csswg-drafts/css3-linebox/Overview.src.html at 381ccc998e95c3f81c0e0c794cc2be4203162864 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">

<html lang="en">
<head>
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS3 module: line</title>

<style type="text/css">
    dt {font-weight: normal}
    /*.issue, .note {font-style: italic}*/
    .figure {font-size: smaller;}

   /* Stolen from CSS2 Recommendation */

   A.noxref:link, A.noxref:visited {color: black; /*color: inherit*/}
    </style>

<style media="print" type="text/css">
    img#edge { width: 80%; height: 70%;}
    dt.label { display: run-in; }
    </style>

<link href="default.css" rel="stylesheet" type="text/css">

<link href="http://www.w3.org/StyleSheets/TR/W3C-WD.css" rel="stylesheet"
type="text/css">
</head>

<body>
<div class="head">
<p><a href="http://www.w3.org/"><img alt="W3C" height="48"
src="http://www.w3.org/Icons/w3c_home" width="72"></a>

<h1 id="css3-module">CSS3 module: line</h1>

<h2 class="no-num no-toc" id="w3c-working">W3C Working Draft 17 March 2003</h2>

<dl>
<dt>This version:

<dd>TBD</dd>

<dt>Latest version:

<dd>TBD</dd>

<dt>Previous version:

<dd>none

<dt>Editors:

<dd><a href="mailto:michelsu@microsoft.com">Michel Suignard (Microsoft)</a>
<dd><a href="http://www.meyerweb.com/eric/">Eric A. Meyer</a>
<dd><a href="http://dbaron.org/">L. David Baron</a>

</dl>

<dl>
<dt>Contributors:

<dd>Original CSS2 authors, and

<dd><a href="mailto:bert@w3.org">Bert Bos (W3C)</a>

<dd><a href="mailto:michelsu@microsoft.com">Michel Suignard (Microsoft)</a>

<dd><a href="mailto:szilles@adobe.com">Steve Zilles (Adobe)</a>

<dd><a href="http://dbaron.org/">L. David Baron</a>

</dl>
<!--begin-include "../copyright.inc"-->
<p class="copyright"><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">
Copyright</a> &copy; [YEAR: 2000] <a href="http://www.w3.org/"><abbr
title="World Wide Web Consortium">W3C</abbr></a><sup>&reg;</sup> (<a
href="http://www.lcs.mit.edu/"><abbr title="Massachusetts Institute of
Technology">MIT</abbr></a>, <a
href="http://www.ercim.org/"><acronym title="European Research
Consortium for Informatics and Mathematics">ERCIM</acronym></a>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>,
<a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> and <a
href="http://www.w3.org/Consortium/Legal/copyright-software">software
licensing</a> rules apply.</p>
<!--end-include-->
<hr title="Separator for header">
</div>

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

<p>This document presents a set of CSS line formatting properties. It also
includes baseline alignment features as well as related functions like initial
line and initial letter effect. It extends on the CSS 2 model <a href="#CSS2"
rel="biblioentry">[CSS2]</a>.

<h2 class="no-num no-toc" id="Status">Status of This Document</h2>

<p>This document is a working draft of the CSS working group which is part of
the <a href="/Style/">Style activity</a> (see <a
href="/Style/Activity">summary</a>). It contains a proposal for features
to be included in CSS level 3.

<p>Feedback is very much welcome. Comments can be sent directly to the
editor, but the mailing list <a
href="/Mail/Lists.html#www-style">www-style@w3.org</a> (see <a
href="/Mail/Request">instructions</a>) is also open and is preferred for
discussion of this and other drafts in the Style area. The mailing list is
archived.<p>This working draft may be updated, replaced or rendered obsolete by other
W3C documents at any time. It is inappropriate to use W3C Working Drafts as
reference material or to cite them as other than "work in progress". Its
publication does not imply endorsement by the W3C membership or the <a
href="/Style/Group/">CSS Working Group</a> (<a
href="http://cgi.w3.org/MemberAccess/AccessRequest">members only</a>).

<p>To find the latest version of this working draft, please follow the
"Latest version" link above, or visit the list of W3C <a
href="/TR/">Technical Reports</a>.

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

<ul class="toc">
<li><a href="#dependencies"><span class="secno">1. </span>Dependencies on
other modules</a>

<li><a href="#introduction"><span class="secno">2. </span>Introduction</a>

<li><a href="#LineBox"><span class="secno">3. </span>Line box, Line
stacking and Content height</a>
<ul class="toc">
<li><a href="#InlineFormatting"><span class="secno">3.1. </span>Inline formatting
context</a>

<li><a href="#LineBoxWrapping"><span class="secno">3.2. </span>Line box
wrapping</a>

<li><a href="#inline1"><span class="secno">3.3. </span>Inline box
progression dimension (height in horizontal flow), the 'text-height' property</a>

<li><a href="#InlineBoxHeight"><span class="secno">3.4. </span>Line height
adjustment: the 'line-height' property</a>

