contains more information on bidirectionality issues.

  <p class=note>Because HTML UAs can turn off CSS styling, we advise HTML
   authors to use the HTML &lsquo;<code class=property>dir</code>&rsquo;
   attribute and &lt;bdo&gt; element to ensure correct bidirectional layout
   in the absence of a style sheet.

  <h3 id=direction><span class=secno>2.1. </span> Specifying Directionality:
   the &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; property</h3>

  <table class=propdef>
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=direction0>direction</dfn>

    <tr>
     <th>Value:

     <td>ltr | rtl

    <tr>
     <th>Initial:

     <td>ltr

    <tr>
     <th>Applies to:

     <td>all elements

    <tr>
     <th>Inherited:

     <td>yes

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed&#160;value:

     <td>specified value
  </table>

  <p>This property specifies the base directionality of text and elements on
   a line, and the directionality of embeddings and overrides (see &lsquo;<a
   href="#unicode-bidi0"><code class=property>unicode-bidi</code></a>&rsquo;)
   for the Unicode bidirectional algorithm. In addition, it affects the
   ordering of <a href="http://www.w3.org/TR/CSS21/tables.html">table</a>
   column layout, the direction of horizontal <a
   href="http://www.w3.org/TR/CSS21/visufx.html#overflow">overflow</a>, and
   the default alignment of text within a line, and other things that depend
   on the base inline base direction.

  <p>Values for this property have the following meanings:

  <dl>
   <dt><dfn id=ltr>ltr</dfn>

   <dd>Left-to-right directionality.

   <dt><dfn id=rtl>rtl</dfn>

   <dd>Right-to-left directionality.
  </dl>

  <p class=note>The &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; property has no effect on bidi
   reordering when specified on inline elements whose &lsquo;<a
   href="#unicode-bidi0"><code class=property>unicode-bidi</code></a>&rsquo;
   property's value is &lsquo;<a href="#normal"><code
   class=css>normal</code></a>&rsquo;.

  <p>The value of the &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; property on the root element is
   also propagated to the initial containing block and, together with the
   &lsquo;<a href="#writing-mode1"><code
   class=property>writing-mode</code></a>&rsquo; property, determines the
   document's principal writing mode. (See <a
   href="#principal-writing-mode">below</a>.)

  <p class=note>Note that the &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; property of the HTML BODY
   element is <em>not</em> propagated to the viewport. That special behavior
   only applies to the background and overflow properties.

  <p class=note>The &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; property, when specified for
   table column elements, is not inherited by cells in the column since
   columns are not the ancestors of the cells in the document tree. Thus, CSS
   cannot easily capture the "dir" attribute inheritance rules described in
   <a href="#HTML401" rel=biblioentry>[HTML401]<!--{{HTML401}}--></a>,
   section 11.3.2.1.

  <h3 id=unicode-bidi><span class=secno>2.2. </span> Embeddings and
   Overrides: the &lsquo;<a href="#unicode-bidi0"><code
   class=property>unicode-bidi</code></a>&rsquo; property</h3>

  <table class=propdef>
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=unicode-bidi0>unicode-bidi</dfn>

    <tr>
     <th>Value:

     <td>normal | embed | [ isolate || bidi-override ] | plaintext

    <tr>
     <th>Initial:

     <td>normal

    <tr>
     <th>Applies to:

     <td>all elements, but see prose

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed&#160;value:

     <td>specified value
  </table>

  <p>Values for this property have the following meanings:

  <dl>
   <dt><dfn id=normal>normal</dfn>

   <dd>The element does not open an additional level of embedding with
    respect to the bidirectional algorithm. For inline elements, implicit
    reordering works across element boundaries.

