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<!DOCTYPE html><html lang=en><head>
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Writing Modes Level 3</title>
<link href=../default.css rel=stylesheet type=text/css>
<link href=../csslogo.ico rel="shortcut icon" type=image/x-icon>
<style>
body {
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
background-attachment: fixed;
color: black;
font-family: sans-serif;
margin: 0 auto;
max-width: 50em;
padding: 2em 1em 2em 70px;
}
:link { color: #00C; background: transparent }
:visited { color: #609; background: transparent }
a[href]:active { color: #C00; background: transparent }
a[href]:hover { background: #ffa }
a[href] img { border-style: none }
h1, h2, h3, h4, h5, h6 { text-align: left }
h1, h2, h3 { color: #005A9C; }
h1 { font: 170% sans-serif }
h2 { font: 140% sans-serif }
h3 { font: 120% sans-serif }
h4 { font: bold 100% sans-serif }
h5 { font: italic 100% sans-serif }
h6 { font: small-caps 100% sans-serif }
.hide { display: none }
div.head { margin-bottom: 1em }
div.head h1 { margin-top: 2em; clear: both }
div.head table { margin-left: 2em; margin-top: 2em }
p.copyright { font-size: small }
p.copyright small { font-size: small }
pre { margin-left: 2em }
dt { font-weight: bold }
ul.toc, ol.toc {
list-style: none;
}
</style>
<script defer src=//test.csswg.org/harness/annotate.js#css-writing-modes-3_dev/css-writing-modes-3 type=text/javascript></script>
<style>
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; padding-left: .5em; padding-right: .5em; }
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;
}
.ascii-art {
display: table;
margin: 1em auto;
}
</style>
</head>
<body class=h-entry>
<div class=head>
<p data-fill-with=logo><a class=logo href=http://www.w3.org/>
<img alt=W3C height=48 src=https://www.w3.org/Icons/w3c_home width=72>
</a>
</p>
<h1 class="p-name no-ref" id=title>CSS Writing Modes Level 3</h1>
<h2 class="no-num no-toc no-ref heading settled" id=subtitle><span class=content>Editor’s Draft,
<time class=dt-updated datetime=20141215>15 December 2014</time></span></h2>
<div data-fill-with=spec-metadata><dl><dt>This version:<dd><a class=u-url href=http://dev.w3.org/csswg/css-writing-modes-3/>http://dev.w3.org/csswg/css-writing-modes-3/</a><dt>Latest version:<dd><a href=http://www.w3.org/TR/css-writing-modes-3/>http://www.w3.org/TR/css-writing-modes-3/</a><dt>Previous Versions:<dd><a href=http://www.w3.org/TR/2014/CR-css-writing-modes-3-20140320/ rel=previous>http://www.w3.org/TR/2014/CR-css-writing-modes-3-20140320/</a><dt>Feedback:<dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-writing-modes%5D%20feedback">www-style@w3.org</a> with subject line “<kbd>[css-writing-modes] <var>… message topic …</var></kbd>” (<a href=http://lists.w3.org/Archives/Public/www-style/ rel=discussion>archives</a>)</span><dt>Test Suite:<dd><a href=http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/>http://test.csswg.org/suites/css-writing-modes-3_dev/nightly-unstable/</a><dt>Issue Tracking:<dd><a href=#issues-index>Inline In Spec</a><dd><span><a href=http://www.w3.org/Style/CSS/Tracker/products/30>http://www.w3.org/Style/CSS/Tracker/products/30</a></span><dt class=editor>Editors:<dd class=editor><div class="p-author h-card vcard"><a class="p-name fn u-url url" href=http://fantasai.inkedblade.net/contact>fantasai</a> (<span class="p-org org">Invited Expert</span>)</div><dd class=editor><div class="p-author h-card vcard"><span class="p-name fn">Koji Ishii</span> (<span class="p-org org">Google</span>)</div><dt>Former Editors:<dd><div class="p-author h-card vcard"><a class="p-name fn u-email email" href=mailto:murakami@antenna.co.jp>Shinyu Murakami</a> (<span class="p-org org"><a href="http://www.antenna.co.jp/">Antenna House</a></span>)</div><dd><div class="p-author h-card vcard"><a class="p-name fn u-email email" href=mailto:paulnel@microsoft.com>Paul Nelson</a> (<span class="p-org org"><a href="http://www.microsoft.com/">Microsoft</a></span>)</div><dd><div class="p-author h-card vcard"><a class="p-name fn u-email email" href=mailto:michelsu@microsoft.com>Michel Suignard</a> (<span class="p-org org"><a href="http://www.microsoft.com/">Microsoft</a></span>)</div></dl></div>
<div data-fill-with=warning></div>
<p class=copyright data-fill-with=copyright><a href=http://www.w3.org/Consortium/Legal/ipr-notice#Copyright>Copyright</a> © 2014 <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>
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc no-ref heading settled" id=abstract><span class=content>Abstract</span></h2>
<div class=p-summary data-fill-with=abstract><p>CSS Writing Modes Level 3 defines CSS 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 href=http://www.w3.org/TR/CSS/>CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.</div>
<h2 class="no-num no-toc no-ref heading settled" id=status><span class=content>Status of this document</span></h2>
<div data-fill-with=status><p>
This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
Don’t cite this document other than as work in progress.
<p>
The (<a href=http://lists.w3.org/Archives/Public/www-style/>archived</a>) public mailing list
<a href="mailto:www-style@w3.org?Subject=%5Bcss-writing-modes%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
(see <a href=http://www.w3.org/Mail/Request>instructions</a>)
is preferred for discussion of this specification.
When sending e-mail,
please put the text “css-writing-modes” in the subject,
preferably like this:
“[css-writing-modes] <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>
This document is governed by the <a href=http://www.w3.org/2014/Process-20140801/>1 August 2014 W3C Process Document</a>.
