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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>CSS Writing Modes Module Level 3</title>
<link rel="stylesheet" type="text/css" href="default.css">
<style type="text/css">
object { vertical-align: middle; }
.sidebar { float: right; background: #eee; border: solid gray; margin: 1em; }
.landscape { height: 10em; }
.portrait { width: 10em; }
.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; }
img, object { max-width: 100%; display: block; margin: 1em auto; }
</style>
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-WD.css">
</head>
<body>
<div class="head">
<!--logo-->
<h1>CSS Writing Modes Module Level 3</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</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/[YEAR]/WD-[SHORTNAME]-[CDATE]">[VERSION]</a>
-->
<dt>Latest version:</dt>
<dd><a
href="http://www.w3.org/TR/css3-writing-modes/">http://www.w3.org/TR/css3-writing-modes/</a></dd>
<dt>Previous version:</dt>
<dd><a href="http://www.w3.org/TR/2010/WD-css3-writing-modes-20101202/">http://www.w3.org/TR/2010/WD-css3-writing-modes-20101202/</a></dd>
<dt>Editors:</dt>
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Invited Expert)</dd>
<dd><a href="mailto:kojiishi@gluesoft.co.jp">Koji Ishii</a> (<a href="http://www.antenna.co.jp/">Antenna House</a>)</dd>
<dd><a href="mailto:murakami@antenna.co.jp">Shinyu Murakami</a> (<a href="http://www.antenna.co.jp/">Antenna House</a>)</dd>
<dt>Previous Editors:</dt>
<dd><a href="mailto:paulnel@microsoft.com">Paul Nelson</a> (<a href="http://www.microsoft.com/">Microsoft</a>)</dd>
<dd><a href="mailto:michelsu@microsoft.com">Michel Suignard</a> (<a href="http://www.microsoft.com/">Microsoft</a>)</dd>
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">Abstract</h2>
<p>CSS3 Writing Modes 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>Inherently bottom-to-top scripts are not handled in this version. See
[[UTN22]] for an explanation of relevant issues.</p>
<h2 class="no-num no-toc" id="status">Status of this document</h2>
<!--status-->
<h2 class="no-num no-toc" id="Contents">Table of Contents</h2>
<!--toc-->
<h2 id="text-flow">Introduction to Writing Modes</h2>
<p>CSS3 Writing Modes 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>writing mode</dfn> in CSS is determined by the 'writing-mode',
'direction', and 'text-orientation' 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">
<object data="diagrams/text-flow-vectors-tb.svg" type="image/svg+xml"
class="landscape"></object>
<p class="caption">Latin-based writing mode
</div>
<div class="figure left">
<object data="diagrams/text-flow-vectors-lr-reverse.svg" type="image/svg+xml"
class="portrait"></object>
<p class="caption">Mongolian-based writing mode
</div>
<div class="figure right">
<object data="diagrams/text-flow-vectors-tb.svg" type="image/svg+xml"
class="landscape"></object>
<object data="diagrams/text-flow-vectors-rl.svg" type="image/svg+xml"
class="portrait"></object>
<p class="caption">Han-based writing mode
</div>
</div>
<p>The <dfn>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 'direction' property specifies the
inline base direction of an element and, together with the 'unicode-bidi'
property and the inherent directionality of any text content, determines
the ordering of inline-level content within a line.
<p>The <dfn>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 'writing-mode' property determines the
block flow direction.</p>
<p>A <dfn>horizontal writing mode</dfn> is one with horizontal lines of
text, i.e. a downward or upward block flow. A <dfn>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>vertical block flow</dfn> (which is a downward or
upward block flow) and <dfn>horizontal block flow</dfn> (which is
leftward or rightward block flow). To avoid confusion, the 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 class="note">See Unicode Technical Note #22 [[UTN22]]
(<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.
<p class="issue">Point at Intro to Text Orientation section once it's written.</p>
<h3 id="placement">Module Interactions</h3>
<p>This module replaces and extends the 'unicode-bidi' and 'direction'
features defined in [[!CSS21]] sections 8.6 and 9.10</var>.
<h3 id="conventions">Document Conventions</h3>
<p>Conformance re
24DF
quirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [[!RFC2119]]</p>
<p>Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with <code>class="example"</code>,
like this:
<div class="example">
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word “Note” and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class="note">Note, this is an informative note.</p>
<h3 id="values">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 [[!CSS21]]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
Other CSS modules may expand the definitions of these value types: for
example [[CSS3COLOR]], 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.