<li><a href="#LineStacking"><span class="secno">3.5. </span>Line Stacking:
the 'line-box-contain' property</a>
</ul>

<li><a href="#baseline">
<span class="secno">4. </span>Baseline alignment</a>
<ul class="toc">
<li><a href="#FontBaseline">
<span class="secno">4.1. </span>Baseline information provided by fonts</a>

<li><a href="#AlignmentBaselineType">
<span class="secno">4.2. </span>Baseline identifiers</a>

<li><a href="#BaselineAlignmentOverview">
<span class="secno">4.3. </span>Overview of the baseline alignment process</a>

<li><a href="#dominant-baseline-prop">
<span class="secno">4.4. </span>Dominant baseline: the <span
class="property">'dominant-baseline'</span> property</a>

<li><a href="#alignment-baseline-prop">
<span class="secno">4.5.
</span>Aligning the alignment point of an element: the <span
class="property">'alignment-baseline'</span> property</a>

<li><a href="#alignment-adjust-prop">
<span class="secno">4.6. </span>Setting the alignment point: the <span class="property">'alignment-adjust'</span>
property</a>

<li><a href="#baseline-shift-prop">
<span class="secno">4.7. </span>Repositioning the dominant baseline: the <span
class="property">'baseline-shift'</span> property</a>

<li><a href="#vertical-align-prop">
<span class="secno">4.8. </span>Vertical alignment: the <span class="property">'vertical-align'</span> shorthand
baseline alignment property</a>

<li><a href="#inline-box-align-prop">
<span class="secno">4.9. </span>Inline box alignment: the <span class="property">'inline-box-align'</span>
property</a>
</ul>

<li><a href="#Initial">
<span class="secno">5. </span>
Initial line and Drop initial</a>

<ul class="toc">
<li><a href="#InitialLine">
<span class="secno">5.1. </span>Initial line</a>

<li><a href="#DropInitial">
<span class="secno">5.2. </span>Drop Initial overview</a>

<li><a href="#DropInitialSize">
<span class="secno">5.3. </span>
Sizing drop initial, the <span class="property">'drop-initial-size'</span> and the <span class="property">'drop-initial-value'</span>
properties</a>

<li><a href="#DropInitialAlign">
<span class="secno">5.4. </span>Aligning drop initial: the <span
class="property">'drop-initial-before-align'</span>, <span
class="property">'drop-initial-before-adjust'</span>, <span
class="property">'drop-initial-after-align'</span> and <span
class="property">'drop-initial-after-adjust'</span> properties</a>

</ul>

<li><a href="#properties"><span class="secno">6. </span>Properties index</a>

<li><a href="#profiles"><span class="secno">7. </span>Profiles</a>

<li><a href="#glossary"><span class="secno">8. </span>Glossary</a>

<li><a href="#BaselineUsage">Appendix A: Usage of baseline alignment
(informative)</a>

<li><a href="#ack">Acknowledgments</a>

<li><a href="#ref">References</a>
<ul class="toc">
<li><a href="#normative">Normative references</a>

<li><a href="#other">Other references</a>
</ul>

</ul>
<!--end-toc-->

<hr>

<h2 id="dependencies"><span class="secno">1. </span>Dependencies on other
modules</h2>

<p>This CSS3 module depends on the following other CSS3 modules:

<ul>
<li>Fonts<li>Text

<li>Syntax and grammar

<li>Values and unit
</ul>

<p>It has non-normative (informative) references to the following other CSS3
modules:

<ul>
<li>Multi-column layout

<li>Ruby
</ul>

<h2 id="introduction"><span class="secno">2. </span>Introduction</h2>

<p>This modules specifies the presentation aspects of line within block elements
and inline elements. It also covers the design of baseline alignment within each
line and the placement of drop initial letter (also known as first letter or
drop caps). Most of the properties are new to CSS3.<p>All described properties, in addition to the noted values, take
'initial' and 'inherit'. These values are not repeated in each of the
property value enumeration.

<p>This module uses extensively the 'before', 'after', 'start' and
'end' notation to specify the four edges of a box relative to its text
advance direction, independently of its absolute positioning in terms of
'top', 'bottom', 'left' and 'right' (corresponding respectively to the
'before', 'after', 'start' and 'end' positions in a typical Western text
layout). This notation is also used extensively in <a href="#XSL1.0"
rel="biblioentry">[XSL1.0]</a> for the same purpose.

<p>Finally, in this document, requirements are expressed using the key words
&quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;,
&quot;SHALL&quot; and &quot;SHALL NOT&quot;. Recommendations are expressed
using the key words &quot;SHOULD&quot;, &quot;SHOULD NOT&quot; and
&quot;RECOMMENDED&quot;. &quot;MAY&quot; and &quot;OPTIONAL&quot; are used to
indicate optional features or behavior. These keywords are used in accordance
with <a href="#RFC2119" rel="biblioentry">[RFC2119]</a>. For legibility these
keywords are used in lowercase form.