</div>
<div data-fill-with=at-risk><p>The following features are at-risk, and may be dropped during the CR period:
<ul><li>The <a class=css data-link-type=maybe href=#valdef-text-orientation-use-glyph-orientation>use-glyph-orientation</a> of <a class=property data-link-type=propdesc href=#propdef-text-orientation>text-orientation</a><li>The <span class=css>digits</span> value of <a class=property data-link-type=propdesc href=#propdef-text-combine-upright>text-combine-upright</a>.<li>The look-ahead/look-behind sequencing rules for <a class=property data-link-type=propdesc href=#propdef-text-combine-upright>text-combine-upright</a>.</ul></div>
<h2 class="no-num no-toc no-ref heading settled" id=contents><span class=content>Table of Contents</span></h2>
<div data-fill-with=table-of-contents role=navigation><ul class=toc role=directory><li><a href=#text-flow><span class=secno>1</span> <span class=content>
Introduction to Writing Modes</span></a><ul class=toc><li><a href=#placement><span class=secno>1.1</span> <span class=content>
Module Interactions</span></a><li><a href=#values><span class=secno>1.2</span> <span class=content>
Values</span></a></ul><li><a href=#text-direction><span class=secno>2</span> <span class=content>
Inline Direction and Bidirectionality</span></a><ul class=toc><li><a href=#direction><span class=secno>2.1</span> <span class=content>
Specifying Directionality: the <span class=property data-link-type=propdesc title=direction>direction</span> property</span></a><li><a href=#unicode-bidi><span class=secno>2.2</span> <span class=content>
Embeddings and Overrides: the <span class=property data-link-type=propdesc title=unicode-bidi>unicode-bidi</span> property</span></a><li><a href=#bidi-example><span class=secno>2.3</span> <span class=content>
Example of Bidirectional Text</span></a><li><a href=#bidi-box-model><span class=secno>2.4</span> <span class=content>
Box model for inline boxes in bidirectional context</span></a></ul><li><a href=#vertical-intro><span class=secno>3</span> <span class=content>
Introduction to Vertical Text</span></a><ul class=toc><li><a href=#block-flow><span class=secno>3.1</span> <span class=content>
Block Flow Direction: the <span class=property data-link-type=propdesc title=writing-mode>writing-mode</span> property</span></a><ul class=toc><li><a href=#svg-writing-mode><span class=secno>3.1.1</span> <span class=content>
SVG1.1 <span class=property data-link-type=propdesc title=writing-mode>writing-mode</span> Values</span></a></ul></ul><li><a href=#inline-alignment><span class=secno>4</span> <span class=content>
Inline-level Alignment</span></a><ul class=toc><li><a href=#intro-baselines><span class=secno>4.1</span> <span class=content>
Introduction to Baselines</span></a><li><a href=#text-baselines><span class=secno>4.2</span> <span class=content>
Text Baselines</span></a><li><a href=#replaced-baselines><span class=secno>4.3</span> <span class=content>
Atomic Inline Baselines</span></a><li><a href=#baseline-alignment><span class=secno>4.4</span> <span class=content>
Baseline Alignment</span></a></ul><li><a href=#intro-text-layout><span class=secno>5</span> <span class=content>
Introduction to Vertical Text Layout</span></a><ul class=toc><li><a href=#text-orientation><span class=secno>5.1</span> <span class=content>
Orienting Text: the <span class=property data-link-type=propdesc title=text-orientation>text-orientation</span> property</span></a><ul class=toc><li><a href=#vertical-font-features><span class=secno>5.1.1</span> <span class=content>
Vertical Typesetting and Font Features</span></a><li><a href=#vertical-orientations><span class=secno>5.1.2</span> <span class=content>
Mixed Vertical Orientations</span></a></ul></ul><li><a href=#abstract-box><span class=secno>6</span> <span class=content>
Abstract Box Terminology</span></a><ul class=toc><li><a href=#abstract-axes><span class=secno>6.1</span> <span class=content>
Abstract Dimensions</span></a><li><a href=#logical-directions><span class=secno>6.2</span> <span class=content>
Flow-relative Directions</span></a><li><a href=#line-directions><span class=secno>6.3</span> <span class=content>
Line-relative Directions</span></a><li><a href=#logical-to-physical><span class=secno>6.4</span> <span class=content>
Abstract-to-Physical Mappings</span></a></ul><li><a href=#abstract-layout><span class=secno>7</span> <span class=content>
Abstract Box Layout</span></a><ul class=toc><li><a href=#vertical-layout><span class=secno>7.1</span> <span class=content>
Principles of Layout in Vertical Writing Modes</span></a><li><a href=#dimension-mapping><span class=secno>7.2</span> <span class=content>
Dimensional Mapping</span></a><li><a href=#orthogonal-flows><span class=secno>7.3</span> <span class=content>
Orthogonal Flows</span></a><ul class=toc><li><a href=#orthogonal-auto><span class=secno>7.3.1</span> <span class=content>
Auto-sizing in Orthogonal Flows</span></a><li><a href=#auto-multicol><span class=secno>7.3.2</span> <span class=content>
Auto-sizing Block Containers in Orthogonal Flows</span></a><li><a href=#orthogonal-layout><span class=secno>7.3.3</span> <span class=content>
Auto-sizing Other Orthogonal Flow Roots</span></a><li><a href=#orthogonal-pagination><span class=secno>7.3.4</span> <span class=content>
Fragmenting Orthogonal Flows</span></a></ul><li><a href=#logical-direction-layout><span class=secno>7.4</span> <span class=content>
Flow-Relative Mappings</span></a><li><a href=#line-mappings><span class=secno>7.5</span> <span class=content>
Line-Relative Mappings</span></a><li><a href=#physical-only><span class=secno>7.6</span> <span class=content>
Purely Physical Mappings</span></a><li><a href=#caption-side><span class=secno>7.7</span> <span class=content>
Table Caption Mappings: the <span class=property data-link-type=propdesc title=caption-side>caption-side</span> keywords</span></a></ul><li><a href=#page-direction><span class=secno>8</span> <span class=content>
Page Flow: the page progression direction</span></a><li><a href=#text-combine><span class=secno>9</span> <span class=content>
Glyph Composition</span></a><ul class=toc><li><a href=#text-combine-upright><span class=secno>9.1</span> <span class=content>
Horizontal-in-Vertical Composition: the <span class=property data-link-type=propdesc title=text-combine-upright>text-combine-upright</span> property</span></a><ul class=toc><li><a href=#text-combine-runs><span class=secno>9.1.1</span> <span class=content>
Text Run Rules</span></a><li><a href=#text-combine-layout><span class=secno>9.1.2</span> <span class=content>
Layout Rules</span></a><li><a href=#text-combine-compression><span class=secno>9.1.3</span> <span class=content>
Compression Rules</span></a><ul class=toc><li><a href=#text-combine-fullwidth><span class=secno>9.1.3.1</span> <span class=content>
Full-width Characters</span></a></ul></ul></ul><li><a href=#changes><span class=secno></span> <span class=content>Changes</span></a><ul class=toc><li><a href=#changes-201311><span class=secno></span> <span class=content>
Changes since the <span>March
2014 CSS Writing Modes Module Level 3 Candidate Recommendation</span></span></a></ul><li><a href=#acknowledgements><span class=secno></span> <span class=content>
Acknowledgements</span></a><li><a href=#character-properties><span class=secno></span> <span class=content>Appendix A.
