csswg-drafts/css-ruby/Overview.src.html at 4bf55c3739acd2aa48b0a63353ce8f03d4758eea · w3c/csswg-drafts · GitHub

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<!DOCTYPE html>
<!--

Issues:
	bidi
	box layout/sizing
	clean up inter-character vs. parallel layout requirements

Redo all examples with consistent font. (M+ 2p?)

-->
<html lang="en">
<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
	<title>CSS Ruby Module Level 1</title>
	<link rel=contents href="#contents">
	<link rel=index href="#index">
	<link rel="stylesheet" type="text/css" href="../default.css">
	<link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
	<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
</head>

<body class="h-entry">

<div class="head">
<!--logo-->

<h1 class="p-name">CSS Ruby Module Level 1</h1>

<h2 class="no-num no-toc">[LONGSTATUS] <time class="dt-updated" datetime="[CDATE]">[DATE]</time> <!-- for HTML4 doctype: <span class="value-title" title="[CDATE]">[DATE]</span></span> --> </h2>
<dl>
	<dt>This version:
		<dd><a class="u-url" href="[VERSION]">[VERSION]</a>

	<dt>Latest version:
		<dd><a href="http://www.w3.org/TR/[SHORTNAME]/">http://www.w3.org/TR/css3-ruby/</a>

	<dt>Editor's draft:
		<dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
		(<a href="https://dvcs.w3.org/hg/csswg/log/tip/[SHORTNAME]/Overview.src.html">change log</a>)

	<dt>Previous version:
		<dd><a href="http://www.w3.org/TR/2011/WD-css3-ruby-20110630/">
		http://www.w3.org/TR/2011/WD-css3-ruby-20110630/</a>

	<dt>Issue Tracking:</dt>
		<dd><a rel="issues" href="http://www.w3.org/Style/CSS/Tracker/products/FIXME">http://www.w3.org/Style/CSS/Tracker/products/FIXME</a>

	<dt>Feedback:</dt>
		<dd><a href="mailto:www-style@w3.org?subject=%5BSHORTNAME%5D%20feedback"
				 >www-style@w3.org</a>
				 with subject line &ldquo;<kbd>[[SHORTNAME]]
				 <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
				 (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/"
					 >archives</a>)

	<dt>Editors:
		<dd class="p-author h-card vcard">
			<a class="p-name fn u-url url" rel="author"
				 href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>,
			<a class="p-org org h-org" href="http://www.mozilla.org/">Mozilla</a>
		<dd class="p-author h-card vcard">
			<a class="p-name fn u-url url" rel="author"
				 href="mailto:koji.a.ishii@mail.rakuten.com">Koji Ishii</a>,
			<span class="p-org org">Rakuten, Inc.</span>
		<dd class="p-author h-card vcard">
			<a class="p-name fn u-url url" rel="author"
				 href="mailto:ishida@w3.org">Richard Ishida</a>,
			<span class="p-org org">W3C</span>

	<dt>Former editors:
		<dd>Michel Suignard, Microsoft
		<dd>Marcin Sawicki, Microsoft
</dl>

<!--copyright-->

<hr title="Separator for header">
</div>

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

	<p>
	<span class="p-summary">
		“Ruby” are short runs of text alongside the base text,
		typically used in East Asian documents to indicate pronunciation
		or to provide a short annotation.
		This module describes the rendering model and formatting controls
		related to displaying ruby annotations in CSS.
	</span>

	<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.

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

<!--status-->

<p>The following features are at risk: &hellip;

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

<!--toc-->

<h2 id="intro">
Introduction</h2>

	<p><em>This section is not normative.</em>

<h3 id="placement">
Module interactions</h3>

	<p>This module extends the inline box model of CSS Level 2 [[!CSS21]]
	to support ruby.

	<p>None of the properties in this module apply to the <code>::first-line</code> or
	<code>::first-letter</code> pseudo-elements.

<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 [[CSS3VAL]], when combined with this module, expands the
	definition of the <var>&lt;length&gt;</var> 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.

<h3 id="diagram-conventions">
Diagram conventions</h3>

	<p>Many typographical conventions in East Asian typography depend
	on whether the character rendered is wide (CJK) or narrow (non-CJK).
	There are a number of illustrations in this document
	for which the following legend is used:

	<dl>
		<dt><img alt="Symbolic wide-cell glyph representation" width="39" height="39" src="images/fullwidth.gif">
		<dd>Wide-cell glyph (e.g. Han) that is the <var>n</var>th character in the text run.
		They are typically sized to 50% when used as annotations.
		<dt><img alt="Symbolic narrow-cell glyph representation" width="19" height="39" src="images/halfwidth.gif">
		<dd>Narrow-cell glyph (e.g. Roman) which is the <var>n</var>th glyph in the text run.
	</dl>

	<p>The orientation which the above symbols assume in the diagrams
	corresponds to the orientation that the glyphs they represent
	are intended to assume when rendered by the user agent.
	Spacing between these characters in the diagrams is incidental,
	unless intentionally changed to make a point.

<h3 id="ruby-def">
What is ruby?</h3>

	<p><dfn>Ruby</dfn> is the commonly-used name for a run of text
	that appears alongside another run of text (referred to as the “base”)
	and serves as an annotation or a pronunciation guide associated with that run of text.

	<p>The following figures show two examples of Ruby,
	a simple case and one with more complicated structure.