<h2 id="text-direction">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" title="bidirectionality (bidi)"><dfn>bidirectionality</dfn></span>, or
"bidi" for short.</p>
<div class="figure">
<pre><span dir="rtl">با Unicode بنویس.</span>
<!-- --><----- ------> <-
<!-- --><================</pre>
<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 <span
class="propinst-direction">'direction'</span> and <span
class="propinst-unicode-bidi">'unicode-bidi'</span> properties allow
authors to specify how the elements and attributes of a document
language map to this algorithm.</p>
<p>User agents that support bidirectional text must apply the Unicode
bidirectional algorithm to every sequence of inline 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>paragraph</dfn>
unit in the bidirectional algorithm.
<p>Except when the ''plaintext'' value of 'unicode-bidi' is in effect,
the paragraph embedding level is set according to the value of the
'direction' 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 'direction' and 'unicode-bidi' 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>
<p>The HTML 4 specification ([[HTML401]], section 8.2) defines
bidirectionality behavior for HTML elements. The HTML 4 specification
also contains more information on bidirectionality issues.</p>
<p class="note">Because HTML UAs can turn off CSS styling, we advise HTML
authors to use the HTML 'dir' attribute and <bdo> element to
ensure correct bidirectional layout in the absence of a style sheet.</p>
<h3 id="direction">Specifying Directionality: the 'direction' property</h3>
<table class="propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn>direction</dfn></td>
</tr>
<tr>
<th>Value:</th>
<td>ltr | rtl </td>
</tr>
<tr>
<th>Initial:</th>
<td>ltr</td>
</tr>
<tr>
<th>Applies to:</th>
<td>all elements</td>
</tr>
<tr>
<th>Inherited:</t
B011
h>
<td>yes</td>
</tr>
<tr>
<th>Percentages:</th>
<td>N/A</td>
</tr>
<tr>
<th>Media:</th>
<td>visual</td>
</tr>
<tr>
<th>Computed value:</th>
<td>specified value</td>
</tr>
</tbody>
</table>
<p>This property specifies the base directionality of text and elements
on a line, and the directionality of embeddings and overrides (see
'unicode-bidi') 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>
<p>Values for this property have the following meanings:</p>
<dl>
<dt><strong>ltr</strong></dt>
<dd>Left-to-right directionality.</dd>
<dt><strong>rtl</strong></dt>
<dd>Right-to-left directionality.</dd>
</dl>
<p class="note">The 'direction' property has no effect on bidi reordering
when specified on inline elements whose 'unicode-bidi' property's
value is ''normal''.</p>
<p>The value of the 'direction' property on the root element is also
propagated to the initial containing block and, together with the
'writing-mode' property, determines the document's principal writing
mode. (See <a href="#principal-writing-mode">below</a>.)
<p class="note">Note that the 'direction' 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 'direction'
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 [[HTML401]], section 11.3.2.1.
<h3 id="unicode-bidi">Embeddings and Overrides: the 'unicode-bidi' property</h3>
<table class="propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn>unicode-bidi</dfn></td>
</tr>
<tr>
<th>Value:</th>
<td>normal | [ [ embed | isolate ] || [ plaintext | bidi-override ] ]</td>
</tr>
<tr>
<th>Initial:</th>
<td>normal</td>
</tr>
<tr>
<th>Applies to:</th>
<td>all elements, but see prose</td>
</tr>
<tr>
<th>Inherited:</th>
<td>no</td>
</tr>
<tr>
<th>Percentages:</th>
<td>N/A</td>
</tr>
<tr>
<th>Media:</th>
<td>visual</td>
</tr>
<tr>
<th>Computed value:</th>
<td>specified value</td>
</tr>
</tbody>
</table>
<p>Values for this property have the following meanings:</p>
<dl>
<dt><strong>normal</strong></dt>
<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.</dd>
<dt><strong>embed</strong></dt>
<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 'direction'
property. Inside the element, reordering is done implicitly. This
corresponds to adding a LRE (U+202A), for 'direction: ltr', or RLE
(U+202B), for 'direction: rtl', 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></dd>
<dt><strong>isolate</strong></dt>
<dd>For the purposes of the Unicode bidirectional algorithm, the
contents of the element are considered to be inside a separate,
independent paragraph, 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.)</dd>
<dt><strong>plaintext</strong></dt>
<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 'direction' as usual, but by following rules P2
and P3 of the Unicode bidirectional algorithm. Note this value
has no effect on inline elements.