Characters and Properties</span></a><li><a href=#bidi-html><span class=secno></span> <span class=content>
Appendix B: Bidi Rules for HTML 4</span></a><li><a href=#script-orientations><span class=secno></span> <span class=content>Appendix C:
Vertical Scripts in Unicode</span></a><li><a href=#conformance><span class=secno></span> <span class=content>
Conformance</span></a><ul class=toc><li><a href=#conventions><span class=secno></span> <span class=content>
Document conventions</span></a><li><a href=#conformance-classes><span class=secno></span> <span class=content>
Conformance classes</span></a><li><a href=#partial><span class=secno></span> <span class=content>
Partial implementations</span></a><li><a href=#experimental><span class=secno></span> <span class=content>
Experimental implementations</span></a><li><a href=#testing><span class=secno></span> <span class=content>
Non-experimental implementations</span></a></ul><li><a href=#references><span class=secno></span> <span class=content>References</span></a><ul class=toc><li><a href=#normative><span class=secno></span> <span class=content>Normative References</span></a><li><a href=#informative><span class=secno></span> <span class=content>Informative References</span></a></ul><li><a href=#index><span class=secno></span> <span class=content>Index</span></a><li><a href=#property-index><span class=secno></span> <span class=content>Property Index</span></a><li><a href=#issues-index><span class=secno></span> <span class=content>Issues Index</span></a></ul></div>
<main>
<h2 class="heading settled" data-level=1 id=text-flow><span class=secno>1. </span><span class=content>
Introduction to Writing Modes</span><a class=self-link href=#text-flow></a></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>
<p>A <dfn data-dfn-type=dfn data-export="" id=writing-mode>writing mode<a class=self-link href=#writing-mode></a></dfn> in CSS is determined by the <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a>,
<a class=property data-link-type=propdesc href=#propdef-direction>direction</a>, and <a class=property data-link-type=propdesc href=#propdef-text-orientation>text-orientation</a> properties. It is defined primarily
in terms of its <i>inline base direction</i> and <i>block flow direction</i>:
<div class=sidebar>
<div class="figure right">
<a href=diagrams/text-flow-vectors-tb.svg>
<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>
<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>
<img alt="Han-based writing mode" class=landscape src=diagrams/text-flow-vectors-tb.png></a>
<a href=diagrams/text-flow-vectors-rl.svg>
<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 data-dfn-type=dfn data-export="" id=inline-base-direction>inline base direction<a class=self-link href=#inline-base-direction></a></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 <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property specifies the
inline base direction of a box and, together with the <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a>
property and the inherent directionality of any text content, determines
the ordering of inline-level content within a line.
<p>The <dfn data-dfn-type=dfn data-export="" id=block-flow-direction>block flow direction<a class=self-link href=#block-flow-direction></a></dfn> is the direction in which
block-level boxes stack and the direction in which line boxes stack
within a block container. The <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> property determines the
block flow direction.</p>
<p>A <dfn data-dfn-type=dfn data-export="" id=horizontal-writing-mode>horizontal writing mode<a class=self-link href=#horizontal-writing-mode></a></dfn> is one with horizontal lines of
text, i.e. a downward or upward block flow.
A <dfn data-dfn-type=dfn data-export="" id=vertical-writing-mode>vertical writing mode<a class=self-link href=#vertical-writing-mode></a></dfn> is one with vertical lines of text,
i.e. a leftward or rightward block flow.
<p class=note role=note>These terms should not be confused with
<dfn data-dfn-type=dfn data-noexport="" id=vertical-block-flow>vertical block flow<a class=self-link href=#vertical-block-flow></a></dfn> (which is a downward or
upward block flow) and <dfn data-dfn-type=dfn data-noexport="" id=horizontal-block-flow>horizontal block flow<a class=self-link href=#horizontal-block-flow></a></dfn> (which is
leftward or rightward block flow). To avoid confusion, CSS
specifications avoid this latter set of terms.</p>
<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 <a class=property data-link-type=propdesc href=#propdef-text-orientation>text-orientation</a> component of the writing mode determines the
<i>line orientation</i>, and controls
details of text layout such as the <i>glyph orientation</i>.
<p class=note role=note>See Unicode Technical Note #22 <a data-biblio-type=informative data-link-type=biblio href=#biblio-utn22 title=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 class="heading settled" data-level=1.1 id=placement><span class=secno>1.1. </span><span class=content>
Module Interactions</span><a class=self-link href=#placement></a></h3>
<p>This module replaces and extends the <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> and <a class=property data-link-type=propdesc href=#propdef-direction>direction</a>
features defined in <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a> sections 8.6 and 9.10.
<h3 class="heading settled" data-level=1.2 id=values><span class=secno>1.2. </span><span class=content>
Values</span><a class=self-link href=#values></a></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 data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>.
Other CSS modules may expand the definitions of these value types: for
example <a data-biblio-type=informative data-link-type=biblio href=#biblio-css3color title=CSS3COLOR>[CSS3COLOR]</a>, when combined with this module, expands the
definition of the <color> value type as used in this specification.</p>
<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.
<p><a id=bidi></a></p>
<h2 class="heading settled" data-level=2 id=text-direction><span class=secno>2. </span><span class=content>
Inline Direction and Bidirectionality</span><a class=self-link href=#text-direction></a></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
<dfn data-dfn-type=dfn data-export="" id=bidirectionality>bidirectionality<a class=self-link href=#bidirectionality></a></dfn>, or "bidi" for short.</p>
<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.
<p>User agents that support bidirectional text must apply the Unicode
bidirectional algorithm to every sequence of inline-level boxes uninterrupted
by any block boundary or
“<a href=http://www.unicode.org/reports/tr9/#Bidirectional_Character_Types>bidi type B</a>”
<dfn data-dfn-type=dfn data-export="" id=forced-paragraph-break>forced paragraph break<a class=self-link href=#forced-paragraph-break></a></dfn>.
This sequence forms the <dfn data-dfn-type=dfn data-export="" id=bidi-paragraph title="bidi paragraph">paragraph<a class=self-link href=#bidi-paragraph></a></dfn> unit
in the bidirectional algorithm.
<p>Two CSS properties, <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> and <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a>,
provide explicit embedding, isolation, and override controls in the CSS layer.
Because the base directionality of a text depends on the structure and
semantics of the document, the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> and <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> properties
should in most cases be used only to map bidi information in the markup
to its corresponding CSS styles.
<strong>If a document language provides markup features to control
bidi, authors and users should use those features instead</strong> and not
specify CSS rules to override them.</p>
<p>In CSS,
the paragraph embedding level must be set
(following <a href=http://www.unicode.org/reports/tr9/#HL1>UAX9 clause HL1</a>)
according to the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property of the paragraph’s containing block
rather than by the heuristic given in steps
<a href=http://www.unicode.org/reports/tr9/#P2>P2</a> and
<a href=http://www.unicode.org/reports/tr9/#P2>P3</a>
of the Unicode algorithm.