	<div class="example">
		<p>In this first example, a single annotation is used to annotate the base text.
		<div class="figure">
			<p><img src="images/licence.png"
			        alt="Example of ruby applied on top of a Japanese expression">
			<p class="caption">Example of ruby used in Japanese (simple case)
		</div>
		<p>In Japanese typography, this case is sometimes called
		<i lang="ja">taigo</i> ruby or group-ruby (per-word ruby),
		because the annotation as a whole is associated
		with multi-character word (as a whole).
	</div>

	<div class="example">
		<p>In this second example,
		two levels of annotations are attached to a base sequence:
		the hiragana characters on top refer to the pronunciation of each of the base kanji characters,
		while the words “Keio” and “University” on the bottom are annotations describing the English translation.
		<div class="figure">
			<p><img src="images/ruby-univ.gif"
			        alt="Example showing complex ruby with annotation text over and under the base characters">
			<p class="caption">Complex ruby with annotation text over and under the base characters
		</div>
		<p>
		<p>Notice that to allow correct association between the hiragana characters and
		their corresponding Kanji base characters,
		the spacing between these Kanji characters is adjusted.
		(This happens around the fourth Kanji character in the figure above.)
		To avoid variable spacing between the Kanji characters in the example above
		the hiragana annotations can be styled as a <i>collapsed annotation</i>,
		which will look more like the group-ruby example earlier.
		However because the base-annotation pairings are recorded in the ruby structure,
		if the text breaks across lines, the annotation characters will stay
		correctly paired with their respective base characters.
	</div>

	<p><i>Ruby</i> formatting as used in Japanese is described in JIS X-4051 [[JIS4051]] (in Japanese)
	and in Requirements for Japanese Text Layout [[JLREQ]] (in English and Japanese)].
	In HTML, ruby structure and markup to represent it is described
	in the Ruby Markup Extension specification.
	This module describes the CSS rendering model
	and formatting controls relevant to ruby layout of such markup.

<h2 id="ruby-model">
Ruby Formatting Model</h2>

	<p>The CSS ruby model is based on
	the <a href="http://darobin.github.io/html-ruby/">HTML Ruby Markup Extension</a>
	and <a href="http://www.w3.org/TR/ruby/">XHTML Ruby Annotation Recommendation</a> [[RUBY]].
	In this model, a ruby structure consists of
	one or more <dfn>ruby base</dfn> elements representing the base (annotated) text,
	associated with one or more levels of <dfn>ruby annotation</dfn> elements representing the annotations.
	The structure of ruby is similar to that of a table:
	there are “rows” (the base text level, each annotation level)
	and “columns” (each <i>ruby base</i> and its corresponding <i>ruby annotations</i>).

	<p>Consecutive bases and annotations are grouped together into <dfn>ruby segments</dfn>.
	Within a <i>ruby segment</i>, a <i>ruby annotation</i> may span multiple <i>ruby bases</i>.

	<p class="note">In HTML, a single <code>&lt;ruby&gt;</code> element may contain multiple <i>ruby segments</i>.
	(In the XHTML Ruby model, a single <code>&lt;ruby&gt;</code> element can only contain one <i>ruby segment</i>.)

<h3 id="ruby-display">
Ruby-specific 'display' property values</h3>

	<p>For document languages (such as XML applications) that do not have pre-defined ruby elements,
	authors must map document language elements to ruby elements;
	this is done with the 'display' property.

	<table class="propdef">
		<tr>
			<th>Name:
			<td>display
		<tr>
			<th><a href="#values">New Values</a>:
			<td>ruby | ruby-base | ruby-text | ruby-base-container | ruby-text-container
	</table>

	<p>The following new 'display' values assign ruby layout roles to an arbitrary element:

	<dl>
		<dt>''ruby''
			<dd>Specifies that an element generates a <dfn title="ruby container | ruby container box">ruby container box</dfn>.
			(Corresponds to HTML/XHTML <code>&lt;ruby&gt;</code> elements.)
		<dt>''ruby-base''
			<dd>Specifies that an element generates a <dfn title="ruby base box | ruby base">ruby base box</dfn>.
			(Corresponds to HTML/XHTML <code>&lt;rb&gt;</code> elements.)
		<dt>''ruby-text''
			<dd>Specifies that an element generates a <dfn title="ruby annotation box | ruby annotation">ruby annotation box</dfn>.
			(Corresponds to HTML/XHTML <code>&lt;rt&gt;</code> elements.)
		<dt>''ruby-base-container''
			<dd>Specifies that an element generates a <dfn title="ruby base container box | ruby base container">ruby base container box</dfn>.
			(Corresponds to XHTML <code>&lt;rbc&gt;</code> elements; always implied in HTML.)
		<dt>''ruby-text-container''
			<dd>Specifies that an element generates a <dfn title="ruby annotation container box | ruby annotation container">ruby annotation container box</dfn>.
			(Corresponds to HTML/XHTML <code>&lt;ruby&gt;</code> elements.)
	</dl>

<h3 id="box-fixup">
Anonymous Ruby Box Generation</h3>

	<p>The CSS model does not require that the document language
	include elements that correspond to each of these components.
	Missing parts of the structure are implied through the anonymous box generation rules
	<a href="http://www.w3.org/TR/CSS21/tables.html#anonymous-boxes">similar to those used to normalize tables</a>. [[!CSS21]]

	<ol>
		<li>Any in-flow block-level boxes directly contained by a
		<i>ruby container</i>,
		<i>ruby base container</i>,
		<i>ruby annotation container</i>,
		<i>ruby base box</i>,
		or <i>ruby annotation box</i>
		are forced to be inline-level boxes,
		and their 'display' value computed accordingly.
		For example,
		the 'display' property of an in-flow element with ''display: block''
		parented by an element with ''display: ruby-text''
		computes to ''inline-block''.
		This computation occurs after any intermediary anonymous-box fixup
		(such as that required by internal table elements).