<hr>

<h2 id="LineBox"><span class="secno">3. </span>Line box, Line stacking and Content height </h2>

<p>This section describes inline boxes formatting, how lines are stacked
together and a mechanism to adjust the content height of inline elements. Two
terms: inline-progression dimension and block-progression dimension are used
frequently in the section and are defined as follows<p>The <b>inline-progression
dimension</b> is a length in the direction of the inline-progression of a box.
For an horizontal layout flow it can be perceived as related to the the width of
the box. However for a vertical layout flow it will be related to the height of
the box. The term inline-progression dimension is the neutral way to express
that dimension independently of the layout flow.&nbsp;

<p>The <b>block-progression dimension</b> is a length in the direction of the
block-progression of a box. For an horizontal layout flow it can be perceived as
related to the height of the box. However for a vertical layout flow it will be
related to the width of the box. The term block-progression dimension is the
neutral way to express that dimension independently of the layout flow.<h3 id="InlineFormatting">
<span class="secno">3.1. </span>Inline formatting context</h3>

<p>In an inline formatting context, boxes are laid out in the inline-progression
direction, one after
the other, following the block-progression within the containing block. Borders,
padding and inline-progression margins are respected between these boxes. The boxes may be aligned
within the inline-progression in different ways: their after-edges or
before-edges may be aligned, or the
baselines of text within them may be aligned. The rectangular area that contains
the boxes that form a line is called a <span class="index-def" title="line box">
<a id="line-box"><dfn>line box</dfn></a></span>.</p>
<p>The inline-progression dimension ('width' in horizontal flow) of a line box is
determined by its containing block. The block-progression dimension ('height' in
horizontal flow) of a line box is determined by the rules given
in the section on
<a href="#line-height">line height
calculations</a>. A line box is generally tall (relative) enough for all of the boxes it
contains. However, it may be taller than the tallest box it contains (if, for
example, boxes are aligned so that baselines line up). The alignment of boxes
within a line box is determined by the
<span class="propinst-vertical-align"><span style="background-position: 0% 0%">
<a href="#baseline">baseline alignment</a></span></span> properties.</p>
<p>In general, when several inline boxes cannot fit within a single line box,
they are distributed among two or more block-progression stacked line boxes. The
exact wrapping of inline boxes at the line ending edge is determined by several
text related property categories such as line-breaking, word-breaking, text
wrapping and white-space management. Thus, a
paragraph is a stack of line boxes.</p>
<p>Line boxes are stacked together in the block-progression direction
without any separation.  The block-progression dimension of the line box
is determined by the contents of the line box (including the root inline
box) and the <span class="property"><a
href="#line-box-contain">'line-box-contain'</a></span> property.</p>
<p>A containing block defines a <b>root inline box</b> which wraps all the
inline children of the block element, including the anonymous inline boxes.
It inherits inheritable properties from the parent block
box (like <span class="property"><a
href="#line-height">'line-height'</a></span>), while non-inherited
properties have their default values.  The root inline box establishes a
baseline for vertical alignment and may affect the height of the line
boxes.</p>
<p>Although margins, borders, and padding of non-replaced elements do not enter
into inline box block-progression dimension calculation (and thus the line box
calculation), they are still rendered around inline boxes. This means that if
the block-progression dimension of a line box is shorter than the
<a href="http://www.w3.org/TR/REC-CSS2/box.html#outer-edge">outer edges</a> of
the boxes it contains, backgrounds and colors of padding and borders may &quot;bleed&quot;
into adjacent line boxes. However, in this case, some user agents may use the
line box to &quot;clip&quot; the border and padding areas (i.e., not render them). </p>