<dt><strong>bidi-override</strong></dt>
<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
'direction' property; the implicit part of the bidirectional algorithm
is ignored. This corresponds to adding a LRO (U+202D), for ''direction:
ltr'', or RLO (U+202E), for ''direction: rtl'', at the start of the
element and a PDF (U+202C) at the end of the element.</dd>
</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. 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>In this process, replaced elements with 'display: inline' are treated
as neutral characters, unless their 'unicode-bidi' property has a
value other than 'normal', in which case they are treated as strong
characters in the 'direction' specified for the element.
All other atomic inline-level boxes are treated as neutral characters
always.</p>
<p>Please 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), more inline boxes (including anonymous inline boxes)
may have to be created, and some inline boxes may have to be split up
and reordered before flowing.</p>
<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">'unicode-bidi'</span> with a value other
than 'normal' unless appropriate. In particular, a value of 'inherit'
should be used with extreme caution. However, for elements that are,
in general, intended to be displayed as blocks, a setting of
'unicode-bidi: isolate' is preferred to keep the element together in
case display is changed to inline (see example below).</p>
<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>
<h3 id="bidi-example">Example of Bidirectional Text</h3>
<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">
<HEBREW>
<PAR>HEBREW1 HEBREW2 english3 HEBREW4 HEBREW5</PAR>
<PAR>HEBREW6 <EMPH>HEBREW7</EMPH> HEBREW8</PAR>
</HEBREW>
<ENGLISH>
<PAR>english9 english10 english11 HEBREW12 HEBREW13</PAR>
<PAR>english14 english15 english16</PAR>
<PAR>english17 <HE-QUO>HEBREW18 english19 HEBREW20</HE-QUO></PAR>
</ENGLISH>
</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">'unicode-bidi'</span> is 'normal' (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">Box model for inline elements in bidirectional context</h3>
<!-- 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).
<p>When the element's 'direction' property is ''ltr'', 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.
<p>When the element's 'direction' property is ''rtl'', 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.
<h2 id="vertical-intro">
Introduction to Vertical Text</h2>
<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 src="vert-horiz-comparison.png"
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."></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, while for Mongolian and Manchu 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 src="vert-latin-layouts.png"
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."></p>
<p class="caption">Examples of Latin in vertical Japanese: Daijirin Viewer 1.4 (iOS)</</p>
</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">
<p><img src="vert-number-layouts.png"
alt="An exerpt 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."></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 src="vert-horiz-combination.png"
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."></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 id="writing-mode">
Block Flow Direction: the 'writing-mode' property</h3>
<table class="propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn>writing-mode</dfn></td>
</tr>
<tr>
<th>Value:</th>
<td>horizontal-tb | vertical-rl | vertical-lr</td>
</tr>
<tr>
<th>Initial:</th>
<td>horizontal-tb</td>
</tr>
<tr>
<th>Applies to:</th>
<td>All elements except table row groups, table column groups, table rows, and table columns</td>
</tr>
<tr>
<th>Inherited:</th>
<td>yes</td>
</tr>
<tr>
<th>Percentages:</th>
<td>N/A</td>
</tr>
<tr>
<th>Media:</th>
<td>visual</td>
</tr>
<tr>
<th>Computed value:</th>
<td>specified value</td>
</tr>
</tbody>
</table>
<p>This property sets the block flow direction. Possible values:</p>
<dl>
<dt>horizontal-tb</dt>
<dd>Top-to-bottom block flow. The writing mode is horizontal.</dd>
<dt>vertical-rl</dt>
<dd>Right-to-left block flow. The writing mode is vertical.</dd>
<dt>vertical-lr</dt>
<dd>Left-to-right block flow. The writing mode is vertical.</dd>
</dl>
<p>SVG1.1 [[!SVG11]] defines some additional values: ''lr'',
''lr-tb'', ''rl'', ''rl-tb'', ''tb'', and ''tb-rl''. These values
are <em>deprecated</em> in any context except SVG1 documents.
