csswg-drafts/css3-linebox/Overview.html at a83a662c13c15d30a256a349c5ad5d58a1e6a861 · 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>CSS Line Layout Module Level 3</title>
  <!--
      FIXME when publishing: copy the current default.css and link to
      "default.css" rather than "../default.css"
   -->
  <link href="../default.css" rel=stylesheet type="text/css">
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">

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    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-ED.css" rel=stylesheet
  type="text/css">

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

   <h1>CSS Line Layout Module Level 3</h1>

   <h2 class="no-num no-toc" id=editors>Editor's Draft 2 March 2012</h2>

   <dl id=authors>
    <dt>This version:

    <dd><a
     href="http://dev.w3.org/csswg/css3-linebox/">http://dev.w3.org/csswg/css3-linebox/</a>
     <!-- <dd><a href="http://www.w3.org/TR/2012/ED-css3-linebox-20120302/">http://www.w3.org/TR/2012/ED-css3-linebox-20120302/</a> -->


    <dt>Latest version:

    <dd><a
     href="http://www.w3.org/TR/css3-linebox/">http://www.w3.org/TR/css3-linebox/</a>

    <dt>Latest editor's draft:

    <dd><a
     href="http://dev.w3.org/csswg/css3-linebox/">http://dev.w3.org/csswg/css3-linebox/</a>

    <dt>Previous version:

    <dd><a
     href="http://www.w3.org/TR/2002/WD-css3-linebox-20020515/">http://www.w3.org/TR/2002/WD-css3-linebox-20020515/</a>

    <dt>Issues List:

    <dd><a
     href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Linebox&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED">CSS3
     Line Layout issues in Bugzilla</a>

    <dt>Discussion:

    <dd><a
     href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
     with subject line “<kbd>[css3-linebox] topic</kbd>”

    <dt>Editors:

    <dd><a href="mailto:jdaggett@mozilla.com">John Daggett (Mozilla)</a>

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

    <dt>Previous Editors:

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

    <dd>Michel Suignard (Microsoft)

    <dd><a href="http://www.meyerweb.com/eric/">Eric A. Meyer</a>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> © 2012 <a href="http://www.w3.org/"><acronym
    title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a
    href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
    of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><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>
    and <a
    href="http://www.w3.org/Consortium/Legal/copyright-documents">document
    use</a> rules apply.</p>
   <!--end-copyright-->
   <hr title="Separator for header">
  </div>

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

  <p>The CSS formatting model provides for a flow of elements and text inside
   of a container to be wrapped into lines. The formatting of elements and
   text within a line, its positioning in the inline progression direction,
   and the breaking of lines are described in <a href="#CSS3TEXT"
   rel=biblioentry>[CSS3TEXT]<!--{{CSS3TEXT}}--></a>. This module describes
   the positioning in the block progression direction both of elements and
   text within lines and of the lines themselves. This positioning is often
   relative to a baseline. It also describes special features for formatting
   of first lines and drop caps. It extends on the model in <a href="#CSS2"
   rel=biblioentry>[CSS2]<!--{{CSS2}}--></a>.

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

  <p>This is a public copy of the editors' draft. It is provided for
   discussion only and may change at any moment. Its publication here does
   not imply endorsement of its contents by W3C. Don't cite this document
   other than as work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss3-linebox%5D%20PUT%20SUBJECT%20HERE">
   www-style@w3.org</a> (see <a
   href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
   discussion of this specification. When sending e-mail, please put the text
   “css3-linebox” in the subject, preferably like this:
   “[<!---->css3-linebox<!---->] <em>…summary of comment…</em>”

  <p>This document was produced by the <a href="/Style/CSS/members">CSS
   Working Group</a> (part of the <a href="/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
   Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
   W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>This document may be available in translations in the future. The
   English version of this specification is the only normative version.