		<li>Any consecutive sequence of <i>ruby bases</i> not parented by a <i>ruby base container</i>
		is wrapped in an anonymous <i>ruby base container</i>.
		Similarly, any consecutive sequence of <i>ruby annotations</i> not parented by a <i>ruby annotation container</i>
		is wrapped in an anonymous <i>ruby annotation container</i>.

		<li>Within each <i>ruby base container</i>,
		each sequence of inline-level boxes is wrapped in an anonymous <i>ruby base box</i>.
		Similarly, within each <i>ruby annotation container</i>,
		each sequence of inline-level boxes is wrapped in an anonymous <i>ruby annotation box</i>.

		<li>A sequence of <i>ruby base containers</i> and/or <i>ruby annotation containers</i>
		not parented by a <i>ruby container</i>
		is wrapped in an anonymous <i>ruby container</i>.
	</ol>

	<p>At this point, all ruby layout structures are properly parented,
	and the UA can start to associate bases with their annotations.

	<p class="note">
	Note that the UA is not required to create any of these anonymous boxes in its internal structures,
	as long as pairing and layout behaves as if they existed.

<h3 id="pairing">
Ruby Pairing and Annotation Levels</h3>

	<p>Within a ruby structure,
	each <i>ruby base</i> is associated with <i>ruby annotations</i>
	and vice versa.
	A <i>ruby base</i> can be associated with at most one <i>ruby annotation</i> per annotation level.
	If there are multiple annotation levels, it can therefore be associated with multiple <i>ruby annotations</i>.
	A <i>ruby annotation</i> is associated with one or more <i>ruby bases</i>;
	annotations can span multiple bases.

	<p><dfn>Annotation pairing</dfn> is the process of associating
	<i>ruby annotations</i> with <i>ruby bases</i>.

	<ol>
		<li>
		<p>First, the ruby structure is divided into <i>ruby segments</i>,
		each consisting of a single <i>ruby base container</i>
		followed by one or more <i>ruby annotation containers</i>.
		If the first child of a <i>ruby container</i> is a <i>ruby annotation container</i>,
		an anonymous, empty <i>ruby base container</i> is assumed to exist before it.
		Similarly, if the <i>ruby container</i> contains consecutive <i>ruby base containers</i>,
		anonymous, empty <i>ruby annotation containers</i> are assumed to exist between them.
		The <i>ruby base container</i> in each segment is thus associated
		with each of the <i>ruby annotation containers</i> in that segment.

		<p>Each <i>ruby annotation containers</i> in a <i>ruby segment</i>
		represents one <dfn title="annotation level | level">level</dfn> of annotation:
		the first one represents the first level of annotation,
		the second one represents the second level of annotation,
		and so on.

		<li>Within each <i>ruby segment</i>,
		each <i>ruby base box</i> in the <i>ruby base container</i>
		is paired with one <i>ruby annotation box</i>
		from each <i>ruby annotation container</i> in its <i>ruby segment</i>.
		If there are not enough <i>ruby annotations</i> in a <i>ruby annotation container</i>,
		the last one is associated with any excess <i>ruby bases</i>.
		If there are not enough <i>ruby bases</i>,
		any remaining <i>ruby annotations</i> are assumed to be associated
		with empty, anonymous bases inserted at the end of the <i>ruby base container</i>.

		<p>If an implementation supports ruby markup with explicit spanning
		(e.g. XHTML Complex Ruby Annotations),
		it must adjust the pairing rules to pair spanning annotations to multiple bases
		appropriately.
	</ol>

	<p>A this point, ruby “columns” are defined,
	each represented by a single <i>ruby base</i>
	and associated with one <i>ruby annotation</i> (possibly an empty, anonymous one)
	from each <i>annotation level</i>.

<h4 id="nested-pairing">
Nested Ruby</h4>

	<p>When <i>ruby containers</i> are nested,
	pairing begins with the deepest <i>ruby container</i>,
	then expands out,
	treating each <i>ruby container</i> nested within another <i>ruby container</i>
	essentially as a single <i>ruby base</i> in the outer <i>ruby container</i>,
	and associating each <i>ruby annotation</i>
	paired with the nested <i>ruby container</i>
	as being associated with (spanning) all of its <i>ruby bases</i>.

	<p>Using nested <i>ruby containers</i> thus allows the representation
	of complex spanning relationships.

	<p class="issue">This has to be Level 1 because HTML5 allows it, so we have to handle it. Yay HTML5.

<h3 id="autohide">
Autohiding Annotations</h3>

	<p>If a <i>ruby annotation</i> has the exact same text content as its base,
	it is <dfn title="hidden ruby annotation | hidden annotation">hidden</dfn>.
	Hiding a <i>ruby annotation</i> does not affect annotation pairing
	or the block-axis positioning of boxes in other <i>levels</i>.
	However the <i>hidden annotation</i> is not visible,
	and it has no impact on layout
	other than to separate adjacent sequences of <i>ruby annotation boxes</i> within its level,
	as if they belonged to separate segments
	and the <i>hidden annotation</i>’s base were not a <i>ruby base</i> but an intervening inline.