<h3 id="LineBoxWrapping"><span class="secno">3.2. </span>Line Box wrapping</h3>

<p>In general, the start edge of a line box touches the start edge of its
containing block and the end edge touches the end edge of its containing block.
However, floating boxes may come between the containing block edge and the line
box edge. Thus, although line boxes in the same inline formatting context
generally have the same inline-progression dimension (that of the
containing block), they may vary if available inline-progression space
is reduced due to <!-- a href="#floats"-->
floats<!--/a-->. Line boxes in the same inline formatting context may vary in
block-progression dimension.</p>
<p>When the total inline-progression dimension of the inline boxes on a line is less than the
inline-progression dimension of
the line box containing them, their inline-progression distribution within the line box
is determined by the
<a class="noxref" href="#text-align">
<span class="property">'text-align'</span></a> property. If that
property has the value 'justify', the user agent may stretch the inline boxes as
well. </p>
<p>When line-breaking opportunities are available within inline boxes and are
not pre-empted by the white-space properties, the inline boxes located at the
ending edge of the line box may be split into several boxes and these boxes
distributed across several line boxes. When an inline box is split, margins, borders, and padding have no visual
effect where the split occurs (or at any split, when they are several). Formatting of margins, borders, and padding may
not be fully defined if the split occurs within a bidirectional embedding. </p>
<p>Inline boxes may also be split into several boxes <em>within the same line
box</em> due to <a href="#direction">bidirectional text processing</a>. </p>
<div class="html-example">
  Here is an example of inline box construction. The following paragraph
  (created by the HTML block-level element P) contains anonymous text
  interspersed with the elements EM and STRONG:
  <pre>&lt;P&gt;Several &lt;EM&gt;emphasized words&lt;/EM&gt; appear
&lt;STRONG&gt;in this&lt;/STRONG&gt; sentence, dear.&lt;/P&gt;
</pre>
  <p>The P element generates a block box that contains five inline boxes, three
  of which are anonymous:</p>
  <ul>
    <li>Anonymous: &quot;Several&quot; </li>
    <li>EM: &quot;emphasized words&quot; </li>
    <li>Anonymous: &quot;appear&quot; </li>
    <li>STRONG: &quot;in this&quot; </li>
    <li>Anonymous: &quot;sentence, dear.&quot; </li>
  </ul>
  <p>To format the paragraph, the user agent flows the five boxes into line
  boxes. In this example, the box generated for the P element establishes the
  containing block for the line boxes. If the containing block is sufficiently
  wide (relative) , all the inline boxes will fit into a single line box: </p>
  <pre> Several <em>emphasized words</em> appear <strong>in this</strong> sentence, dear.
</pre>
  <p>If not, the inline boxes will be split up and distributed across several
  line boxes. The previous paragraph might be split as follows: </p>
  <pre>Several <em>emphasized words</em> appear
<strong>in this</strong> sentence, dear.
</pre>
  <p>or like this: </p>
  <pre>Several <em>emphasized</em>
<em>words</em> appear <strong>in this</strong>
sentence, dear.
</pre>
</div>
<p>In the previous example, the EM box was split into two EM boxes (call them
&quot;split1&quot; and &quot;split2&quot;). Margins, borders, padding, or text decorations have no
visible effect after split1 or before split2. </p>
<div class="html-example">
  Consider the following example:
  <pre>&lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 4.0//EN&quot;&gt;
&lt;HTML&gt;
  &lt;HEAD&gt;
    &lt;TITLE&gt;Example of inline flow on several lines&lt;/TITLE&gt;
    &lt;STYLE type=&quot;text/css&quot;&gt;
      EM {
        padding: 2px;
        margin: 1em;
        border-width: medium;
        border-style: dashed;
        line-height: 2.4em;
      }
    &lt;/STYLE&gt;
  &lt;/HEAD&gt;
  &lt;BODY&gt;
    &lt;P&gt;Several &lt;EM&gt;emphasized words&lt;/EM&gt; appear here.&lt;/P&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</pre>
  <p>Depending on the width of the P, the boxes may be distributed as follows:</p>
  <div class="figure">
    <img alt="Image illustrating the effect of line breaking on the
display of margins, borders, and padding." src="inline-layout.gif" width="260" height="99"><span class="dlink">&nbsp;&nbsp;&nbsp;<a title="Long description for illustration of margin/border
behavior on split inline box" href="http://www.w3.org/TR/REC-CSS2/images/longdesc/inline-layout-desc.html" name="img-inline-layout">[D]</a></span></div>
  <ul>
    <li>The margin is inserted before &quot;emphasized&quot; and after &quot;words&quot;. </li>
    <li>The padding is inserted before, above, and below &quot;emphasized&quot; and after,
    above, and below &quot;words&quot;. A dashed border is rendered on three sides in each
    case. </li>
  </ul>
</div>

<h3 id="inline1"><span class="secno">3.3. </span>Inline box progression dimension (height in horizontal flow), the
<span class="property"><a href="#text-height">'text-height'</a></span> property</h3>

<p>Each inline box has a block-progression dimension which is derived from the
following parameters: the 'text-height' and 'font-size' properties for
non-replaced elements, the height or the width for replaced elements (depending
on the text flow being horizontal or vertical respectively) and the stacked
block-progression dimension for inline-block elements (identical to what is done
for block-level elements).</p>
<p>The block-progression dimension determines the position of the padding,
border and margin for the element but does not necessarily determine the line
stacking progression as this is also affected by the 'line-box-contain'
property.</p>

<table class="propdef" id="name-61">
<tr>
<td><em>Name:</em>

<td><dfn id="text-height">text-height</dfn><tr>
<td><em>Value:</em>

<td>auto | font-size | text-size | max-size | &lt;number&gt;<tr>
<td><em>Initial:</em>

<td>auto<tr>
<td><em>Applies to:</em>

<td>inline elements and parents of element with display:ruby-text<tr>
<td><em>Inherited:</em>

<td>yes

<tr>
<td><em>Percentages:</em>

<td>N/A<tr>
<td><em>Media:</em>

<td><a class="noxref"
href="/TR/REC-CSS2/media.html#visual-media-group">visual</a>

<tr>
<td><em>Computed value:</em>

<td>specified value (except for initial and inherit)</table>

<p>The 'text-height' property determine the block-progression dimension of the
text content area of an inline box (non-replaced elements) . Possible values:</p>

<dl>
<dt><strong>auto</strong>

<dd>The block-progression dimension is based either on the em square determined
by the computed element font-size property value or the cell-height (ascender + descender)
related to the computed element font-size as chosen by the user agent.