  <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="#conformance"><span class=secno>2 </span>Conformance</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>Inline-progression: Line Box wrapping</a>

     <li><a href="#inline1"><span class=secno>3.3 </span>Block-progression
      dimensions: the <span class=property>‘<code
      class=property>text-height</code>’</span> property</a>

     <li><a href="#LineBoxHeight"><span class=secno>3.4 </span>Block
      progression positioning and spacing</a>
      <ul class=toc>
       <li><a href="#InlineBoxHeight"><span class=secno>3.4.1 </span>Line
        height adjustment: the <span class=property>‘<code
        class=property>line-height</code>’</span> property</a>

       <li><a href="#LineStacking"><span class=secno>3.4.2 </span>Line
        Stacking: the <span class=property>‘<code
        class=property>line-box-contain</code>’</span> property</a>
      </ul>
    </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>‘<code
      class=property>dominant-baseline</code>’</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>‘<code
      class=property>alignment-baseline</code>’</span> property</a>

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

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

     <li><a href="#vertical-align-prop"><span class=secno>4.8 </span>Vertical
      alignment: the <span class=property>‘<code
      class=property>vertical-align</code>’</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>‘<code
      class=property>inline-box-align</code>’</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>‘<code
      class=property>drop-initial-size</code>’</span> and the <span
      class=property>‘<code
      class=property>drop-initial-value</code>’</span> properties</a>

     <li><a href="#DropInitialAlign"><span class=secno>5.4 </span>Aligning
      drop initial: the <span class=property>‘<code
      class=property>drop-initial-before-align</code>’</span>, <span
      class=property>‘<code
      class=property>drop-initial-before-adjust</code>’</span>, <span
      class=property>‘<code
      class=property>drop-initial-after-align</code>’</span> and <span
      class=property>‘<code
      class=property>drop-initial-after-adjust</code>’</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 class=no-num><a href="#BaselineUsage">Appendix A: Usage of baseline
    alignment (informative)</a>

   <li class=no-num><a href="#ack">Acknowledgments</a>

   <li class=no-num><a href="#ref">References</a>
    <ul class=toc>
     <li class=no-num><a href="#normative">Normative references</a>

     <li class=no-num><a href="#other">Other references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>
  </ul>
  <!--end-toc-->

  <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 <a href="#CSS3-FONTS"
    rel=biblioentry>[CSS3-FONTS]<!--{{!CSS3-FONTS}}--></a>

   <li>Text <a href="#CSS3TEXT"
    rel=biblioentry>[CSS3TEXT]<!--{{!CSS3TEXT}}--></a>

   <li>Syntax and grammar <a href="#CSS3SYN"
    rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a>

   <li>Values and units <a href="#CSS3VAL"
    rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a>

   <li>Cascade <a href="#CSS3CASCADE"
    rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a>
  </ul>

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

  <ul>
   <li>Multi-column layout <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>

   <li>Ruby <a href="#CSS3RUBY"
    rel=biblioentry>[CSS3RUBY]<!--{{CSS3RUBY}}--></a>
  </ul>

  <h2 id=conformance><span class=secno>2 </span>Conformance</h2>

  <p class=issue>[Add some real conformance text here.]

  <p>All described properties, in addition to the noted values, take ‘<code
   class=property>initial</code>’ and ‘<code
   class=property>inherit</code>’. These values are not repeated in each of
   the property value enumeration.

  <p>This module uses extensively the ‘<code
   class=property>before</code>’, ‘<code class=property>after</code>’,
   ‘<code class=property>start</code>’ and ‘<code
   class=property>end</code>’ notation to specify the four edges of a box
   relative to its text advance direction, independently of its absolute
   positioning in terms of ‘<code class=property>top</code>’, ‘<code
   class=property>bottom</code>’, ‘<code class=property>left</code>’
   and ‘<code class=property>right</code>’ (corresponding respectively to
   the ‘<code class=property>before</code>’, ‘<code
   class=property>after</code>’, ‘<code class=property>start</code>’
   and ‘<code class=property>end</code>’ positions in a typical Western
   text layout). This notation is also used extensively in <a href="#XSL10"
   rel=biblioentry>[XSL10]<!--{{XSL10}}--></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]<!--{{!RFC2119}}--></a>. For legibility these
   keywords are used in lowercase form.

  <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.

  <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 <a class=index-def href="#line-box0" id=x
   title="line box"> </a><a id=line-box><dfn id=line-box0>line box</dfn></a>.