There is, however, one exception:
when the computed <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> of the paragraph’s containing block is <a class=css data-link-type=maybe href=#valdef-unicode-bidi-plaintext>plaintext</a>,
the Unicode heuristics in P2 and P3 are used as described in <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>,
without the HL1 override.
<p>The HTML specifications (<a data-biblio-type=informative data-link-type=biblio href=#biblio-html401 title=HTML401>[HTML401]</a>, section 8.2, and <a data-biblio-type=informative data-link-type=biblio href=#biblio-html5 title=HTML5>[HTML5]</a>, section 10.3.5) define
bidirectionality behavior for HTML elements.</p>
<p class=note role=note>Because HTML UAs can turn off CSS styling, we advise HTML
authors to use the HTML <code>dir</code> attribute and <bdo> element to
ensure correct bidirectional layout in the absence of a style sheet.</p>
<h3 class="heading settled" data-level=2.1 id=direction><span class=secno>2.1. </span><span class=content>
Specifying Directionality: the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property</span><a class=self-link href=#direction></a></h3>
<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-direction>direction<a class=self-link href=#propdef-direction></a></dfn><tr><th>Value:<td class=prod>ltr <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> 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 value:<td>specified value<tr><th>Animatable:<td>no<tr><th>Canonical order:<td>n/a</table>
<p>This property specifies the inline base direction or directionality
of any bidi paragraph, embedding, isolate, or override established by the box.
(See <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a>.)
In addition, it informs 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 layout effects
that depend on the box’s inline base direction.</p>
<p>Values for this property have the following meanings:</p>
<dl data-dfn-for=direction data-dfn-type=value>
<dt><dfn class=css data-dfn-for=direction data-dfn-type=value data-export="" id=valdef-direction-ltr>ltr<a class=self-link href=#valdef-direction-ltr></a></dfn></dt>
<dd>Left-to-right directionality.</dd>
<dt><dfn class=css data-dfn-for=direction data-dfn-type=value data-export="" id=valdef-direction-rtl>rtl<a class=self-link href=#valdef-direction-rtl></a></dfn></dt>
<dd>Right-to-left directionality.</dd>
</dl>
<p class=note role=note>The <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property has no effect on bidi reordering
when specified on inline boxes whose <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> property’s
value is <a class=css data-link-type=maybe href=#valdef-unicode-bidi-normal>normal</a>, because the box does not open an additional level
of embedding with respect to the bidirectional algorithm.</p>
<p>The value of the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property on the root element is also
propagated to the initial containing block and, together with the
<a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> property, determines the document’s principal writing
mode. (See <a href=#principal-writing-mode>below</a>.)
<p class=note role=note>Note that the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> 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 role=note>The <a class=property data-link-type=propdesc href=#propdef-direction>direction</a>
property, when specified for table column boxes, 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 data-biblio-type=informative data-link-type=biblio href=#biblio-html401 title=HTML401>[HTML401]</a>, section 11.3.2.1.
<h3 class="heading settled" data-level=2.2 id=unicode-bidi><span class=secno>2.2. </span><span class=content>
Embeddings and Overrides: the <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> property</span><a class=self-link href=#unicode-bidi></a></h3>
<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-unicode-bidi>unicode-bidi<a class=self-link href=#propdef-unicode-bidi></a></dfn><tr><th>Value:<td class=prod>normal <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> embed <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> isolate <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> bidi-override <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> isolate-override <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> 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 value:<td>specified value<tr><th>Animatable:<td>no</table>
<p>Normally (i.e. when <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> is <a class=css data-link-type=maybe href=#valdef-unicode-bidi-normal>normal</a>)
an inline box is transparent to the unicode bidi algorithm;
content is ordered as if the box’s boundaries were not there.
Other values of the <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> property cause inline boxes
to create scopes within the algorithm,
and to override the intrinsic directionality of text.
<p>The following informative table summarizes the box-internal and
box-external effects of <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a>:
<table class=data>
<caption>Effect of non-<a class=css data-link-type=maybe href=#valdef-unicode-bidi-normal>normal</a> values of <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> on inline boxes</caption>
<colgroup span=2></colgroup>
<colgroup span=2></colgroup>
<thead>
<tr><th colspan=2 rowspan=2>
<th colspan=2 scope=rowgroup><abbr title="To surrounding contents, the box behaves as if its boundary were...">Outside</abbr>
<tr><th><abbr title="a strong character of the box’s 'direction'.">strong</abbr>
<th><abbr title="a neutral character.">neutral</abbr>
</thead>
<tbody>
<tr><th rowspan=3 scope=colgroup><abbr title="Within the box, content is ordered as if...">Inside</abbr>
<th><abbr title="the box’s boundaries were strong characters of the box’s 'direction'.">scoped</abbr>
<td><a class=css data-link-type=maybe href=#valdef-unicode-bidi-embed>embed</a>
<td><a class=css data-link-type=maybe href=#valdef-unicode-bidi-isolate>isolate</a>
<tr><th><abbr title="all text consisted of strong characters of the box’s 'direction'.">override</abbr>
<td><a class=css data-link-type=maybe href=#valdef-unicode-bidi-bidi-override>bidi-override</a>
<td><a class=css data-link-type=maybe href=#valdef-unicode-bidi-isolate-override>isolate-override</a>
<tr><th><abbr title="the box were a standalone paragraph ordered using UAX9 heuristics.">plaintext</abbr>
<td>—
<td><a class=css data-link-type=maybe href=#valdef-unicode-bidi-plaintext>plaintext</a>
</tbody>
</table>
<p>Values for this property have the following (normative) meanings:</p>
<dl>
<dt><dfn class=css data-dfn-for=unicode-bidi data-dfn-type=value data-export="" id=valdef-unicode-bidi-normal>normal<a class=self-link href=#valdef-unicode-bidi-normal></a></dfn></dt>
<dd>The box does not open an additional level of embedding with
respect to the bidirectional algorithm. For inline boxes,
implicit reordering works across box boundaries.</dd>
<dt><dfn class=css data-dfn-for=unicode-bidi data-dfn-type=value data-export="" id=valdef-unicode-bidi-embed>embed<a class=self-link href=#valdef-unicode-bidi-embed></a></dfn></dt>
<dd>If the box is inline, this value creates a <dfn data-dfn-type=dfn data-export="" id=directional-embedding>directional embedding<a class=self-link href=#directional-embedding></a></dfn>
by opening an additional level of embedding with respect to the bidirectional algorithm.
The direction of this embedding level is given by the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a>
property. Inside the box, reordering is done implicitly.