<dt>font-size</dt>
<dd>The block-progression dimension is based on the em square as determined by
the computed element font-size. </dd>
<dt>text-size</dt>
<dd>The block-progression dimension is based on the cell-height (ascender +
descender) related to the computed element font-size.</dd>
<dt>max-size</dt>
<dd>The block-progression dimension is based on the maximum extents toward the
before-edge and after-edge of the box obtained by considering all children
elements located on the same line, ruby annotations (elements with 'display:ruby-text')
and baseline shifted elements.</dd>
<dt>&lt;number&gt;</dt>
<dd>The block progression dimension is based on &lt;number&gt; times the em square as
determined by the computed font-size.</dd>
</dl>

<p>When more than one font-size is used (this could happen when glyphs are found
in different fonts), it is recommended that the largest font-size provides the em square and the cell-height.</p>

<p>Although the 'text-height' property does not apply directly to block
elements, it applies to their anonymous children inline boxes (if any).</p>

<p>For replaced elements, the block-progression dimension is determined by the
outer edges (i.e. margin edge).</p>

<h3 id="InlineBoxHeight"><span class="secno">3.4. </span>Line height adjustment: the
<span class="property"><a href="#line-height">'line-height'</a></span> property</h3>

<p>The 'line-height' property controls the amount of leading space
which is added before and after the block-progression dimension of an
inline box (not including replaced inline boxes, but including the
root inline box) to determine the extended block-progression dimension
of the inline box.
The value of the 'line-box-contain' property determines how the
block-progression dimension and the extended block-progression dimension
affect the height of the line box.
The sum of the possible leading space and the block-progression dimension is called the <b>extended block-progression dimension</b>.
That extended value is not used for border, margin and padding placement but is used for vertical alignment (relative) and line box block-progression dimension.
</p>

<p>The <span class="index-def" title="leading"><em><a name="x15">leading</a></em></span><span title="leading">
for an inline non-replaced element
is defined as the d</span>ifference between the block-progression dimension as
determined by the 'text-height' property and the computed value
of
<a class="noxref" href="http://www.w3.org/TR/REC-CSS2/visudet.html#propdef-line-height">
<span class="propinst-line-height">'line-height'</span></a>. Half the leading is
called the <span class="index-def" title="half-leading"><em><a name="x16">
half-leading</a></em></span>, each half-leading is
located before and after the block-progression dimension of the element.
The 'line-height' value
will specify the <em>exact</em> extended block-progression dimension of each box
generated by the element.
(Depending on the value of 'line-box-contain', the extended block-progression dimension may be ignored.)
(For inline
<a href="http://www.w3.org/TR/REC-CSS2/conform.html#replaced-element">replaced</a>
elements, both the block-progression dimension and the extended
block-progression dimension of the box are given by the
outer edges (i.e. margin edge).)
Anonymous inline boxes use the 'text-height' and
'line-height' property values specified for their parent.</p>
<p>Depending on the
value of the 'line-stacking-strategy', the 'line-height' value may provide a
minimum or exact extended block-progression dimension for each generated inline box and
the line boxes. How the leading space is distributed between the before-edge and
the after-edge of inline elements within the line box depends on their
vertical-align property value.
<span class="issue">[LDB: I don't understand this paragraph.]</span>
</p>
<p>Empty inline elements generate empty inline boxes, but these boxes still have
a line height, and thus may influence the line box block-progression dimension,
as if the empty box contained an infinitely narrow letter.</p>
<p>A inline box which otherwise would influence the line box extended
progression dimension may be removed from the computation by using a special
value of the line-height property: 'none'.</p>

<table class="propdef
7B3D
" id="name-62">
<tr>
<td><em>Name:</em>

<td><dfn id="line-height">line-height</dfn><tr>
<td><em>Value:</em>

<td>normal |
        <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-number">
        <span class="value-inst-number">&lt;number&gt;</span></a> |
        <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-length">
        <span class="value-inst-length">&lt;length&gt;</span></a> |
        <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-percentage">
        <span class="value-inst-percentage">&lt;percentage&gt;</span></a> | none

<tr>
<td><em>Initial:</em>

<td>normal<tr>
<td><em>Applies to:</em>

<td>all elements<tr>
<td><em>Inherited:</em>

<td>yes

<tr>
<td><em>Percentages:</em>

<td>refer to the font size of the element itself <tr>
<td><em>Media:</em>

<td><a class="noxref"
href="/TR/REC-CSS2/media.html#visual-media-group">visual</a>