   <dt><dfn id=embed>embed</dfn>

   <dd>If the element is inline, this value opens an additional level of
    embedding with respect to the bidirectional algorithm. The direction of
    this embedding level is given by the &lsquo;<a href="#direction0"><code
    class=property>direction</code></a>&rsquo; property. Inside the element,
    reordering is done implicitly. This corresponds to adding a LRE (U+202A),
    for &lsquo;<code class=css>direction: ltr</code>&rsquo;, or RLE (U+202B),
    for &lsquo;<code class=css>direction: rtl</code>&rsquo;, at the start of
    the element and a PDF (U+202C) at the end of the element. <span
    class=note>This value has no effect on elements that are not
    inline.</span>

   <dt><dfn id=isolate>isolate</dfn>

   <dd>For the purposes of the Unicode bidirectional algorithm, the contents
    of the element are considered to be inside a separate, independent
    paragraph with a base directionality given by the element's &lsquo;<a
    href="#direction0"><code class=property>direction</code></a>&rsquo;
    property, and for the purpose of bidi resolution in its containing bidi
    paragraph (if any), the element itself is treated as if it were an Object
    Replacement Character (U+FFFC). (If the element is broken across multiple
    lines, then each box of the element is treated as an Object Replacement
    Character.)

   <dt><dfn id=bidi-override>bidi-override</dfn>

   <dd>For inline elements this creates an override. For block-container
    elements this creates an override for inline-level descendants not within
    another block container element. This means that inside the element,
    reordering is strictly in sequence according to the &lsquo;<a
    href="#direction0"><code class=property>direction</code></a>&rsquo;
    property; the implicit part of the bidirectional algorithm is ignored.
    This corresponds to adding a LRO (U+202D), for &lsquo;<code
    class=css>direction: ltr</code>&rsquo;, or RLO (U+202E), for &lsquo;<code
    class=css>direction: rtl</code>&rsquo;, at the start of the element and a
    PDF (U+202C) at the end of the element.

   <dt><dfn id=plaintext>plaintext</dfn>

   <dd>
    <p>For the purposes of the Unicode bidirectional algorithm, the base
     directionality of each bidi paragraph for which the element forms the
     containing block is determined not by the element's computed &lsquo;<a
     href="#direction0"><code class=property>direction</code></a>&rsquo; as
     usual, but by following the heuristic in rules P2 and P3 of the Unicode
     bidirectional algorithm. For inline elements, this value behaves as for
     &lsquo;<a href="#isolate"><code class=css>isolate</code></a>&rsquo;,
     except, as with block containers, the base directionality is determined
     by following the Unicode heuristic instead of by using the &lsquo;<a
     href="#direction0"><code class=property>direction</code></a>&rsquo;
     value.
  </dl>

  <p>The final order of characters within in each bidi paragraph is the same
   as if the bidi control codes had been added as described above, markup had
   been stripped, and the resulting character sequence had been passed to an
   implementation of the Unicode bidirectional algorithm for plain text that
   produced the same line-breaks as the styled text.

  <p>In this process, replaced elements with &lsquo;<code class=css>display:
   inline</code>&rsquo; are treated as neutral characters, unless their
   &lsquo;<a href="#unicode-bidi0"><code
   class=property>unicode-bidi</code></a>&rsquo; property has a value other
   than &lsquo;<a href="#normal"><code
   class=property>normal</code></a>&rsquo;, in which case they are treated as
   strong characters in the &lsquo;<a href="#direction0"><code
   class=property>direction</code></a>&rsquo; specified for the element. All
   other atomic inline-level boxes are treated as neutral characters always.

  <p>If an inline element is broken around a bidi paragraph boundary (e.g. if
   split by a block or forced paragraph break), then the bidi control codes
   corresponding to the end of the element are added before the interruption
   and the codes corresponding to the start of the element are added after
   it. (In other words, any embedding levels or overrides started by the
   element are closed at the paragraph break and reopened on the other side
   of it.)

  <p>Because the Unicode algorithm has a limit of <em title="According to
   unicode 3.0, chapter 3, section 12, definition BD2. Specifically, page 58
   here: http://www.unicode.org/unicode/uni2book/ch03.pdf"> 61 levels</em> of
   embedding, care should be taken not to use <span
   class=propinst-unicode-bidi>&lsquo;<a href="#unicode-bidi0"><code
   class=property>unicode-bidi</code></a>&rsquo;</span> with a value other
   than &lsquo;<a href="#normal"><code
   class=property>normal</code></a>&rsquo; unless appropriate. In particular,
   a value of &lsquo;<code class=property>inherit</code>&rsquo; should be
   used with extreme caution. However, for elements that are, in general,
   intended to be displayed as blocks, a setting of &lsquo;<code
   class=css>unicode-bidi: isolate</code>&rsquo; is preferred to keep the
   element together in case display is changed to inline (see example below).