<p>Following clause <a href=http://www.unicode.org/reports/tr9/#HL3>HL3</a> <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>,
this corresponds to inserting
a LRE (U+202A), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: ltr</a>,
or RLE (U+202B), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: rtl</a>,
at the start of the inline box
and a PDF (U+202C) at the end of the box.
<p class=note role=note>This value has no effect on boxes that are not inline.
<dt><dfn class=css data-dfn-for=unicode-bidi data-dfn-type=value data-export="" id=valdef-unicode-bidi-isolate>isolate<a class=self-link href=#valdef-unicode-bidi-isolate></a></dfn></dt>
<dd>On an inline box, this <dfn data-dfn-type=dfn data-export="" id=bidi-isolate title="bidi-isolate|bidi-isolated|bidi isolation|isolation">bidi-isolates<a class=self-link href=#bidi-isolate></a></dfn> its contents.
This is similar to a directional embedding (and increases the embedding level accordingly)
except that each sequence of inline-level boxes
uninterrupted by any block boundary or <i>forced paragraph break</i>
is treated as an <dfn data-dfn-type=dfn data-noexport="" id=isolated-sequence>isolated sequence<a class=self-link href=#isolated-sequence></a></dfn>:
<ul>
<li>the content within the sequence is ordered
as if inside an independent paragraph
with the base directionality specified by the box’s <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property.
<li>for the purpose of bidi resolution in its containing bidi paragraph,
the sequence is treated as if it were a single Object Replacement Character (U+FFFC).
</ul>
In effect, neither is the content inside the box bidi-affected
by the content surrounding the box,
nor is the content surrounding the box bidi-affected by the
content or specified directionality of the box.
However, <i>forced paragraph breaks</i> within the box still create
a corresponding break in the containing paragraph.
<p>Following clause <a href=http://www.unicode.org/reports/tr9/#HL3>HL3</a> <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>,
this corresponds to inserting
an LRI (U+2066), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: ltr</a>,
or RLI (U+2067), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: rtl</a>,
at the start of the inline box,
and a PDI (U+2069) at the end of the box. <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>
<p class=note role=note>This value has no effect on boxes that are not inline.
<dt><dfn class=css data-dfn-for=unicode-bidi data-dfn-type=value data-export="" id=valdef-unicode-bidi-bidi-override>bidi-override<a class=self-link href=#valdef-unicode-bidi-bidi-override></a></dfn></dt>
<dd>This value puts the box’s immediate inline content in a <dfn data-dfn-type=dfn data-export="" id=directional-override>directional override<a class=self-link href=#directional-override></a></dfn>.
For an inline, this means that the box acts like a <i>directional embedding</i>
in the bidirectional algorithm,
except that reordering within it is strictly in sequence according to the
<a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property; the implicit part of the bidirectional algorithm
is ignored.
For a block container, the override is applied
to an anonymous inline box that surrounds all of its content.
<p>Following clause <a href=http://www.unicode.org/reports/tr9/#HL3>HL3</a> <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>,
this corresponds to inserting
an LRO (U+202D), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: ltr</a>,
or RLO (U+202E), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: rtl</a>,
at the start of the inline box
and a PDF (U+202C) at the end of the box. <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>
<dt><dfn class=css data-dfn-for=unicode-bidi data-dfn-type=value data-export="" id=valdef-unicode-bidi-isolate-override>isolate-override<a class=self-link href=#valdef-unicode-bidi-isolate-override></a></dfn></dt>
<dd>This combines the <i>isolation</i> behavior of <a class=css data-link-type=maybe href=#valdef-unicode-bidi-isolate>isolate</a>
with the <i>directional override</i> behavior of <a class=css data-link-type=maybe href=#valdef-unicode-bidi-bidi-override>bidi-override</a>:
to surrounding content, it is equivalent to <a class=css data-link-type=maybe href=#valdef-unicode-bidi-isolate>isolate</a>,
but within the box content is ordered as if <a class=css data-link-type=maybe href=#valdef-unicode-bidi-bidi-override>bidi-override</a> were specified.
<p>Following clause <a href=http://www.unicode.org/reports/tr9/#HL3>HL3</a> <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>,
for inline boxes this corresponds to inserting
an FSI (U+2068) followed by
an LRO (U+202D), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: ltr</a>,
or RLO (U+202E), for <a class=css data-link-type=propdesc href=#propdef-direction title=direction>direction: rtl</a> at the start of the box,
and a PDF (U+202C) followed by PDI (U+2069) at the end of the box. <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>
For other boxes, this value is exactly equivalent to <a class=css data-link-type=maybe href=#valdef-unicode-bidi-bidi-override>bidi-override</a>.
<dt><dfn class=css data-dfn-for=unicode-bidi data-dfn-type=value data-export="" id=valdef-unicode-bidi-plaintext>plaintext<a class=self-link href=#valdef-unicode-bidi-plaintext></a></dfn></dt>
<dd><p>This value behaves as <a class=css data-link-type=maybe href=#valdef-unicode-bidi-isolate>isolate</a> except that for the purposes of
the Unicode bidirectional algorithm, the base directionality of each
of the box’s <i>bidi paragraphs</i> (if a block container)
or <i>isolated sequences</i> (if an inline)
is determined by following the heuristic in rules P2 and P3
of the Unicode bidirectional algorithm
(rather than by using the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property of the box).
<p>Following cause <a href=http://www.unicode.org/reports/tr9/#HL3>HL3</a> <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>,
for inline boxes this corresponds to inserting
an FSI (U+2068) at the start of the box,
and a PDI (U+2069) at the end of the box. <a data-biblio-type=normative data-link-type=biblio href=#biblio-uax9 title=UAX9>[UAX9]</a>
</dl>
<p class=note role=note>Because the <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> property does not inherit,
setting <a class=css data-link-type=maybe href=#valdef-unicode-bidi-bidi-override>bidi-override</a> or <a class=css data-link-type=maybe href=#valdef-unicode-bidi-plaintext>plaintext</a> on a block box will
not affect any descendant blocks. Therefore these values are best
used on blocks and inlines that do not contain any block-level
structures.
<p class=note role=note>Note that <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> does not affect the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a>
property even in the case of <a class=css data-link-type=maybe href=#valdef-unicode-bidi-plaintext>plaintext</a>, and thus does not affect
<a class=property data-link-type=propdesc href=#propdef-direction>direction</a>-dependent layout calculations.
<p>The final order of characters within each <i>bidi paragraph</i> 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 <a class=css data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display title=display>display: inline</a>
are treated as neutral characters,
unless their <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> property is either <a class=css data-link-type=maybe href=#valdef-unicode-bidi-embed>embed</a> or <a class=css data-link-type=maybe href=#valdef-unicode-bidi-bidi-override>bidi-override</a>,
in which case they are treated as strong characters
in the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> specified for the element.
