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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;utf-8">
<title>CSS Text Layout Module Level 3</title>
<link rel="stylesheet" type="text/css" href="../default.css">
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/tr/W3C-ED.css">
<style type="text/css">
.data, .proptable {
margin: 1em auto;
border-collapse: collapse;
border: solid #005A9B;
}
.data caption {
width: 100%;
text-align: center;
}
.data td, .data th,
.proptable td, .proptable th {
border: thin solid;
padding: 0.2em;
text-align: center;
}
.data thead th[scope="row"],
.proptable thead th[scope="row"] {
text-align: right;
background: #A4C8E2;
color: inherit;
}
.data thead,
.proptable thead {
background: #EEEEEE;
color: inherit;
}
.data tbody th:first-child,
.proptable tbody th:first-child {
text-align: right;
background: #EEEEEE;
color: inherit;
}
.data thead,
.data tbody,
.data tfoot,
.data colgroup {
border: solid;
}
table.propdef {
table-layout: auto;
}
.propdef th {
font-style: italic;
font-weight: normal;
text-align: left;
width: 3em;
}
dt dfn code {
font-size: inherit;
}
.egbidiwsaA,.egbidiwsbB,.egbidiwsaB,.egbidiwsbC
{ white-space:pre;font-size:80%;font-family:monospace; vertical-align:2px; margin:1px }
.egbidiwsaA { background:lime;padding:2px; }
.egbidiwsbB { border:2px solid blue }
.egbidiwsaB { background:yellow;border:2px dotted white }
.egbidiwsbC { border:2px dotted red }
</style>
</head>
<body>
<div class="head">
<!--logo-->
<h1>CSS Text Layout Module Level 3</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE: 15 February 2008]</h2>
<dl>
<dt>This version:</dt>
<dd><a href="http://dev.w3.org/csswg/css3-text-layout/">http://dev.w3.org/csswg/css3-text-layout/</a>
<dt>Latest version:</dt>
<dd><a
href="http://www.w3.org/TR/css3-text-layout/">http://www.w3.org/TR/css3-text-layout/</a></dd>
<dt>Previous version:</dt>
<dd><a
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/">http://www.w3.org/tr/2003/CR-css3-text-20030514/</a></dd>
<dt>Editors:</dt>
<dd><a href="mailto:paulnel@microsoft.com">Paul Nelson</a> (<a href="http://www.microsoft.com/">Microsoft</a>)</dd>
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a></dd>
<dt>Previous Editor:</dt>
<dd><a href="mailto:michelsu@microsoft.com">Michel Suignard</a> (<a href="http://www.microsoft.com/">Microsoft</a>)</dd>
</dl>
<!--begin-copyright-->
<p>[Here will be included the file "../copyright.inc"]</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id="abstract">Abstract</h2>
<p>This module specifies the text layout model in CSS and the properties
that control it. It covers bidirectionality, vertical text, grid layout,
and other special inline text layout effects.</p>
<h2 class="no-num no-toc" id="status">Status of this document</h2>
<!--status-->
<h2 class="no-num no-toc" id="Contents">Contents</h2>
<!--toc-->
<h2 id="intro">Text layout introduction</h2>
<p>This section describes the text layout features supported by CSS, which
includes support for various international writing directions, such as
left-to-right (e.g., Latin scripts), right-to-left (e.g., Hebrew or Arabic),
bidirectional (e.g., mixing Latin with Arabic) and vertical (e.g., Asian
scripts). In addition, this section defines a set of properties that provide
for a document grid layout system that is commonly used with East Asian text
layout.</p>
<h2 id="text-flow">Text flow</h2>
<p>Text flow is defined in terms of inline progression and block progression.