	<div class="example">
		<p>This is to allow correct inlined display of annotations
		for Japanese words that are a mix of kanji and hirangana.
		For example, the word <i>振り仮名</i> should be inlined as
		<p class="figure">振り仮名(ふりがな)
		<p>and therefore marked up as
		<pre>
<!--		-->&lt;ruby>
<!--		-->  &lt;rb>振&lt;/rb>&lt;rb>り&lt;/rb>&lt;rb>仮&lt;/rb>&lt;rb>名&lt;/rb>
<!--		-->  &lt;rp>(&lt;/rp>&lt;rt>ふ&lt;/rt>&lt;rt>り&lt;/rt>&lt;rt>が&lt;/rt>&lt;rt>な&lt;/rt>&lt;rp>)&lt;/rp>
<!--		-->&lt;ruby></pre>
		<p>However, when displayed as ruby, the “り” should be hidden
		<div class="figure">
			<p><img src="images/furigana-separate.png"
			        alt="Hiragana annotations for 振り仮名 appear, each above its base character.">
			<p class="caption">Hiragana ruby for 振り仮名
		</div>
	</div>

	<p class="note">
		Future levels of CSS Ruby may add controls for this,
		but in this level it is always forced.

	<p>The content comparison for this auto-hiding behavior
	takes place prior to white space collapsing
	and ignores elements (considers only the <code>textContent</code> of the boxes).
	<p class="issue">Is before or after white space collapsing easier? We should do whatever is easier, as it really doesn't matter much which way to go.

<h3 id="white-space">
White Space</h3>

	<p><i>Collapsible</i> white space within a ruby structure is discarded
	<ul>
		<li>at the beginning and end of a <i>ruby container</i>, <i>ruby annotation container</i>, or <i>ruby base container</i>,
		<li>at the beginning/end of a <i>ruby annotation box</i> or <i>ruby base box</i> if white space is not its only contents,
		<li>between a <i>ruby base container</i> and its following <i>ruby annotation container</i>,
		<li>between <i>ruby annotation containers</i>.
	</ul>

	<p>Between <i>ruby segments</i>, between <i>ruby bases</i>, and between <i>ruby annotations</i>, however,
	white space is not discarded.

	<p>Where undiscarded white space is <i>collapsible</i>, it will collapse
	following the standard <a href="http://www.w3.org/TR/css3-text/#white-space-rules">white space processing rules</a>. [[!CSS3TEXT]]
	For <i>collapsible</i> white space between <i>ruby segments</i>, however,
	the contextual text for determining collapsing behavior is given by the <i>ruby bases</i> on either side,
	not the text on either side of the white space in the source document.

	<div class="note">
		<p>Note that the white space processing rules
		cause a white space sequence containing a <i>segment break</i> (such as a line feed)
		to <a href="http://www.w3.org/TR/css3-text/#line-break-transform">collapse to nothing</a> between CJK characters.
		This means that CJK ruby can safely use white space for indentation of the ruby markup.
		For example, the following markup will display without any spaces:
		<pre>
<!--		-->&lt;ruby>
<!--		-->  &lt;rb>東&lt;/rb>&lt;rb>京&lt;/rb>
<!--		-->  &lt;rt>とう&lt;/rt>&lt;rt>きょう&lt;/rt>
<!--		-->&lt;/ruby></pre>
		<p>However, white space that does not contain a <i>segment break</i> does not collapse completely away,
		so this markup will display with a space between the first and second ruby pairs:
		<pre>
<!--		-->&lt;ruby>
<!--		-->  &lt;rb>東&lt;/rb>	&lt;rb>京&lt;/rb>
<!--		-->  &lt;rt>とう&lt;/rt>	&lt;rt>きょう&lt;/rt>
<!--		-->&lt;/ruby></pre>
	</div>

	<p>Any preserved white space is then wrapped in an anonymous box belonging to
	the <i>ruby base container</i> (if between <i>ruby bases</i>),
	<i>ruby annotation container</i> (if between <i>ruby annotations</i>),
	or <i>ruby container</i> (if between <i>ruby segments</i>).
	In the latter case, the text is considered part of the <i>base level</i>.
	Such anonynmous boxes do not take part in pairing.
	They merely ensure separation between adjacent bases/annotations.

	<p class="issue">Specify how these anonymous white space boxes impact layout.

	<div class="example">
		<p>These rules allow ruby to be used with space-separated scripts such as Latin.
		For example,
		<pre>
<!--		-->&lt;ruby>
<!--		-->  &lt;rb>W&lt;/rb>&lt;rb>W&lt;/rb>&lt;rb>W&lt;/rb>
<!--		-->  &lt;rt>World&lt;/rt> &lt;rt>Wide&lt;/rt> &lt;rt>Web&lt;/rt>
<!--		-->&lt;/ruby></pre>
		<p>They also ensure that annotated white space is preserved. For example,
		<pre>
<!--		-->&lt;ruby>
<!--		-->  &lt;rb>Aerith&lt;/rb>&lt;rb> &lt;/rb>&lt;rb>Gainsboro&lt;/rb>
<!--		-->  &lt;rt>エアリス&lt;/rt>&lt;rt>・&lt;/rt>&lt;rt>ゲインズブール&lt;/rt>
<!--		-->&lt;/ruby></pre>
	</div>

<h3 id="ruby-layout">
Ruby layout</h3>

	<p>When a ruby structure is laid out,
	its base level is laid out on the line,
	aligned according to its 'vertical-align' property
	exactly as if its <i>bases</i> were a regular sequence of <i>inline</i> boxes.
	Each <i>ruby base container</i> is sized and positioned
	to contain exactly the full height of its <i>ruby bases</i>.