All other atomic inline-level boxes are treated as neutral characters
always.</p>
<p>If an inline box is broken around a <i>bidi paragraph</i> boundary
(e.g. if split by a block or <i>forced paragraph break</i>),
then the <a href=http://www.unicode.org/reports/tr9/#HL3>HL3</a> bidi control codes assigned to the end of the box
are also added before the interruption
and the codes assigned to the start of the box are also added after it.
(In other words, any embedding levels, isolates, or overrides started by the box
are closed at the paragraph break and reopened on the other side of it.)
<div class=example>
<p>For example, where <BR/> is a <i>forced paragraph break</i>
the bidi ordering is identical between
<pre><para>...<i1><i2>...<BR/>...</i2></i1>...</para></pre>
<p>and
<pre><para>...<i1><i2>...</i2></i1><BR/><i1><i2>...</i2></i1>...</para></pre>
<p>for all values of <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> on inline elements <i1> and <i2>.
</div>
<p class=note role=note>
Because the Unicode algorithm has a limit of 125 levels of embedding,
care should be taken not to use <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a>
with a value other than <a class=css data-link-type=maybe href=#valdef-unicode-bidi-normal>normal</a> unless appropriate.
In particular, a value of <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-cascade-3/#valuedef-inherit>inherit</a>
should be used with extreme caution. However, for elements that are,
in general, intended to be displayed as blocks, a setting of
<a class=css data-link-type=propdesc href=#propdef-unicode-bidi title=unicode-bidi>unicode-bidi: isolate</a> is preferred to keep the element together
in case the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display>display</a> is changed to <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-display-3/#valdef-display-inline>inline</a>
(see example below).</p>
<h3 class="heading settled" data-level=2.3 id=bidi-example><span class=secno>2.3. </span><span class=content>
Example of Bidirectional Text</span><a class=self-link href=#bidi-example></a></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.</p>
<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 below in logical backing store order.</p>
<pre class=xml-example><section dir=rtl>
<para>HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5</para>
<para>HEBREW6 <emphasis>HEBREW7</emphasis> HEBREW8</para>
</section>
<section dir=ltr>
<para>english9 english10 english11 HEBREW12 HEBREW13</para>
<para>english14 english15 english16</para>
<para>english17 <quote dir=rtl>HEBREW18 english19 HEBREW20</quote></para>
</section>
</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 */
[dir=ltr] {direction: rtl; unicode-bidi: isolate; }
[dir=rtl] {direction: ltr; unicode-bidi: isolate; }
/* Rules for presentation */
section, para {display: block;}
emphasis {font-weight: bold;}
quote {font-style: italic;}
</pre>
<p>If the line length is long,
the formatting of this text might look like this:</p>
<pre class=ascii-art> 5WERBEH 4WERBEH english3 2WERBEH 1WERBEH
8WERBEH <b>7WERBEH</b> 6WERBEH
english9 english10 english11 13WERBEH 12WERBEH
english14 english15 english16
english17 <i title="">20WERBEH english19 18WERBEH</i>
</pre>
<p>The first <code><section></code> element is a block with a right-to-left base direction,
the second <code><section></code> element is a block with a left-to-right base direction.
The <code><para></code>s are blocks that inherit the base direction from their parents.
Thus, the first two <code><para></code>s are read starting at the top right,
the final three are read starting at the top left.</p>
<p>The <code><emphasis></code> element is inline-level,
and since its value for <a class=property data-link-type=propdesc href=#propdef-unicode-bidi>unicode-bidi</a> is <a class=css data-link-type=maybe href=#valdef-unicode-bidi-normal>normal</a> (the initial value),
it has no effect on the ordering of the text.
<p>The <code><quote></code> element, on the other hand,
creates an <i>isolated sequence</i> with the given internal directionality.
Note that this causes <samp>HEBREW18</samp> to be to the right of <samp>english19</samp>.
<p>If lines have to be broken, the same text might format 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 <i title="">18WERBEH
20WERBEH english19</i>
</pre>
<p>Notice that because <samp>HEBREW18</samp> must be read before <samp>english19</samp>,
it is on the line above <samp>english19</samp>.
Just breaking the long line from the earlier formatting would not have worked.
<p>Note also that the first syllable from <samp>english19</samp>
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.
</div>
<h3 class="heading settled" data-level=2.4 id=bidi-box-model><span class=secno>2.4. </span><span class=content>
Box model for inline boxes in bidirectional context</span><a class=self-link href=#bidi-box-model></a></h3>
<p>Since bidi reordering can split apart and reorder text that is
logically contiguous, bidirectional text can cause an inline box
to be split and reordered within a line.
<p class=note role=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>
<p>For each line box, UAs must take the fragments of each inline box
and render the margins, borders and padding in visual order (not logical order).
The <i>start</i>-most fragment on the first line box in which the box appears
has the <i>start</i> edge’s margin, border, and padding;
and the end-most fragment on the last line box in which the box appears
has the <i>end</i> edge’s margin, border, and padding.
For example, in the <a class=css data-link-type=maybe href=#valdef-writing-mode-horizontal-tb>horizontal-tb</a> writing mode:
<ul>
<li>When the parent’s <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property is <a class=css data-link-type=maybe href=#valdef-direction-ltr>ltr</a>,
the left-most box fragment on the first line box in which the box appears
has the left margin, left border and left padding,
and the right-most box fragment on the last line box in which the box appears
has the right padding, right border and right margin.
<li>When the parent’s <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property is <a class=css data-link-type=maybe href=#valdef-direction-rtl>rtl</a>,
the right-most fragment of the first line box in which the box appears
has the right padding, right border and right margin,
and the left-most fragment of the last line box in which the box appears
has the left margin, left border and left padding.
</ul>
<p>Analogous rules hold for vertical writing modes.</p>
<p class=note role=note>The <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-break-3/#box-decoration-break>box-decoration-break</a> property can override this behavior
to draw box decorations on both sides of each fragment. <a data-biblio-type=normative data-link-type=biblio href=#biblio-css3-break title=CSS3-BREAK>[CSS3-BREAK]</a>
<h2 class="heading settled" data-level=3 id=vertical-intro><span class=secno>3. </span><span class=content>
Introduction to Vertical Text</span><a class=self-link href=#vertical-intro></a></h2>
<p><em>This subsection is non-normative.</em></p>
<p>In addition to extensions to CSS2.1’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 role=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>
<p>The change from horizontal to vertical writing can affect not just the
layout, but also the typesetting. For example, the position of a punctuation
mark within its spacing box can change from the horizontal to the
vertical case, and in some cases alternate glyphs are used.