Inline progression is the way elements flow on the line and defines on which
end of the line the "start" and "end" are located. The <span class="property">
<a href="#direction">'direction'</a></span> and <span class="property">
<a href="#unicode-bidi">'unicode-bidi'</a></span> properties determine the
inline-progression and bidirectional layout. The <span class="property">
<a href="#block-progression">'block-progression'</a></span> property
determines the line progression, or way lines stack, in a block. The
<span class="property"><a href="#writing-mode">'writing-mode'</a></span>
shorthand combines inline and block progression together. </p>
<p>Some examples of text flow are:</p>
<ul>
<li>Latin scripts are typically written using a left to right inline progression
with a top to bottom block progression.</li>
<li>Mongolian script languages are typically written using a top to bottom
inline progression with a left to right block progression.</li>
<li>Japanese is commonly written using both a left to right inline progression
9927
with a top to bottom block progression, <strong>and</strong> a top to bottom
inline progression with a right to left block progression. Many magazines
and newspapers will mix these two text flows on the same page.</li>
</ul>
<h3 id="text-direction">Bidirectionality</h3>
<p><a href="conform.html#conformance">Conforming</a> user agents that do
not support bidirectional text may ignore the <span
class="propinst-direction">'direction'</span> and <span
class="propinst-unicode-bidi">'unicode-bidi'</span> properties
described in this section. This exception includes UAs that render
right-to-left characters simply because a font on the system contains them
but do not support the concept of right-to-left text direction.</p>
<p>The characters in 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>
<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 directionality of 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 line break or block boundary. This sequence forms the
"paragraph" unit in the bidirectional algorithm. The paragraph embedding
level is set according to the value of the <span
class="propinst-direction">'direction'</span> property of the containing
block rather than by the heuristic given in steps P2 and P3 of the Unicode
algorithm.</p>
<p>Because the directionality of a text depends on the structure and
semantics of the document language, these properties should in most
cases be used only by designers of document type descriptions (Dtds),
or authors of special documents. If a default style sheet specifies
these properties, authors and users should not specify rules to
override them.</p>
<p>The HTML 4 specification ([HTML4], section 8.2) defines
bidirectionality behavior for HTML elements. The style sheet
rules that would achieve the bidi behaviors specified in [[HTML4]] are
given below. The HTML 4 specification also contains more information on
bidirectionality issues.</p>
<pre>
*[dir="ltr"] { direction: ltr; unicode-bidi: embed; }
*[dir="rtl"] { direction: rtl; unicode-bidi: embed; }
bdo[dir="ltr"] { direction: ltr; unicode-bidi: bidi-override; }
bdo[dir="rtl"] { direction: rtl; unicode-bidi: bidi-override; }
</pre>
<p class="note">Because HTML UAs can turn off CSS styling, we advise HTML
authors to use the HTML 'dir' property 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:</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 specifies the inline progression of text and elements
on a line, and the direction of embeddings and overrides (see <span
class="propinst-unicode-bidi">'unicode-bidi'</span>) for the Unicode
bidirectional algorithm. In addition, it specifies the direction of <a
href="http://www.w3.org/TR/2006/WD-CSS21-20060411/tables.html">table</a> column layout, the direction of
horizontal <a href="http://www.w3.org/TR/2006/WD-CSS21-20060411/visufx.html#overflow">overflow</a>, and the
position of an incomplete last line in a block in case of 'text-align:
justify'.</p>
<p>Values for this property have the following meanings:</p>
<dl>
<dt><strong>ltr</strong></dt>
<dd>Left-to-right direction.</dd>
<dt><strong>rtl</strong></dt>
<dd> Right-to-left direction.</dd>
</dl>
<p>For the <span class="propinst-direction">'direction'</span>
property to affect reordering in inline-level elements, the <span
class="propinst-unicode-bidi">'unicode-bidi'</span> property's value
must be 'embed' or 'override'.</p>
<p class="note">The <span class="propinst-direction">'direction'</span>
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 [[HTML4]], section 11.3.2.1.
<h3 id="unicode-bidi">Embeddings and Overrides: the 'uncode-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 | 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-level elements,
implicit reordering works across element boundaries.</dd>
<dt><strong>embed</strong></dt>
<dd>If the element is inline-level, this value
opens an additional level of embedding with respect to the
bidirectional algorithm. The direction of this embedding level is
given by the <span class="propinst-direction">'direction'</span>
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.</dd>
<dt><strong>bidi-override</strong></dt>
<dd>For inline-level elements this creates an override.