	<p><i>Ruby annotations</i> associated with the base level
	are then positioned with respect to their <i>ruby base boxes</i>
	according to the applicable 'ruby-position' values.
	<i>Ruby annotations</i> within a level (within a single <i>ruby container</i>)
	are aligned to each other as if they were inline boxes
	participating in the same inline formatting context.
	Each <i>ruby annotation container</i> is sized and positioned
	to contain exactly the full height of its <i>ruby annotations</i>.

	<p>A ruby container (or fragment thereof)
	measures as wide as the content of its widest level.
	Similarly, <i>ruby base boxes</i> and <i>ruby annotation boxes</i>
	within a ruby “column” have the measure of the widest content in that “column”.
	In the case of spanning <i>annotations</i>
	(whether actually spanning or pretending to span per 'ruby-collapse'),
	the measures of the <i>ruby annotation box</i> and
	the sum of its associated <i>ruby base boxes</i> must match.

	<p>How the extra space is distributed
	when ruby content is narrower than the measure of its box
	is specified by the 'ruby-align' property.

<h4 id="inter-character-layout">
Inter-character ruby layout</h4>

	<p>Inter-character annotations have special layout.
	When 'ruby-position' indicates ''inter-character'' annotations,
	the affected <i>annotation boxes</i>
	are spliced into and measured as part of the layout of the base level.
	The <i>ruby base container</i> must be sized to include both the <i>base boxes</i>
	as well as the ''inter-character'' <i>annotation boxes</i>.
	The affected <i>ruby annotation container</i> is similarly sized
	so that its content box coincides with that of the <i>ruby base container</i>.

	<p>For the purpose of laying out other levels of annotations,
	an ''inter-character'' annotation effectively becomes part of its base.
	<span class="issue">Or should it become a quasi-base between two bases?</span>
	A spanning ''inter-character'' annotation is placed after
	all the bases that it spans.

<h3 id="box-style">
Styling Ruby Boxes</h3>

	<p>In most respects, ruby boxes can be styled similar to inline boxes.
	However, the UA is not required to support
	any of the box properties (borders, margins, padding),
	any of the background properties or outline properties,
	or any other property that illustrates the bounds of the box
	on <i>ruby base container boxes</i>, <i>ruby annotation container boxes</i>,
	or <a href="#nested-pairing">ruby-internal <i>ruby container boxes</i></a>.
	The UA may implement these boxes simply as abstractions for inheritance
	and control over the layout of their contents.

<h3 id="line-breaks">
Ruby box and line breaking</h3>

	<p>When there is not enough space for an entire <i>ruby container</i> to fit on the line,
	the ruby may be broken wherever all levels simultaneously allow a break.
	Ruby most often breaks between base-annotation sets,
	but if the line-breaking rules allow it, can also break within a <i>ruby base</i>
	(and, in parallel, its associated <i>annotation boxes</i>).

	<p>Whenever ruby breaks across lines, <i>ruby annotations</i> must stay
	with their respective <i>bases</i>.
	The line <em>must not</em> break between a <i>ruby base</i> and its <i>annotations</i>,
	even in the case of ''inter-character'' <i>annotations</i>.

	<div class="figure">
		<img src="images/r-break-b.gif"
		     alt='Diagram showing the line breaking opportunity in a "Bopomofo" ruby'>
		<p class="caption">''inter-character'' ruby line breaking opportunity
	</div>

<h4 id="break-between">
Breaking between bases</h4>

	<p>In typical cases, <i>ruby base boxes</i> and <i>ruby annotation boxes</i>
	are styled to forbid internal line wrapping and do not contain forced breaks.
	(See <a href="#default-stylesheet">Appendix A</a>.)
	In such cases the <i>ruby container</i> can only break between adjacent <i>ruby bases</i>,
	and only if no <i>ruby annotations</i> span those <i>ruby bases</i>.

	<div class="figure">
		<p><img src="images/r-break-a.gif"
		     alt="Diagram showing the line breaking opportunity in a complex ruby">
		<p class="caption">Ruby line breaking opportunity
	</div>

	<p>Whether ruby can break between two adjacent <i>ruby bases</i>
	is controlled by normal line-breaking rules for the base text,
	exactly as if the <i>ruby bases</i> were adjacent <i>inline</i> boxes.
	(The annotations are ignored when determining soft wrap opportunities for the base level.)

	<div class="example">
		<p>For example, if two adjacent ruby bases are “蝴” and “蝶”,
		the line may break between them,
		because lines are normally allowed to break between two Han characters.
		However, if 'word-break' is ''keep-all'', that line break is forbidden.
		<pre>&lt;ruby>蝴&lt;rt>hú&lt;/rt>蝶&lt;rt>dié&lt;/rt></pre>
	</div>

	<p>Inter-base white space is significant for evaluating line break opportunities between <i>ruby bases</i>.
	As with white space between inlines, it collapses when the line breaks there.
	Similarly, annotation white space is also trimmed at a line break.