  <h3 id=bidi-example><span class=secno>2.3. </span> Example of Bidirectional
   Text</h3>

  <p>The following example shows an XML document with bidirectional text. It
   illustrates an important design principle: document language designers
   should take bidi into account both in the language proper (elements and
   attributes) and in any accompanying style sheets. The style sheets should
   be designed so that bidi rules are separate from other style rules, and
   such rules should not be overridden by other style sheets so that the
   document language's bidi behavior is preserved.

  <div class=example>
   <p>In this example, lowercase letters stand for inherently left-to-right
    characters and uppercase letters represent inherently right-to-left
    characters. The text stream is shown in logical backing store order.</p>

   <pre class=xml-example><code class=xml>
&lt;HEBREW&gt;
  &lt;PAR&gt;HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5&lt;/PAR&gt;
  &lt;PAR&gt;HEBREW6 &lt;EMPH&gt;HEBREW7&lt;/EMPH&gt; HEBREW8&lt;/PAR&gt;
&lt;/HEBREW&gt;
&lt;ENGLISH&gt;
  &lt;PAR&gt;english9 english10 english11 HEBREW12 HEBREW13&lt;/PAR&gt;
  &lt;PAR&gt;english14 english15 english16&lt;/PAR&gt;
  &lt;PAR&gt;english17 &lt;HE-QUO&gt;HEBREW18 english19 HEBREW20&lt;/HE-QUO&gt;&lt;/PAR&gt;
&lt;/ENGLISH&gt;
    </code></pre>

   <p>Since this is arbitrary XML, the style sheet is responsible for setting
    the writing direction. This is the style sheet:</p>

   <pre>
/* Rules for bidi */
HEBREW, HE-QUO  {direction: rtl; unicode-bidi: embed;}
ENGLISH         {direction: ltr; unicode-bidi: embed;}

/* Rules for presentation */
HEBREW, ENGLISH, PAR  {display: block;}
EMPH                  {font-weight: bold;}
    </pre>

   <p>The HEBREW element is a block with a right-to-left base direction, the
    ENGLISH element is a block with a left-to-right base direction. The PARs
    are blocks that inherit the base direction from their parents. Thus, the
    first two PARs are read starting at the top right, the final three are
    read starting at the top left. Please note that HEBREW and ENGLISH are
    chosen as element names for explicitness only; in general, element names
    should convey structure without reference to language.</p>

   <p>The EMPH element is inline-level, and since its value for <span
    class=propinst-unicode-bidi>&lsquo;<a href="#unicode-bidi0"><code
    class=property>unicode-bidi</code></a>&rsquo;</span> is &lsquo;<a
    href="#normal"><code class=property>normal</code></a>&rsquo; (the initial
    value), it has no effect on the ordering of the text. The HE-QUO element,
    on the other hand, creates an embedding.</p>

   <p>The formatting of this text might look like this if the line length is
    long:</p>

   <pre class=ascii-art>
               5WERBEH 4WERBEH english3 2WERBEH 1WERBEH

                                8WERBEH <b>7WERBEH</b> 6WERBEH

english9 english10 english11 13WERBEH 12WERBEH

english14 english15 english16

english17 20WERBEH english19 18WERBEH
    </pre>

   <p>Note that the HE-QUO embedding causes HEBREW18 to be to the right of
    english19.</p>

   <p>If lines have to be broken, it might be more like this:</p>

   <pre class=ascii-art>
       2WERBEH 1WERBEH
  -EH 4WERBEH english3
                 5WERB