<p>Vertical text that includes Latin script text or text from other scripts
normally displayed horizontally can display that text in a number of
ways. For example, Latin words can be rotated sideways, or each letter
can be oriented upright:
<div class=figure>
<p><img alt="A dictionary definition for ヴィルス
might write the English word 'virus' rotated 90° clockwise,
but stack the letters of the initialisms 'RNA' and 'DNA' upright." src=vert-latin-layouts.png></p>
<p class=caption>Examples of Latin in vertical Japanese: Daijirin Viewer 1.4 (iOS)
</div>
<p>In some special cases such as two-digit numbers in dates, text is fit
compactly into a single vertical character box:
<div class=figure id=fig-mac>
<p><img alt="An excerpt from MacFan shows several possible vertical layouts
for numbers: the two-digit month and day are written as
horizontal-in-vertical blocks; the years are written with
each character upright; except in the English phrase
“for Mac 2011”, where the date is rotated to
match the rotated Latin." src=vert-number-layouts.png></p>
<p class=caption>Mac Fan, December 2010, p.49</p>
</div>
<p>Layouts often involve a mixture of vertical and horizontal elements:
<div class=figure>
<p><img alt="Magazines often mix horizontal and vertical layout; for
example, using one orientation for the main article text
and a different one for sidebar or illustrative content." src=vert-horiz-combination.png></p>
<p class=caption>Mixture of vertical and horizontal elements</p>
</div>
<p>Vertical text layouts also need to handle bidirectional text layout;
clockwise-rotated Arabic, for example, is laid out bottom-to-top.
<h3 class="heading settled" data-level=3.1 id=block-flow><span class=secno>3.1. </span><span class=content>
Block Flow Direction: the <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> property</span><a class=self-link href=#block-flow></a></h3>
<table class="definition propdef"><tr><th>Name:<td><dfn class=css data-dfn-type=property data-export="" id=propdef-writing-mode>writing-mode<a class=self-link href=#propdef-writing-mode></a></dfn><tr><th>Value:<td class=prod>horizontal-tb <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> vertical-rl <a data-link-for="" data-link-type=grammar href=http://dev.w3.org/csswg/css-values-3/#comb-one>|</a> vertical-lr<tr><th>Initial:<td>horizontal-tb<tr><th>Applies to:<td>All elements except table row groups, table column groups, table rows, and table columns<tr><th>Inherited:<td>yes<tr><th>Percentages:<td>n/a<tr><th>Media:<td>visual<tr><th>Computed value:<td>specified value<tr><th>Animatable:<td>no<tr><th>Canonical order:<td>n/a</table>
<p>This property specifies whether lines of text are laid out horizontally
or vertically and the direction in which blocks progress. Possible
values:</p>
<dl data-dfn-for=writing-mode data-dfn-type=value>
<dt><dfn class=css data-dfn-for=writing-mode data-dfn-type=value data-export="" id=valdef-writing-mode-horizontal-tb>horizontal-tb<a class=self-link href=#valdef-writing-mode-horizontal-tb></a></dfn></dt>
<dd>Top-to-bottom <i>block flow direction</i>.
The <i>writing mode</i> is horizontal.</dd>
<dt><dfn class=css data-dfn-for=writing-mode data-dfn-type=value data-export="" id=valdef-writing-mode-vertical-rl>vertical-rl<a class=self-link href=#valdef-writing-mode-vertical-rl></a></dfn></dt>
<dd>Right-to-left <i>block flow direction</i>.
The <i>writing mode</i> is vertical.</dd>
<dt><dfn class=css data-dfn-for=writing-mode data-dfn-type=value data-export="" id=valdef-writing-mode-vertical-lr>vertical-lr<a class=self-link href=#valdef-writing-mode-vertical-lr></a></dfn></dt>
<dd>Left-to-right <i>block flow direction</i>.
The <i>writing mode</i> is vertical.</dd>
</dl>
<p>The <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> property specifies the <i>block flow direction</i>,
which determines the ordering direction of block-level boxes in a block formatting context;
the ordering direction of line boxes in a block container that contains inlines;
the ordering direction of rows in a table; etc.
By virtue of determining the stacking direction of line boxes,
the <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> property also determines whether the line boxes' orientation (and thus the <i>writing mode</i>)
is horizontal or vertical.
The <a class=property data-link-type=propdesc href=#propdef-text-orientation>text-orientation</a> property then determines how text is laid out within the line box.
<p>The <dfn data-dfn-type=dfn data-export="" id=principal-writing-mode>principal writing mode<a class=self-link href=#principal-writing-mode></a></dfn> of the
document is determined by the <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> and <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> values
specified on the root element. This writing mode is used, for example,
to determine the default page progression direction. (See <a data-biblio-type=informative data-link-type=biblio href=#biblio-css3page title=CSS3PAGE>[CSS3PAGE]</a>.)
Like <a class=property data-link-type=propdesc href=#propdef-direction>direction</a>, the <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> value of the root element is also
propagated to the initial containing block and sets the block flow
direction of the initial block formatting context.
<p class=note role=note>Note that the <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> 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>If a box has a different block flow direction than its containing
block:
<ul>
<li>If the box has a specified <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display>display</a> of <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-display-3/#valdef-display-inline>inline</a>, its <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display>display</a>
computes to <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-display-3/#valdef-display-inline-block>inline-block</a>. <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>
<li>If the box has a specified <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display>display</a> of <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-display-3/#valdef-display-outside-run-in>run-in</a>, its <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display>display</a>
computes to <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-display-3/#valdef-display-inside-block>block</a>. <a data-biblio-type=normative data-link-type=biblio href=#biblio-css21 title=CSS21>[CSS21]</a>
<li>If the box is a block container, then it establishes a new block
formatting context.
</ul>
<p>The content of replaced elements do not rotate due to the writing mode:
images, for example, remain upright. However replaced content
involving text (such as MathML content or form elements) should match
the replaced element’s writing mode and line orientation if the UA
supports such a vertical writing mode for the replaced content.
<div class=example>
<p>In the following example, two block elements (1 and 3) separated
by an image (2) are presented in various flow writing modes.</p>
<p>Here is a diagram of horizontal writing mode (<code>writing-mode: horizontal-tb</code>):</p>
<p><img alt="Diagram of horizontal layout: blocks 1, 2, and 3 are stacked top-to-bottom" height=300 src=horizontal.png width=219></p>
<p>Here is a diagram for the right-to-left vertical writing mode commonly
used in East Asia (<code>writing-mode: vertical-rl</code>):</p>
<p><img alt="Diagram of a right-to-left vertical layout: blocks 1, 2,
and 3 are arranged side by side from right to left" height=191 src=vertical-rl.png width=297></p>
<p>And finally, here is a diagram for the left-to-right vertical
writing mode used for Manchu and Mongolian (<code>writing-mode: vertical-lr</code>):</p>
<p><img alt="Diagram of left-to-right vertical layout: blocks 1, 2,
and 3 are arranged side by side from left to right" height=191 src=vertical-lr.png width=300></p>
</div>
<div class=example>
<p>In the following example, some form controls are rendered inside
a block with <a class=css data-link-type=maybe href=#valdef-writing-mode-vertical-rl>vertical-rl</a> writing mode. The form controls are
rendered to match the writing mode.