For block-level, table-cell, table-caption, or inline-block elements
this creates an override for inline-level descendents not within
another block-level, table-cell, table-caption, or inline-block
element. This means that inside the element, reordering is strictly in sequence
according to the <span class="propinst-direction">'direction'</span>
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 in each block-level element 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. In this process, non-textual entities such as images are
treated as neutral characters, unless their <span
class="propinst-unicode-bidi">'unicode-bidi'</span> property has a
value other than 'normal', in which case they are treated as strong
characters in the <span class="propinst-direction">'direction'</span>
specified for the element.</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: embed' 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: <span
class="index-inst" title="Dtd">Dtd</span> 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. The bidi rules
should not be overridden by other style sheets so that the document
language's or Dtd'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 XML, the style sheet is responsible for setting the
writing direction. This is the style sheet:</p>
<pre class="example">
/* 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="block-progression">Block progression direction: the 'block-flow' property</h3>
<table class="propdef">
<tbody>
<tr>
<th>Name:</th>
<td><dfn>block-flow</dfn></td>
</tr>
<tr>
<th>Value:</th>
<td>tb | rl | lr</td>
</tr>
<tr>
<th>Initial:</th>
<td>tb</td>
</tr>
<tr>
<th>Applies to:</th>
<td><span class="issue">??</span></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 class="issue">Should we change the name to something more understandable?
"block-flow"? "block-direction"? We need to make sure terminology for
inline progression and block progression ("layout" vs "flow" vs "orientation"
vs "direction", "horizontal" vs "vertical") is consistent throughout the
document as well.)</p>
<p>This property sets the block-progression value and the layout orientation.
Possible values:</p>
<dl>
<dt>tb</dt>
<dd>Top-to-bottom block flow. The layout orientation is horizontal.</dd>
<dt>rl</dt>
<dd>Right-to-left block flow. The layout orientation is vertical.</dd>
<dt>lr</dt>
<dd>Left-to-right block flow. The layout orientation is vertical.</dd>
</dl>
<p>An inline-level element that has a different 'block-flow' from
its containing block becomes an 'inline-block' element <a href="#CSS3-box" rel="biblioentry">[CSS3-box]</a>.
<span class="issue">switch to CSS2.1</span></p>
<h3>Details</h3>
<p>Box layout in vertical orientations is exactly analogous to layout in
the horizontal orientation.</p>
<ul>
<li>Calculations that refer to 'width' use 'height' instead, and vice
versa.</li>
<li>Calculations that refer to the '*-left' and '*-right' box properties
(border, margin, padding, outline) use '*-top' and '*-bottom'
instead, and vice versa. In RL layout '*-right' is analogous to
TB's '*-top'; in LR layout '*-left' is analogous to TB's '*-top'.
The logic for this is clear if you lock an arrow representing the
TB block flow to a compass of the four directions and rotate it to
represent RL and LR flow.</li>
<li>Layout effects that depend on the directionality to choose between
left and right (e.g. overflow, overconstraint resolution, 'start'
values for 'text-align' and other properties, table column ordering)
depend instead on the effective physical directionality
(top-to-bottom or bottom-to-top) to choose between top and bottom.</li>
</ul>
<p>When an element has a different 'block-progression' from its
containing block two cases are possible:</p>
<em class="issue">These descriptions need work</em>
<ul>
<li>The two block-progressions are perpendicular to each other. (For
example, 'tb' and 'lr'). In such cases, <span class="issue">detail
calculation of width/height and determination of containing block dim</span></li>
<li>The two block-progressions are parallel to each other. (For example,
'rl' and 'lr').</li>
</ul>
<h3>Example</h3>
<div class="example">
<p>In the following example, two blocks elements (1 and 3) separated
by an image (2) are presented in various flow orientations.</p>
<p>Here is a diagram of horizontal layout (<code>block-progression: 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 layout commonly
used in East Asia (<code>block-progression: 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
layout used for Uighur and Mongolian (<code>block-progression: 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>
<h3 id="writing-mode">Text flow short cut: the 'writing-mode' property</h3>
<table class="propdef" id="name-">
<tr>
<td><em>Name:</em></td>
<td><dfn id="writing-mode">writing-mode</dfn></td>
</tr>
<tr>
<td><em>Value:</em></td>
<td>lr-tb | rl-tb | tb-rl | bt-rl | tb-lr | bt-lr</td>
</tr>
<tr>
<td><em>Initial:</em></td>
<td>not defined for shorthand properties</td>
</tr>
<tr>
<td><em>Applies to:</em></td>
<td>all elements and generated content</td>
</tr>
<tr>
<td><em>Inherited:</em></td>
<td>yes</td>
</tr>
<tr>
<td><em>Percentages:</em></td>
<td>N/A</td>
</tr>
<tr>
<td><em>Media:</em></td>
<td>visual</td>
</tr>
<tr>
<td><em>Computed value:</em></td>
<td>see individual properties</td>
</tr>
</table>
<p>The <span class="property"><a
href="#writing-mode">'writing-mode'</a></span> property is a shorthand
property for the <span class="property"><a
href="#direction">'direction'</a></span> property and the <span class="property"> <a
href="#block-progression">'block-progression'</a></span> property. Although
strictly speaking, the property has no initial value, it is equivalent to 'lr-tb'.