   -EH <b>7WERBEH</b> 6WERBEH
                 8WERB

english9 english10 en-
glish11 12WERBEH
13WERBEH

english14 english15
english16

english17 18WERBEH
20WERBEH english19
    </pre>

   <p>Because HEBREW18 must be read before english19, it is on the line above
    english19. Just breaking the long line from the earlier formatting would
    not have worked. Note also that the first syllable from english19 might
    have fit on the previous line, but hyphenation of left-to-right words in
    a right-to-left context, and vice versa, is usually suppressed to avoid
    having to display a hyphen in the middle of a line.</p>
  </div>
  <!-- example -->

  <h3 id=bidi-box-model><span class=secno>2.4. </span> Box model for inline
   elements in bidirectional context</h3>

  <p>Since bidi reordering can split apart and reorder text that is logically
   contiguous, bidirectional text can cause an inline boxes to be split and
   reordered within a line.

  <p class=note>Note that in order to be able to flow inline boxes in a
   uniform direction (either entirely left-to-right or entirely
   right-to-left), anonymous inline boxes may have to be created.</p>
  <!-- CSS2.1 8.6 -->

  <p>For each line box, UAs must take the inline boxes generated for each
   element and render the margins, borders and padding in visual order (not
   logical order). The <a href="#start"><i>start</i></a>-most box on the
   first line box in which the element appears has the <a
   href="#start"><i>start</i></a> edge's margin, border, and padding; and the
   end-most box on the last line box in which the element appears has the <a
   href="#end"><i>end</i></a> edge's margin, border, and padding. For
   example, in the &lsquo;<a href="#horizontal-tb"><code
   class=css>horizontal-tb</code></a>&rsquo; writing mode:

  <ul>
   <li>When the parent's &lsquo;<a href="#direction0"><code
    class=property>direction</code></a>&rsquo; property is &lsquo;<a
    href="#ltr"><code class=css>ltr</code></a>&rsquo;, the left-most
    generated box of the first line box in which the element appears has the
    left margin, left border and left padding, and the right-most generated
    box of the last line box in which the element appears has the right
    padding, right border and right margin.

   <li>When the parent's &lsquo;<a href="#direction0"><code
    class=property>direction</code></a>&rsquo; property is &lsquo;<a
    href="#rtl"><code class=css>rtl</code></a>&rsquo;, the right-most
    generated box of the first line box in which the element appears has the
    right padding, right border and right margin, and the left-most generated
    box of the last line box in which the element appears has the left
    margin, left border and left padding.
  </ul>

  <p>Analogous rules hold for vertical writing modes.

  <p class=note>The &lsquo;<code
   class=property>box-decoration-break</code>&rsquo; property can override
   this behavior to draw box decorations on both sides of each box. <a
   href="#CSS3BG" rel=biblioentry>[CSS3BG]<!--{{!CSS3BG}}--></a>

  <h2 id=vertical-intro><span class=secno>3. </span> Introduction to Vertical
   Text</h2>

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

  <p>In addition to extensions to CSS2.1&rsquo;s support for bidirectional
   text, this module introduces the rules and properties needed to support
   vertical text layout in CSS.

  <p>Unlike languages that use the Latin script which are primarily laid out
   horizontally, Asian languages such as Chinese and Japanese can be laid out
   vertically. The Japanese example below shows the same text laid out
   horizontally and vertically. In the horizontal case, text is read from
   left to right, top to bottom. For the vertical case, the text is read top
   to bottom, right to left. Indentation from the left edge in the
   left-to-right horizontal case translates to indentation from the top edge
   in the top-to-bottom vertical case.

  <div class=figure>
   <p><img alt="A comparison of horizontal and vertical Japanese shows that
    although the lines rotate, the characters remain upright. Some glyphs,
    however change: a period mark shifts from the bottom left of its glyph
    box to the top right. Running headers, however, may remain laid out
    horizontally across the top of the page." src=vert-horiz-comparison.png></p>

   <p class=caption>Comparison of vertical and horizontal Japanese: iBunko
    application (iOS)</p>
  </div>

  <p class=note>For Chinese and Japanese lines are ordered either right to
   left or top to bottom, while for Mongolian and Manchu lines are ordered
   left to right.

  <p>The change from horizontal to vertical writing can affect not just the