<tr>
<td><em>Computed value:</em>

<td>see prose</table>

<p>Values for this property have the following meanings:</p>
<dl>
  <dt><strong>normal</strong> </dt>
  <dd>Tells user agents to set the
  <a href="http://www.w3.org/TR/REC-CSS2/cascade.html#computed-value">computed
  value</a> to a &quot;reasonable&quot; value based on the font size of the element. The
  value has the same meaning as <span class="index-inst" title="&lt;number&gt;">
  <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-number" name="x18">
  <span class="value-inst-number">&lt;number&gt;</span></a></span>. We recommend a
  computed value for 'normal' between 1.0 to 1.2. The user agent may allow the
  &lt;number&gt; to vary depending on the metrics of the font(s) being used.</dd>
  <dt><span class="index-inst" title="&lt;length&gt;">
  <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-length" name="x19">
  <span class="value-inst-length"><strong>&lt;length&gt;</strong></span></a></span>
  </dt>
  <dd>The box height is set to this length. Negative values are illegal. The
  length is the computed value.</dd>
  <dt><span class="index-inst" title="&lt;number&gt;">
  <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-number" name="x20">
  <span class="value-inst-number"><strong>&lt;number&gt;</strong></span></a></span>
  </dt>
  <dd>The <a href="http://www.w3.org/TR/REC-CSS2/cascade.html#computed-value">
  computed value</a> of the property is this number multiplied by the element's
  font size. Negative values are illegal. However, the number, not the
  <a href="http://www.w3.org/TR/REC-CSS2/cascade.html#computed-value">computed
  value</a>, is inherited. </dd>
  <dt><span class="index-inst" title="&lt;percentage&gt;">
  <a class="noxref" href="http://www.w3.org/TR/REC-CSS2/syndata.html#value-def-percentage" name="x21">
  <span class="value-inst-percentage"><strong>&lt;percentage&gt;</strong></span></a></span>
  </dt>
  <dd>The <a href="http://www.w3.org/TR/REC-CSS2/cascade.html#computed-value">
  computed value</a> of the property is this percentage multiplied by the
  element's computed font size. Negative values are illegal.</dd>
  <dt>none</dt>
  <dd>For inline-level elements, the element does not influence the <i>extended
  block-progression dimension</i> of the line. The computed value is the
  specified value ('none').  For block-level elements, equivalent to 'normal'
  (and the computed value is 'normal').
  <span class="issue">This definition should be briefer, and should
  refer to the next section.</span></dd>
</dl>
<div class="example">
  <p style="DISPLAY: none">Example(s):</p>
  <p>The three rules in the example below have the same resultant line height:
  </p>
  <pre>DIV { line-height: 1.2; font-size: 10pt }     /* number */
DIV { line-height: 1.2em; font-size: 10pt }   /* length */
DIV { line-height: 120%; font-size: 10pt }    /* percentage */
</pre>
</div>
<p>When an element contains text that is rendered in more than one font, user
agents should determine the
<a class="noxref" href="http://www.w3.org/TR/REC-CSS2/visudet.html#propdef-line-height">
<span class="propinst-line-height">'line-height'</span></a> value according to
the largest font size (when appropriate). </p>
<p>Note that replaced elements have a
<a class="noxref" href="http://www.w3.org/TR/REC-CSS2/fonts.html#propdef-font-size">
<span class="propinst-font-size">'font-size'</span></a> and a
<a class="noxref" href="http://www.w3.org/TR/REC-CSS2/visudet.html#propdef-line-height">
<span class="propinst-line-height">'line-height'</span></a> property, even if
they are not used directly to determine the extended block-progression dimension of the box: 'em' and 'ex'
values are relative to values of 'font-size' and percentage values for
<a class="noxref" href="http://www.w3.org/TR/REC-CSS2/visudet.html#propdef-vertical-align">
<span class="propinst-vertical-align">'vertical-align'</span></a> are relative
to values of the extended block-progression dimension.  <span class="issue">[LDB: The second part of this sentence is a significant change from CSS2, and also doesn't mauke senes in the context of the sentence.]</span></p>

<p>When the
<a class="noxref" href="http://www.w3.org/TR/REC-CSS2/visudet.html#propdef-line-height">
<span class="propinst-line-height">'line-height'</span></a> value is less than
the font size, the final inline box extended block-progression dimension will be less than the font size and
the rendered glyphs will &quot;bleed&quot; outside the box. If such a box touches the edge
of a line box, the rendered glyphs will also &quot;bleed&quot; into the adjacent line box.</p>

<h3 id="LineStacking"><span class="secno">3.5. </span>Line Stacking: the
 <span class="property"><a href="#line-box-contain">'line-box-contain'</a></span> property</h3>


<table class="propdef" id="name-30">
<tbody>
<tr>
<td><em>Name:</em>

<td><dfn id="line-box-contain">line-box-contain</dfn>

<tr>
<td><em>Value:</em>

<td>[ block || inline || font || text || replaced || inline-box ] | none |
inherit | initial

<tr>
<td><em>Initial:</em>

<td>block inline replaced

<tr>
<td><em>Applies to:</em>

<td>block-level elements

<tr>
<td><em>Inherited:</em>

<td>yes

<tr>
<td><em>Percentages:</em>

<td>N/A

<tr>
<td><em>Media:</em>

<td>visual

<tr>
<td><em>Computed value:</em>

<td>specified value (except for 'inherit' and 'initial')
</table>

<p>This property enumerates which aspects of the elements in a line box
contribute to the height height of that line box.