	<div class="example">
		<p>For example, given the following markup:
		<pre>&lt;ruby>&lt;rb>one&lt;/rb> &lt;rb>two&lt;/rb> &lt;rt>1&lt;/rt> &lt;rt>2&lt;/rt>&lt;/ruby></pre>
		<p>Due to the space, the line may break between “one” and “two“.
		If the line breaks there, that space&mdash;and the space between “1” and “2”&mdash;disappears,
		in accordance with standard CSS white 
6D50
space processing rules. [[CSS3TEXT]]
	</div>

<h4 id="break-within">
Breaking within bases</h4>

	<p>For longer base texts, it is sometimes appropriate to allow breaking within a base-annotation pair.
	For example, if an English sentence is annotated with its Japanese translation,
	allowing the text to wrap allows for reasonable line breaking behavior in the paragraph.

	<p class="issue">
	Insert scanned example so people don't think this is just the ramblings of an insane spec-writer.

	<p>Line-breaking within a <i>ruby base</i> is only allowed if the 'white-space' property
	of the <i>ruby base</i> and all its parallel <i>annotations</i> allow it,
	and there exists a <i>soft wrap opportunity</i> <em>within</em> (i.e. not at the start or end)
	the content of each base/annotation box.
	Since there is no structural correspondance between fragments of content
	within <i>ruby bases</i> and <i>annotations</i>,
	the UA may break at any set of opportunities;
	but it is recommended that the UA attempt to proportionally balance
	the amount of content inside each fragment.

	<p>There are no line breaking opportunities within ''inter-character'' <i>annotations</i>.

	<p>Ruby alignment takes place within each fragment, after line-breaking.

<h3 id="ruby-bidi">
Bidi Reordering</h3>

	<p class="issue">
	Constraints:
	Text within a ruby base must remain contiguous,
	and bases belonging to a single annotation must remain contiguous.
	Still figuring out exactly to enforce these limitations in a sensible manner.

<!--

	<p>The Unicode bidirectional algorithm orders logically-stored text for visual presentation
	when characters from scripts of opposing directionalities are mixed
	within a single paragraph.
	(See [[CSS3-WRITING-MODES]] for a more in-depth discussion of bidirectional text in CSS.)
	Bidi reordering of ruby-annotation pairs is controlled by the ordering of the base text, as follows:

<h4 id="bidi-B">Proposal A</h4>

	<p class="note">This proposal is simpler, but will require more frequent tagging of mixed-directionality content.

	<p>To avoid the interference of the <i>annotations</i> in the ordering of base text,
	all annotations are ignored for the purpose of resolving neutral characters in the base level.

	<p>To preserve the correspondance of <i>ruby annotations</i>
	to their respective <i>ruby bases</i>,
	a few restrictions are imposed:
	<ul>
		<li>The contents of a <i>ruby base</i> or <i>ruby annotation</i> must remain contiguous.
			To this end, the contents of each <i>ruby base</i> and <i>ruby annotation</i> are treated as if within a <i>bidi isolation</i>.
		<li>For the purpose of ordering <i>ruby bases</i> within a <i>ruby container</i>,
			each <i>ruby base</i> is treated as a strong character of its specified 'direction'.
			(<i>Ruby annotations</i> are positioned with respect to their <i>bases</i>,
			so are affected by reordering, but do not themselves participate in reordering.)
		<li>All <i>bases</i> spanned by a single <i>annotation</i> must remain contiguous.
			To this end, the embedding level of all bases spanned by a spanning <i>annotation</i>
			is increased by two prior to reordering.
	</ul>

<h4 id="bidi-B">Proposal B</h4>

	<p class="note">This preserves some aspects of implicit bidi.
	For example, annotating each half of "first-second" would not cause the word to reverse itself to "second-first" within an opposite-order paragraph.

	<p>To avoid the interference of the <i>annotations</i> in the ordering of the base text,
	all annotations are ignored for the purpose of resolving neutral characters in the base text;
	and the base text is resolved exactly as if each <i>ruby base</i> were
	just a normal <i>inline box</i> embedded in the <i>ruby container</i>'s inline formatting context.

	<p>Furthermore, to preserve the correspondance of <i>ruby annotations</i>
	to their respective <i>ruby bases</i>,
	a few restrictions are imposed:
	<ul>
		<li>The contents of a <i>ruby base</i> must remain contiguous.
		<li>All <i>bases</i> spanned by a single <i>annotation</i> must remain contiguous.
	</ul>
	<p>To this end, the text of <i>ruby base</i> boxes that are not <i>bidi-isolated</i>
	must have its embedding level increased by two;
	and if an <i>annotation</i> spans more than one <i>ruby base</i>,
	the spanned text's embedding level
	(or the embedding level assigned to the isolation, if the <i>ruby base</i> is <i>bidi-isolated</i>)
	is further increased by two.

-->

<h3 id="line-height">
Ruby box and line stacking</h3>

	<p>The 'line-height' property controls spacing between lines in CSS.
	When inline content on line is shorter than the 'line-height',
	half-leading is added on either side of the content,
	as specificed in <a href="http://www.w3.org/TR/CSS21/visudet.html#line-height">CSS2.1&sect;10.8</a>. [[!CSS21]]

	<p>In order to ensure consistent spacing of l
9E70
ines,
	documents with ruby typically ensure that the 'line-height' is large enough
	to accommodate ruby between lines of text.
	Therefore, ordinarily, <i>ruby annotation containers</i> and <i>ruby annotation boxes</i>
	do not contribute to the measured height of a line's inline contents;
	any alignment (see 'vertical-align') and line-height calculations
	are performed using only the <i>ruby base container</i>,
	exactly as if it were a normal inline.