Implementations that wish to support them in the context of CSS
must treat these values as follows:
<table class="data">
<thead>
<tr><th>SVG1</th> <th>CSS</th></tr>
</thead>
<tbody>
<tr><td>lr</td> <td rowspan=3>horizontal-tb</td></tr>
<tr><td>lr-tb</td></tr>
<tr><td>rl</td></tr>
<tr><td>tb</td> <td rowspan=2>vertical-rl</td></tr>
<tr><td>tb-rl</td></tr>
</tbody>
</table>
<p class="note">In SVG1.1, these values set the <dfn>inline progression
direction</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 'direction' property (which is independent of
'writing-mode'). (See <a href="http://www.w3.org/TR/SVG/text.html#RelationshipWithBiDirectionality">Relationship
with bidirectionality</a>. [[!SVG11]]) 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>
<p class="issue">See <a href="test-writing-mode-direction.svg">this demo</a>
to check out your implementation's interpretation! (Note that most
SVG implementations don't support the 'direction' property, and
thus your results may be skewed on that account. Examine the red
line of text: if the numbers are not in reverse order, your
implementation doesn't support "direction: rtl".)</p>
<p>The 'writing-mode' property determines the direction of block flow.
This determines the progression of block-level boxes in a block formatting
context; the progression of line boxes in a block container that contains
inlines; and the progression of rows in a table. By virtue of determining
the stacking direction of line boxes, the 'writing-mode' property also
determines whether line boxes and thus the writing mode is horizontal or
vertical.
<p>When set on the root element, the 'writing-mode' property together
with the 'direction' property determines the
<dfn id="principal-writing-mode">principal writing mode</dfn>
of the document. This writing mode is used, for example,
to determine the default page progression direction. (See [[CSS3PAGE]].)
The 'writing-mode' 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">Note that the 'writing-mode' 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 an element has a different block flow direction than its containing
block:
<ul>
<li>If the element has a specified 'display' of ''inline'', its 'display'
computes to 'inline-block'. [[!CSS21]]
<li>If the element has a specified 'display' of ''run-in'', its 'display'
computes to 'block'. [[!CSS21]]
</ul>
<p>If such an element is a block container, then it establishes a new
block formatting context.
<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 blocks 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"
src="horizontal.png" width="219" height="300" ></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"
src="vertical-rl.png" height="191" 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"
src="vertical-lr.png" height="191" width="300" ></p>
</div>
<h2 id="inline-alignment">
Inline-level Alignment</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 'vertical-align'
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 'vertical-align' property to
determine the alignment of inline-level content.
<h3 id="intro-baselines">
Introduction to Baselines</h3>
<p><em>This section is non-normative.</em></p>
<p>A <dfn>baseline</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">
[Picture of alphabetic text in two font sizes with the baseline and
emboxes indicated.]
</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>
<p>A well-constructed font contains a <dfn>baseline table</dfn>, which
indicates the position of one or more baselines within the font's
design coordinate space. (The design coordinate space is scaled with
the font size.)
<div class="figure">
<p><img alt=""
src="baselines.gif"></p>
<p class="caption">In a well-designed mixed-script font, the glyphs are
positioned in the coordinate space to harmonize with one another
when typeset together. The baseline table is then constructed to
match the shape of the glyphs, each baseline positioned to match
the glyphs from its preferred scripts.</p>
</div>
<p>The baseline table is a property of the font, and the positions
of the various baselines apply to all glyphs in the font.
<p>Different baseline tables can be provided for alignment in
horizontal and vertical text.
<h3 id="text-baselines">
Text Baselines</h3>
<p>In this specification, only the following baselines are considered:
<dl>
<dt>alphabetic</dt>
<dd>The <dfn>alphabetic baseline</dfn>, which typically aligns with the
bottom of uppercase Latin glyphs. In horizontal typographic mode,
this is the dominant baseline.
</dd>
<dt>central</dt>
<dd>The <dfn>central baseline</dfn>, which typically crosses the center
of the em box. In vertical typographic mode, this is the
dominant baseline. If the font is missing this baseline,
it is assumed to be halfway between the ascender (<i>over</i>)
and descender (<i>under</i>) edges of the em box.