<dl>
<dt>block

<dd>The extended block progression dimension of the root inline box
must fit within the line box.

<dt>inline

<dd>The extended block progression dimension of all non-replaced
inline boxes whose 'line-height' is not 'none' in the line box must fit within the line box.

<dt>font

<dd>The block progression dimension of all non-replaced inline boxes in
the line whose 'line-height' is not 'none' that directly (i.e., within
the box but not within one of its descendants) contain non-removed text
must fit within the line box, where non-removed text is any characters
not removed based on the values of the 'linefeed-treatment',
'white-space-treatment', and 'all-space-treatment' properties.  <span
class="issue">[LDB: This isn't quite right.  What about zwnj,
etc.?</span>

<dt>replaced

<dd>The margin box of all <span title="replaced element"><a
href="#replaced">replaced elements</a></span> within the line
must fit within the line box.

<dt>inline-box

<dd>The margin-box of all non-replaced inline elements in the line whose
'line-height' is not 'none' must fit within the line box.  <span
class="issue">Should this split into 'inline-border' and
'inline-margin'?</span>

</dl>

<p>If the property has no value for all elements within a line, then the line
box has 0 height, and the line within the anonymous inline established by the
block that coincides with the line block is the baseline of that anonymous
inline block (or should it be determined by the 'vertical-align' property of
the block?). <span class="issue">[This is arbitrary. Does anyone have better
ideas?]</span>

<p class="note">Note that the CSS2 model is equivalent to 'block inline
replaced' but the backwards-compatible HTML model is similar to (but not
exactly) 'font replaced' [1].

<p class="issue">Concerns: * How does this work with the cascade? * What
about determining the visual size of inline boxes?

<p class="issue">[1] I believe the differences are restricted to the first
line of LI elements, the last line of LI, DD, and DT elements, and issues
concerning whitespace around images. [DB]

<p>The height of each line box is established as follows (we describe the
case for <em><a href="#horizontal">horizontal flow</a></em>, but vertical
flow is analogous). First align all the boxes on the line relative to each
other according to the rules for <span
class="property">'vertical-align'</span>. The line box must satisfy all of
the following constraints:

<ol>
<li>For each box on the line that has <span class="css">'font'</span> in its
value of <span class="property"><a
href="#line-box-contain">'line-box-contain'</a></span>, the line box must be
high enough to contain the <span title="top of the text"><a
href="#top-of">top and bottom of that box's text</a></span>.

<li>For each box that has <span class="css"><a
href="#inline">'inline'</a></span> in <span class="property"><a
href="#line-box-contain">'line-box-contain'</a></span>, the top of the line
box must be at least as high as the <span title="top of the text"><a
href="#top-of">top of the box's text</a></span> plus the box's <em><a
href="#half-leading">half-leading</a></em>. The bottom of the line box must
be at least as low as the bottom of the text plus the half-leading.

<li>For each box that has <span class="css">'inline-box'</span> in <span
class="property"><a href="#line-box-contain">'line-box-contain'</a></span>,
the top of the line box must be at least as high as the margin-top of this
box. The bottom of the line box must be at least as low as the margin-bottom.

<li>For each <em>replaced box</em> that has <span
class="css">'replaced'</span> in its <span class="property"><a
href="#line-box-contain">'line-box-contain'</a></span>, the line box must
contain the margin-top and margin-bottom of this box. <span
class="issue">[The value 'replaced' is not strictly needed. 'Inline-box' also
does the job, except that it is not automatically restricted to replaced
elements.]</span>

<li>For each box that has <span class="css">'text'</span> in <span
class="property"><a href="#line-box-contain">'line-box-contain'</a></span>,
the top of the line box must be at least as high as the top of each glyph in
the box (excluding those in child elements). The bottom of the line box must
be at least as low as the bottom of each glyph in the box (excluding child
elements).

<li>If the enclosing block for this line has <span class="css"><a
href="#block">'block'</a></span> in <span class="property"><a
href="#line-box-contain">'line-box-contain'</a></span>, then the top of the
line box must be at least as high as the text top plus the half-leading of an
<em><a href="#anonymous">anonymous inline element</a></em> and the bottom of
the line box as low as the text bottom plus the half leading. This must hold
whether or not this line actually contains such an element.
</ol>

<p class="note">Thus 'block' can be used to set an overall minimum line
height (viz. the value of 'line-height' of the element itself) for all lines
in an element, independent of the actual contents of each line. In
particular, setting 'line-box-contain' to just 'block' and no other values
will ensure that all lines are the same height, at the possible risk of some
tall inline elements overlapping with lines above or below.

<p>The <dfn id="half-leading">half-leading</dfn> of a box is defined as half
the computed value of 'line-height' minus half the computed value of
'font-size', i.e., (line-height - font-size)/2.