<pre><style>
form { writing-mode: vertical-rl; }
</style>
...
<form>
<p><label>姓名 <input value="艾俐俐"></label>
<p><label>语言 <select><option>English
<option>français
<option>فارسی
<option>中文
<option>日本語</select></label>
</form></pre>
<p><img alt="Screenshot of vertical layout: the input element is
laid lengthwise from top to bottom and its contents
rendered in a vertical writing mode, matching the
labels outside it. The drop-down selection control
after it slides out to the side (towards the after
edge of the block) rather than downward as it would
in horizontal writing modes." src=vertical-form.png></p>
</div>
<div class=example>
<p>In this example, <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> sets the list markers upright
using the <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-lists-3/#selectordef-marker>::marker</a> pseudo-element. Vertical alignment ensures
that longer numbers will still align with the right of the first
line of text. <a data-biblio-type=informative data-link-type=biblio href=#biblio-css3list title=CSS3LIST>[CSS3LIST]</a>
<pre>::marker { writing-mode: horizontal-tb; vertical-align: text-top;
color: blue; }</pre>
<div class=figure>
<p><img alt="Diagram showing list markers of '1.', '2.', '3.' sitting
upright atop sideways vertical Latin list item text." class=example src=vertical-horizontal-list-markers.png>
<p class=caption>Example of horizontal list markers in a vertical list</p>
</div>
</div>
<h4 class="heading settled" data-level=3.1.1 id=svg-writing-mode><span class=secno>3.1.1. </span><span class=content>
SVG1.1 <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> Values</span><a class=self-link href=#svg-writing-mode></a></h4>
<p>SVG1.1 <a data-biblio-type=normative data-link-type=biblio href=#biblio-svg11 title=SVG11>[SVG11]</a> defines some additional values: <span class=css>lr</span>,
<span class=css>lr-tb</span>, <span class=css>rl</span>, <span class=css>rl-tb</span>, <span class=css>tb</span>, and <span class=css>tb-rl</span>.
<p>These values are <em>deprecated</em> in any context except SVG1 documents.
Implementations that wish to support these values in the context of CSS
must treat them as follows:
<table class=data>
<thead>
<tr><th>SVG1/Obsolete</th> <th>CSS</tr>
</thead>
<tbody>
<tr><td><span class=css>lr</span></td> <td rowspan=3><a class=css data-link-type=maybe href=#valdef-writing-mode-horizontal-tb>horizontal-tb</a></tr>
<tr><td><span class=css>lr-tb</span></tr>
<tr><td><span class=css>rl</span></tr>
<tr><td><span class=css>tb</span></td> <td rowspan=2><a class=css data-link-type=maybe href=#valdef-writing-mode-vertical-rl>vertical-rl</a></tr>
<tr><td><span class=css>tb-rl</span></tr>
</tbody>
</table>
<p class=note role=note>The SVG1.1 values were also present in an older revision
of the CSS <a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a> specification, which is obsoleted by this
specification. The additional <span class=css>tb-lr</span> value of that revision is
replaced by <a class=css data-link-type=maybe href=#valdef-writing-mode-vertical-lr>vertical-lr</a>.
<p>In SVG1.1, these values set the <dfn data-dfn-type=dfn data-noexport="" id=inline-progression-direction>inline progression direction<a class=self-link href=#inline-progression-direction></a></dfn>,
in other words, the direction the current text position
advances each time a glyph is added. This is a geometric process that
happens <em>after</em> bidi reordering, and thus has no effect on the
interpretation of the <a class=property data-link-type=propdesc href=#propdef-direction>direction</a> property (which is independent of
<a class=property data-link-type=propdesc href=#propdef-writing-mode>writing-mode</a>). (See <a href=http://www.w3.org/TR/SVG/text.html#RelationshipWithBiDirectionality>Relationship
with bidirectionality</a>. <a data-biblio-type=normative data-link-type=biblio href=#biblio-svg11 title=SVG11>[SVG11]</a>)
<p class=note role=note>There are varying interpretations
on whether this process causes "writing-mode: rl" to merely shift the
text string or reverse the order of all glyphs in the text.</p>
<h2 class="heading settled" data-level=4 id=inline-alignment><span class=secno>4. </span><span class=content>
Inline-level Alignment</span><a class=self-link href=#inline-alignment></a></h2>
<p>When different kinds of inline-level content are placed together on a
line, the baselines of the content and the settings of the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visudet.html#propdef-vertical-align>vertical-align</a>
property control how they are aligned in the transverse direction of the
line box. This section discusses what baselines are, how to find them,
and how they are used together with the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visudet.html#propdef-vertical-align>vertical-align</a> property to
determine the alignment of inline-level content.
<h3 class="heading settled" data-level=4.1 id=intro-baselines><span class=secno>4.1. </span><span class=content>
Introduction to Baselines</span><a class=self-link href=#intro-baselines></a></h3>
<p><em>This section is non-normative.</em></p>
<p>A <dfn data-dfn-type=dfn data-noexport="" id=baseline>baseline<a class=self-link href=#baseline></a></dfn> is a line along the <i>inline axis</i> of a line box
along which individual glyphs of text are aligned. Baselines guide the
design of glyphs in a font (for example, the bottom of most alphabetic
glyphs typically align with the alphabetic baseline), and they guide
the alignment of glyphs from different fonts or font sizes when typesetting.
<div class=figure>
<p><img alt="Picture of alphabetic text in two font sizes with the baseline and em-boxes" src=alphabetic-baseline-in-two-font-sizes.svg width=480></p>
<p class=caption>Alphabetic text in two font sizes with the baseline and em-boxes</p>
</div>
<p>Different writing systems prefer different baseline tables.</p>
<div class=figure>
<p><img alt="Latin prefers the alphabetic baseline, on top of which most
letters rest, though some have descenders that dangle below it.
Indic scripts are sometimes typeset with a hanging baseline,
since their glyph shapes appear to be hanging from a
horizontal line.
Han-based systems, whose glyphs are designed to fill a square,
tend to align on their bottoms." src=script-preferred-baselines.gif></p>
<p class=caption>Preferred baselines in various writing systems</p>
</div>