</dd>
</dl>
<p class="note">A future CSS module will deal with baselines in more
detail and allow the choice of other dominant baselines and alignment
options.</p>
<h4 id="opentype-baselines">
Baselines in TrueType/OpenType fonts</h4>
<p><em>This section is non-normative.</em></p>
<p>The alphabetic baseline is listed as the <var>romn</var>
baseline in TrueType/OpenType baseline tables. If missing, it can
sometimes be computed from the ascender and descender metrics. If it
cannot be computed from other metrics, it may be assumed to correspond
to the zero coordinate.</p>
<p>For the ascender and descender metrics, it is recommended to use the
<var>sTypoAscender</var> and <var>sTypoDescender</var> metrics from the
font's OS/2 table. These should match the <var>idtp</var> and <var>ideo</var>
baselines (respectively), which, if present, may be used instead. In the
absence of these metrics, the <var>Ascent</var> and <var>Descent</var>
metrics from the HHEA/VHEA tab
2F7B
le should be used. The Win metrics can be
used as a fallback; however these are intended as clipping bounds and
are not always appropriate for use as em box measurements.</em>
<h3 id="replaced-baselines">
Atomic Inline Baselines</h3>
<p>If an <a href="http://www.w3.org/TR/CSS21/visuren.html#inline-boxes">atomic
inline</a> (such as an inline-block, inline-table, or replaced inline element)
is not capable of providing its own baseline information, then the
UA synthesizes a baseline table thus:
<dl>
<dt>alphabetic</dt>
<dd>The alphabetic baseline is assumed to be at the <i>under</i>
margin edge.</dd>
<dt>central</dt>
<dd>The central baseline is assumed to be halfway between the
<i>under</i> and <i>over</i> margin edges of the box.
</dl>
<h3 id="baseline-alignment">
Baseline Alignment</h3>
<p>The dominant baseline is used in CSS for alignment in two cases:
<ul>
<li>Aligning glyphs from different fonts within the same inline box.
The glyphs are aligned by matching up the positions of the dominant
baseline.
<li>Aligning a child inline within its parent. The child is aligned to
the parent by matching the parent's dominant baseline to the same
baseline in the child. (E.g. if the parent's dominant baseline is
alphabetic, then the child's alphabetic baseline is matched to the
parent's alphabetic baseline, even if the child's dominant baseline
is something else.)
<div class="example">
<p>Given following sample markup:
<pre><p><span class="outer">Ap <span class="inner"><i>ji</i></span></span></p></pre>
<p>And the following style rule:
<pre>span.inner { font-size: .75em; }</pre>
<p>The baseline tables of the parent (<code>.outer</code>) and the child
(<code>.inner</code>) will not mach up due to the font size difference.
Since the dominant baseline is the alphabetic baseline, the child box
is aligned to its parent by matching up their alphabetic baselines.
<div class="figure">
<p><img alt="" src="baseline-align-sizes.gif">
</div>
</div>
<p>Basline alignment in this fashion is used with the following values of
'vertical-align': ''baseline'', ''sub'', ''super'', <length>,
<percentage>. In the latter cases, the baselines are aligned as
for ''baseline'', but the child is shifted according to the offset given
by its 'vertical-align' value.
<div class="example">
<p>If we assign ''vertical-align: super'' to the <code>.inner</code>
element from the example above, the same rules are used to align
the <code>.inner</code> child to its parent; the only difference
is in addition to the baseline alignment, the child is shifted to
the superscipt position.
<pre>span.inner { vertical-align: super; font-size: .75em; }</pre>
<div class="figure">
<p><img alt="In this example, the resulting alignment is equivalent
to shifting the parent baseline table upwards by the
superscript offset, and then aligning the child's
alphabetic baseline to the shifted position of the
parent's alphabetic baseline."
src="baseline-align-super.gif">
</div>
</div>
</ul>
<h2 id="intro-text-layout">
Introduction to Intrinsic Text Layout</h2>
<p>Each writing system has one or more native orientations. Modern scripts
can therefore be classified into three orientational categories:</p>
<dl>
<dt>horizontal-only</dt>
<dd>Scripts that have horizontal, but not vertical, native orientation.
Includes: Latin, Arabic, Hebrew, Devanagari
<dt>vertical-only</dt>
<dd>Scripts that have vertical, but not horizontal, native orientation.
Includes: Mongolian, Manchu
<dt>bi-orientational</dt>
<dd>Scripts that have both vertical and horizontal native orientation.
Includes: Han, Hangul, Japanese Kana
</dl>
<p>In modern typographic systems, all glyphs are assigned a horizontal
orientation, which is used when laying out text horizontally.
To lay out vertical text, the UA needs to transform the text from its
horizontal orientation. This transformation is the <dfn>bi-orientational
transform</dfn>, and there are two types:
<dl>
<dt>rotate</dt>
<dd>Rotate the glyph from horizontal to vertical
<object data="diagrams/glyph-right.svg" class="figure" type="image/svg+xml" height="80" width="110"></object>