The definition of the property values are established by the following table,
which shows the setting of the constituent properties and example of common
usage.</p>
<div class="figure">
<table border="1" style="border-collapse: collapse" cellpadding="0" cellspacing="0" width="100%">
<tr>
<th>writing-mode:</th>
<th>direction:</th>
<th>block-progression:</th>
<th>Common Usage:</th>
</tr>
<tr>
<td>lr-tb</td>
<td>ltr</td>
<td>tb</td>
<td>Latin-based, Greek, Cyrillic writing systems (and many others)</td>
</tr>
<tr>
<td>rl-tb</td>
<td>rtl</td>
<td>tb</td>
<td>Arabic, Hebrew writing systems</td>
</tr>
<tr>
<td>tb-rl</td>
<td>ltr</td>
<td>rl</td>
<td>East Asian writing systems in vertical mode</td>
</tr>
<tr>
<td>bt-rl</td>
<td>ltr</td>
<td>rl</td>
<td>Arabic script block quote embedded in East Asian vertical text</td>
</tr>
<tr>
<td>tb-lr</td>
<td>ltr</td>
<td>lr</td>
<td>Mongolian script writing system</td>
</tr>
<tr>
<td>bt-lr</td>
<td>rtl</td>
<td>lr</td>
<td>Arabic script block quote embedded in Mongolian script document</td>
</tr>
</table>
</div>
<h3 id="writing-mode-mixed"><span class="secno">2.4.</span> Mixing the 'writing-mode' in normal usage</h3>
<p>In East Asian documents, it is often preferred to display certain
Latin-based strings, such as numerals in a year, always in a horizontal
layout orientation regardless of the flow orientation of the line of
text these strings appear in, as in:</p>
<div class="figure">
<p><img alt="Diagram of Tate Naka Yoko, showing a group of glyphs
appearing horizontally in a vertical column of text"
class="example" src="lf-tny.gif" height="170" width="90">
<img alt="Example of Tate Naka Yoko, showing the year 1996
appearing horizontally in a column of vertical text"
class="example" src="tny.gif" height="132" width="37">
<p>Horizontal in vertical ("<span lang="ja">Tate-chu-yoko</span>")
</div>
<p>In Japanese, this effect is known as
<a href="#tatechuyoko" lang="ja">"Tate-chu-yoko"</a>. In order to
achieve it in a CSS document, the Latin string should be enclosed
in an inline element with a horizontal layout orientation, as in:
<pre>.date {block-progression: tb;}
<!-- --><span class="date">1996</span></pre>
</div>
<h2 id="document-grid">Document grid</h2>
<h3 id="grid-introduction">Document grid introduction</h3>
<p>Documents written in East Asian languages, such as Chinese or Japanese,
are commonly laid out on the page according to a specified one- or
two-dimensional grid. The concept of grid can also be used with other
non-ideographic contexts such as Braille script or monospaced layout. </p>
<p>The diagram below represents a fragment of horizontal text on a page with
mixed wide-cell and narrow-cell glyphs that a Japanese content author intended
to be laid out on a grid system that has 9 glyphs per line. Gray grid lines
are shown for clarity):
<div class=figure>
<p><img class=example height=89
alt="Example of strict grid layout (genko) applied to mixed Japanese and English in horizontal layout."