	<p>However, if the 'line-height' specified on the <i>ruby container</i>
	is less than the distance between
	the top of the top <i>ruby annotation container</i>
	and the bottom of the bottom <i>ruby annotation container</i>,
	then additional leading is added
	on the appropriate side of the <i>ruby base container</i>
	such that if a block consisted of three lines
	each containing ruby identical to this,
	none of the <i>ruby containers</i> would overlap.

	<p class="note">Note that this does not ensure that the <i>ruby annotations</i> remain within the line box.
	It merely ensures that <em>if all lines had equal spacing</em>
	and equivalent amounts and positioning of <i>ruby annotations</i>,
	there would be enough room to avoid overlap.

	<p>Authors should ensure appropriate 'line-height' and 'padding' to accommodate ruby,
	and be particularly careful at the beginning or end of a block
	and when a line contains inline-level content
	(such as images, inline blocks, or elements shifted with 'vertical-align')
	taller than the paragraph's default font size.

	<div class="figure">
		<p><img src="images/rlh-a.gif"
		        alt="The content of each line sits in the middle of its line height;
		             the additional space on each side is called half-leading.
		             Ruby fits between lines if it is smaller than twice the half-leading,
		             but this means that it occupies space belonging to the half-leading of the previous line.">
		<p class="caption">Ruby annotations will often overflow the line;
		authors should ensure content over/under a ruby-annotated line
		is adequately spaced to leave room for the ruby.
	</div>

	<p class="note">More control over how ruby affects alignment and line layout
	will be part of the CSS Line Layout Module Level 3.
	Note, it is currently in the process of being rewritten;
	the current drafts should not be relied upon.

<h2 id="ruby-props">
Ruby Properties</h2>

	<p>The following properties are introduced to control ruby positioning and alignment.

<h3 id="rubypos">
Ruby positioning: the 'ruby-position' property</h3>

	<table class="propdef">
		<tr>
			<th>Name:
			<td><dfn>ruby-position</dfn>
		<tr>
			<th><a href="#values">Value</a>:
			<td>[ over | under | inter-character ] && [ right | left ]
		<tr>
			<th>Initial:
			<td>over right
		<tr>
			<th>Applies to:
			<td>ruby annotation containers
		<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><abbr title="follows order of property value definition">per grammar</abbr>
	</table>

	<p>This property controls position of the ruby text with respect to its base.
	Values have the following meanings:

	<p class="issue"><span class="issuehead">Issue-107:&nbsp;</span> Roland Steiner has requested the addition of an auto value as default. See <a href="http://www.w3.org/Search/Mail/Public/advanced_search?keywords=&amp;hdr-1-name=subject&amp;hdr-1-query=ruby-position%3A+undesirable+default+value+%27before%27+for+complex+ruby&amp;hdr-2-name=from&amp;hdr-2-query=&amp;hdr-3-name=message-id&amp;hdr-3-query=&amp;period_month=&amp;period_year=&amp;index-grp=Public__FULL&amp;index-type=t&amp;type-index=www-style&amp;resultsperpage=20&amp;sortby=date">this thread</a> and <a href="http://www.w3.org/Search/Mail/Public/advanced_search?keywords=&amp;hdr-1-name=subject&amp;hdr-1-query=Styling+of+complex+Ruby&amp;hdr-2-name=from&amp;hdr-2-query=&amp;hdr-3-name=message-id&amp;hdr-3-query=&amp;period_month=&amp;period_year=&amp;index-grp=Public__FULL&amp;index-type=t&amp;type-index=public-i18n-core&amp;resultsperpage=20&amp;sortby=date">this one</a>.</p>
	<dl>
		<dt><dfn title="ruby-position:over">''over''</dfn>
		<dd>The ruby text appears <i>over</i> the base in horizontal text.

			<div class="figure">
				<p><img src="images/shinkansen-top.gif"
				        alt="Diagram of ruby glyph layout in horizontal mode with ruby text appearing above the base">
				<p class="caption">Ruby over Japanese base text in horizontal layout
			</div>
		</dd>

		<dt><dfn title="ruby-position:right">''right''</dfn>
		<dd>The ruby text appears on the right side of the base in vertical text.
			<div class="figure">
				<p><img src="images/shinkansen-right.gif" width="33"
				        alt="Diagram of ruby glyph layout in vertical mode with ruby text apearing vertically on the right of the base">
				<p class="caption">Ruby to the right of Japanese base text in vertical layout
			</div>
		</dd>

		<dt><dfn title="ruby-position:under">''under''</dfn>
		<dd>The ruby text appears under the base in horizontal text.
			This is a relatively rare setting used in ideographic East Asian writing systems,
			most easily found in educational text.

			<div class="figure">
				<p><img src="images/shinkansen-bottom.gif"
				        alt="Diagram of ruby glyph layout in horizontal mode with ruby text appearing below the base">
				<p class="caption">Ruby under Japanese base text in horizontal layout
			</div>
		</dd>

		<dt><dfn title="ruby-position:left">''left''</dfn>
		<dd>The ruby text appears on the left side of the base in vertical text.

			<div class="figure">
				<p><img src="images/shinkansen-left.gif"
				        alt="Diagram of ruby glyph layout in vertical mode with ruby text apearing vertically on the left of the base">
				<p class="caption">Ruby to the left of Japanese base text in vertical layout
			</div>
		</dd>

		<dt><dfn title="ruby-position:inter-character">''inter-character''</dfn></dt>
		<dd>
			<p>The ruby text appears on the right of the base in horizontal text.
			This value forces the 'writing-mode' of the <i>ruby annotation</i> to be vertical.