<p>The <dfn id="top-of">top of the text</dfn> is the top of the em-box of a
box's nominal font, whether or not there actually is any letter that tall.
Replaced elements, for example, have no text, but still have a nominal font
and are thus a text top. The rules above refer to the position of the text
top after the box has been aligned with 'vertical-align'.


<h4>Old, remove this:</h4>

<p>Line stacking is the mechanism by which a line box is determined for each
line in a block and then these lines are stacked in the block-progression
direction resolving any spacing constraints between adjacent lines. The
line-stacking strategy covers both the determination of the block-progression
dimension (height in horizontal flow) of a line box and the rules for spacing
line boxes.</p>
<p>All line stacking strategies make use of the properties: 'font-size',
'text-height' and 'line-height'.</p>
<p>The line stacking properties are made of the following properties:</p>
<ol>
  <li>The <span class="property"><a href="#line-stacking-strategy">'line-stacking-strategy'</a></span> property determines the overall stacking
  method,</li>
  <li>The <span class="property"><a href="#line-stacking-ruby">'line-stacking-ruby</a>'</span> property determines how elements with
  'display=ruby' are stacked,</li>
  <li>The <span class="property"><a href="#line-stacking-shift">'line-stacking-shift</a>'</span> property determines how elements which are
  baseline-shifted are stacked,</li>
  <li>Finally the <span class="property"><a href="#line-stacking">'line-stacking</a>'</span> property is a short-hand for the previous
  properties.</li>
</ol>
<table class="propdef" id="name-162">
<tr>
<td><em>Name:</em>

<td><dfn id="line-stacking-strategy">line-stacking-strategy</dfn>

<tr>
<td><em>Value:</em>

<td>inline-line-height | block-line-height | max-height | grid-height

<tr>
<td><em>Initial:</em>

<td>inline-line-height<tr>
<td><em>Applies to:</em>

<td>block-level elements

<tr>
<td><em>Inherited:</em>

<td>yes

<tr>
<td><em>Percentages:</em>

<td>N/A<tr>
<td><em>Media:</em>

<td><a class="noxref"
href="/TR/REC-CSS2/media.html#visual-media-group">visual</a>

<tr>
<td><em>Computed value:</em>

<td>specified value (except for initial and inherit)</table>

<p>This property determines the line stacking strategy for stacked line boxes
within a containing block element. The term <b>stack-height</b> is used in the
context of this property description to indicate the block-progression dimension
for the line boxes. Possible values:<dl>
<dt><strong>inline-line-height</strong>

<dd>The <i id="stack-height">stack-height</i> is the smallest value that contains the <i>extended
block-progression dimension</i> of all the inline elements on that line when those
elements are properly aligned. Since the line spacing information is already
included in the computation of the line box, these line boxes are simply stacked
adjacent to one another in the formatted block to which they belong. The
'line-height' property value is taken into account for both the inline elements
and the block elements. For inline elements, it defines the extended
block-progression dimensio
871
n. For block elements, it defines the <i>minimum
extended block-progression dimension</i>.<dt>block-line-height</dt>
<dd>The <i>stack-height</i> is determined by the block element computed 'line-height' property
value. The 'line-height' property value is ignored for inline elements. For
alignment purpose, this case is similar to the <i>minimum extended
block-progression dimension</i> case (strut model). This is the only line-stacking
strategy that may cause inline boxes within the line to bleed before and after
the line b
5F4C
ox because the line box is not constrained by its inline boxes. If the
block computed 'line-height' is 'none', the stack-height is determined as if
'line-height: normal'.</dd>
<dt>max-height</dt>
<dd>The <i>stack-height</i> is the smallest value that contains the
block-progression dimension of all the inline elements on that line when those
elements are properly aligned. The 'line-height' property value is taken into
account only for the block elements and it defines the <i>minimum
extended block-progression dimension</i>.</dd>
<dt>grid-height</dt>
<dd>The <i>stack-height</i> is the smallest multiple of the block element 'line-height'
computed value that can contain the block-progression of all the inline
elements on that line when those elements are properly aligned. The
'line-height' property value is ignored for inline elements. If the block
computed 'line-height' is 'none', the stack-height is determined as if
'line-height: normal'.</dd>
</dl>

<p>When the line-stack strategy dictates that the inline element line-height be
ignored, this means that for those elements only their block-progression
dimensions are considered for the <i>stack-height</i>, not their extended
block-progression dimensions.</p>

<p class="note"><b>Note.</b> XSL has a similar property with the same name which
use different but equivalent values: 'line-height' instead of
'inline-line-height', 'font-height' instead of 'block-line-height'. It also uses
'max-height'. The value 'grid-height' is new to the CSS3 property.<p>&nbsp;<table class="propdef" id="name-60">
<tr>
<td><em>Name:</em>

<td><dfn id="line-stacking-ruby">line-stacking-ruby</dfn><tr>
<td><em>Value:</em>

<td>exclude-ruby | include-ruby

<tr>
<td><em>Initial:</em>

<td>exclude-ruby<tr>
<td><em>Applies to:</em>

<td>block-level elements

<tr>
<td><em>Inherited:</em>