src="lg-s-ex.gif" width=274> <br>
'<span lang=ja>Genko</span>' grid applied to mixed text </p>
</div>
<p>The grid behavior can be set on the inline-progression, on the
block-progression or both. The grid on the block-progression dimension is
determined by the following properties:</p>
<ul>
<li>'line-stacking-strategy' <a
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#CSS3-line"
rel=biblioentry>[CSS3-line]</A> set to the 'grid-height' value, </li>
<li>'line-height' <a
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#CSS3-line"
rel=biblioentry>[CSS3-line]</a> of the block element set to the desired grid
block-progression value,</li>
<li>'text-height' <a
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#CSS3-line"
rel=biblioentry>[CSS3-line]</a> which defines the block-progression value for
inline boxes. Depending on the content size, one or a multiple of
'line-height' will be necessary to accommodate a given inline box. </LI>
</ul>
<p>The block-progression grid is not described in this section as it can be
achieved simply by using the properties mentioned above and described in the
CSS3 Line module.</p>
<p>The grid on the inline-progression dimension is obtained by altering the
glyphs advance width (or inline-progression value) of inline elements. There are
several modes:</p>
<ul>
<li>Adding extra space between each grapheme cluster by using the <span
class=property><a
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#letter-spacing">'letter-spacing'</a></span>
property. This is sometimes called "loose" grid.</li>
<li>Giving a fixed advance width to ideograph grapheme clusters only. Other
grapheme clusters are spaced normally. This is called "<span
lang=ja>genko</span>" in Japanese typography. </li>
<li>Giving a fixed advance width to all grapheme clusters, excepted connected
glyphs (like Arabic). </li>
</ul>
<p>Two properties control this advance width modification: <span
class=property><a
href="#line-grid-mode">'line-grid-mode'</a></span>
enables it and <span class=property><a
href="#line-grid-progression">'line-grid-progression'</a></span>
determines its value. The shorthand <span class=property><a
href="#line-grid">'line-grid'</a></span>
allows setting both together.</p>
<h3 id="line-grid-mode">Line grid mode: the 'line-grid-mode' property</h3>
<table class="propdef">
<tr>
<th>Name:
<td><dfn>line-grid-mode</dfn>
<tr>
<th>Value:
<td>none | ideograph | all
<tr>
<th>Initial:
<td>none
<tr>
<th>Applies to:
<td>block-level and inline-block elements
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
</table>
<p>Specifies the line grid behavior. Each line grid mode value entails a
different set of rules for rendering inline contents (the term 'horizontal' is
used in the following description to indicate the inline-progression dimension).
Possible values: </p>
<dl>
<dt>none </dt>
<dd>
<p>No line grid. Standard text alignments apply to the block element. </p>
</dd>
<dt>ideograph </dt>
<dd>Content is divided into units that we will call strips. Each strip is
horizontally centered within the smallest number of grid spaces that contain
the strip. The width of the grid space is determined by the <span
class=property><A
href="#line-grid-progression">'line-grid-progression'</A></span>
setting.