			<p>This value is provided for the special case of traditional Chinese
			as used especially in Taiwan:
			ruby (made of <a href="#g-bopomofo">bopomofo</a> glyphs) in that context
			appears vertically along the right side of the base glyph,
			even when the layout of the base characters is horizontal:

				<div class="figure">
					<p><img src="images/bopomofo.gif"
					        alt="Example of Taiwanese-style ruby">
					<p class="caption">“Bopomofo” ruby in traditional Chinese
					(ruby text shown in blue for clarity) in horizontal layout
				</div>
			<p class="note">
				Note that the user agent is responsible for ensuring the correct relative alignment and positioning of the glyphs,
				including those corresponding to the tone marks, when displaying.
				Tone marks are spacing characters that occur (in memory) at the end of the ruby text for each base character.
				They are usually displayed in a separate column to the right of the bopomofo characters,
				and the height of the tone mark depends on the number of characters in the syllable.
				One tone mark, however, is placed above the bopomofo, not to the right of it.
			<!-- See Taiwanese requirements doc for EPUB at http://epub-revision.googlecode.com/files/EGLS_TW_eng.ppt -->
		</dd>
	</dl>

	<p>If multiple <i>ruby annotation containers</i> have the same 'ruby-position',
	they stack along the block axis,
	with lower levels of annotation closer to the base text.

<h3 id="collapsed-ruby">
Collapsed Ruby Annotations: the 'ruby-merge' property</h3>

	<table class="propdef">
		<tr>
			<th>Name:
			<td><dfn>ruby-merge</dfn>
		<tr>
			<th><a href="#values">Value</a>:
			<td>separate | collapse | auto
		<tr>
			<th>Initial:
			<td>separate
		<tr>
			<th>Applies to:
			<td>ruby annotation containers
		<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><abbr title="follows order of property value definition">per grammar</abbr>
	</table>

	<p>
		This property controls how ruby annotation boxes should be rendered
		when there are more than one in a ruby container box.

	<p>Possible values:</p>
	<dl>
		<dt><dfn title="ruby-merge:separate">''separate''</dfn>
		<dd>
			<p>
				Each ruby annotation box is rendered in the same column(s) as its corresponding base box(es).
				This style is called “mono ruby” in [[JLREQ]].

			<div class="example">
				<p>For example, the following two markups render the same:
				<pre>&lt;ruby&gt;無&lt;rt&gt;む&lt;/ruby&gt;&lt;ruby&gt;常&lt;rt&gt;じょう&lt;/ruby&gt;</pre>
				<p>and:
				<pre>&lt;ruby style="ruby-merge:separate"&gt;&lt;rb&gt;無&lt;rb&gt;常&lt;rt&gt;む&lt;rt&gt;じょう&lt;/ruby&gt;</pre>
			</div>
		</dd>

		<dt><dfn title="ruby-merge:collapse">''collapse''</dfn>
		<dd>
			<p>
				All <i>ruby annotation boxes</i> within the same <i>ruby segment</i> on the same line are concatenated,
				and laid out as if their contents belonged to a single <i>ruby annotation box</i>
				spanning all their associated <i>ruby base boxes</i>.
				This style renders similar to “group ruby” in [[JLREQ]],
				except that <i>ruby annotations</i> are kept together with their respective <i>ruby bases</i> when breaking lines.
			</p>

			<div class="example">
				<p>The following two markups render the same both characters fit on one line:
				<pre>&lt;ruby&gt;無常&lt;rt&gt;むじょう&lt;/ruby&gt;</pre>
				<p>and:
				<pre>&lt;ruby style="ruby-merge:collapse"&gt;&lt;rb&gt;無&lt;rb&gt;常&lt;rt&gt;む&lt;rt&gt;じょう&lt;/ruby&gt;</pre>
				<p>However, the second one renders the same as ''ruby-position: separate''
				when the two bases are split across lines.
			</div>
		</dd>

		<dt><dfn title="ruby-merge:auto">''auto''</dfn></dt>
		<dd>
			<p>
				The user agent may use any algorithm to determine how each ruby annotation box
				is rendered to its corresponding base box,
				with the intention that if all annotations fit over their respective bases,
				the result is identical to “mono ruby”,
				but if some annotations are wider than their bases
				the space is shared in some way
				to avoid forcing space between bases.
			<div class="example">
			<p>
				One possible algorithm is described as “jukugo ruby” in [[JLREQ]].
			<p>
				Another, more simplified algorithm of “jukugo ruby” is
				to render as ''separate'' if all ruby annotation boxes fit
				within the advances of their corresponding base boxes,
				and render as ''collapse'' otherwise.
			</p>
			</div>
		</dd>
	</dl>

<h3 id="ruby-align-property"><a id="rubyalign"></a>
Ruby Text Distribution: the 'ruby-align' property</h3>

	<table class="propdef">
		<tr>
			<th>Name:
			<td><dfn>ruby-align</dfn>
		<tr>
			<th><a href="#values">Value</a>:
			<td>start | center | space-between | space-around
		<tr>
			<th>Initial:
			<td>space-around
		<tr>
			<th>Applies to:
			<td>ruby bases, ruby annotations, ruby base containers, ruby annotation containers
		<tr>
			<th>Inherited:
			<td>yes
		<tr>
			<th>Percentages:
			<td>N/A
		<tr>
			<th>Media:
			<td>visual