<p>Each grapheme cluster with a wide base character is a strip. Each grapheme
cluster with a narrow <A
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#g-kana"><span
lang=ja>kana</span></A> character as its base is a strip. Each non-breakable
object (e.g. an image) is a strip. Other grapheme clusters are treated as a
single strip bounded by the strips described prior. That single strip may be
decomposed in several strips if line breaking occurs within it.</p>
<p>The strips are arranged in the grid as follows: <p>
<ul>
<li>Each strip corresponding to wide base character (as well as narrow <A
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#g-kana"><span
lang=ja>kana</span></A> character) that can fit within a single grid space
is rendered in the horizontal center of the grid space. </li>
<li>Each strip corresponding to other characters are placed in the center of
the smallest number of grid spaces necessary for it to fit. If a line break
occurs within such a strip, the strip is treated as two or more separate
strips whose individual placement follows the same rules as those for a
single strip.</li>
<li>Strips corresponding to non-breakable objects and strips corresponding
to wide base character (as well as narrow <A
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#g-kana"><span
lang=ja>kana</span></A> character) that are wider than a single grid space,
are each centered within the smallest number of grid cells necessary for
them to fit. </li>
</ul>
<div class=figure>
<p><img class=example height=69 alt="Mixed glyph layout in strict grid"
src="lg-s.gif" width=282>
<p>Mixed glyph layout in strict grid </p>
</div>
<div class=figure>
<p><img class=example height=67
alt="Object layout in strict grid. Large rectangular object is centered horizontally within 2 grid spaces"
src="lg-s2.gif" width=282>
<p>Object layout in strict grid </p>
</div>
<p>The 'ideograph' mode disables all special text justification and glyph
width adjustment normally applied to the contents of the block element. </p>
<p>If a line break opportunity cannot be found in a text run going over the
line boundary, then that text run will be pushed down to the next line and the
last part of the previous line will be left blank. </p>
<div class=example>
<p>Here is an example of mixed text in 'ideograph' grid mode:
<div class=figure>
<p><img class=example height=89
alt="Example of strict grid applied to mixed Japanese and English text in horizontal layout"
src="lg-s-ex.gif" width=274>
<p>Strict grid applied to mixed text </p>
</div>
</div>
<dt>all </dt>
<dd>This type of grid can be used to achieve mono-spaced layout. As with
'ideograph', content is divided into strips and each strip is horizontally
centered within the smallest number of grid spaces that can contain the grid.
The rules for determining strips differs.
<p>Each grapheme cluster with a non-joining base character is a strip. Each
non-breakable object (e.g. an image) is a strip. Each run of grapheme clusters
with joining base characters that join to each other is a strip.</p>
<div class=figure>
<p><img class=example height=68
alt="Layout in fixed grid mode. All glyphs equally spread out."
src="lg-f.gif" width=336>
<p>Mixed glyph layout in fixed grid </p>
</div>
<div class=example>
<p>For example: </p>
<div class=figure>
<p><img class=example height=89
alt="Example of fixed grid mode in mixed Japanese and English text in horizontal layout"
src
7BFB
="lg-f-ex.gif" width=274>
<p>Fixed grid applied to mixed text </p>
</div>
</div>
</dd>
</dl>
<p>The <span class=property><a
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#letter-spacing">'letter-spacing'</A></span>
property does not apply to characters in a grid but does apply for all
characters not in the grid (i.e. all characters for 'line-grid-mode: none',
non-ideographs for 'line-grid-mode: ideograph' and all non-connected glyphs for
'line-grid-mode: all').</p>
<h3 id="line-grid-progression">Line grid progression: the 'line-grid-progression' property</h3>
<table class="propdef">
<tr>
<th>Name:
<td><dfn id=line-grid-progression>line-grid-progression</dfn>
<tr>
<th>Value:
<td>text-height | line-height | <length>
<tr>
<th>Initial:
<td>text-height
<tr>
<th>Applies to:
<td>block-level and inline-block elements
<tr>
<th>Inherited:
<td>yes
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td><length>
</tbody>
</table>
<p>This property affects the inline-progression dimension of characters which
are subject to the fixed advance width as determined by the <span
class=property><a
href="#line-grid-mode">'line-grid-mode'</A></span>
property.</p>
<p>Possible values: </p>
<dl>
<dt>text-height </dt>
<dd>The computed value of the block element 'text-height' <A
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#CSS3-line"
rel=biblioentry>[CSS3-line]</A> is used. </dd>
<dt>line-height </dt>
<dd>The computed value of the block element 'line-height' <A
href="http://www.w3.org/tr/2003/CR-css3-text-20030514/#CSS3-line"
rel=biblioentry>[CSS3-line]</A> is used. </dd>
<dt><length> </dt>
<dd>inline-progression dimension of the line grid's unit space. </dd>
</dl>
<div class=example>
<p>For example:</p>
<pre>div.section1 { line-grid-progression: .5in }</pre>
<p>The rule set above would make grid spaces 0.5 inches long in a
<samp>div</samp> element in the section1 class. If the element has horizontal
flow, it would like the following (without the grid lines, which are shown for
clarity).</p>
<div class=figure>
<p><img class=example height=122
alt="Example of a line-grid-char setting applied to mixed Japanese and English text in horizontal layout"