  <p>The inline-progression dimension (‘<code
   class=property>width</code>’ in horizontal flow) of a line box is
   determined by its containing block. The block-progression dimension
   (‘<code class=property>height</code>’ 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 <a href="#baseline">baseline alignment</a>
   properties.

  <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>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>‘<code class=property><a
   href="#line-box-contain">line-box-contain</a></code>’</span> property.

  <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>‘<code class=property><a
   href="#line-height">line-height</a></code>’</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>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).

  <h3 id=LineBoxWrapping><span class=secno>3.2 </span>Inline-progression:
   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>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 <span class=property>‘<code class=property><a
   href="#text-align">text-align</a></code>’</span> property. If that
   property has the value ‘<code class=property>justify</code>’, the user
   agent may stretch the inline boxes as well.

  <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>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>.

  <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>EM: &quot;emphasized words&quot;

    <li>Anonymous: &quot;appear&quot;

    <li>STRONG: &quot;in this&quot;

    <li>Anonymous: &quot;sentence, dear.&quot;
   </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.

  <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." height=99
    src=inline-layout.gif width=260><span class=dlink>   <a
    href="http://www.w3.org/TR/REC-CSS2/images/longdesc/inline-layout-desc.html"
    name=img-inline-layout title="Long description for illustration of
    margin/border behavior on split inline box">[D]</a></span></div>

   <ul>
    <li>The margin is inserted before &quot;emphasized&quot; and after
     &quot;words&quot;.

    <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.
   </ul>
  </div>

  <h3 id=inline1><span class=secno>3.3 </span>Block-progression dimensions:
   the <span class=property>‘<code class=property><a
   href="#text-height">text-height</a></code>’</span> property</h3>

  <p>Each inline box has a block-progression dimension which is derived from
   the following parameters: the ‘<code class=property><a
   href="#text-height">text-height</a></code>’ and ‘<code
   class=property><a href="#font-size">font-size</a></code>’ 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>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 ‘<code
   class=property><a href="#line-box-contain">line-box-contain</a></code>’
   property.

  <table class=propdef id=nametext-height>
   <tbody>
    <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>visual

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

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

  <p>The ‘<code class=property><a
   href="#text-height">text-height</a></code>’ property determine the
   block-progression dimension of the text content area of an inline box
   (non-replaced elements) . Possible values:

  <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

   <dd>The block-progression dimension is based on the em square as
    determined by the computed element font-size.

   <dt>text-size

   <dd>The block-progression dimension is based on the cell-height (ascender
    + descender) related to the computed element font-size.

   <dt>max-size

   <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 ‘<code class=css>display:ruby-text</code>’) and baseline shifted
    elements.

   <dt>&lt;number&gt;

   <dd>The block progression dimension is based on &lt;number&gt; times the
    em square as determined by the computed font-size.
  </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>Although the ‘<code class=property><a
   href="#text-height">text-height</a></code>’ property does not apply
   directly to block elements, it applies to their anonymous children inline
   boxes (if any).

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

  <h3 id=LineBoxHeight><span class=secno>3.4 </span>Block progression
   positioning and spacing</h3>

  <h4 id=InlineBoxHeight><span class=secno>3.4.1 </span>Line height
   adjustment: the <span class=property>‘<code class=property><a
   href="#line-height">line-height</a></code>’</span> property</h4>

  <p>The ‘<code class=property><a
   href="#line-height">line-height</a></code>’ 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 ‘<code class=property><a
   href="#line-box-contain">line-box-contain</a></code>’ 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>The <span class=index-def id=leading 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 ‘<code
   class=property><a href="#text-height">text-height</a></code>’ property
   and the computed value of <span class=property>‘<code class=property><a
   href="#line-height">line-height</a></code>’</span>. Half the leading is
   called the <a class=index-def href="#half-leading" id=half-leading0
   title=half-leading><em><a name=x16> half-leading</a></em></a>, each
   half-leading is located before and after the block-progression dimension
   of the element. The ‘<code class=property><a
   href="#line-height">line-height</a></code>’ value will specify the
   <em>exact</em> extended block-progression dimension of each box generated
   by the element. (Depending on the value of ‘<code class=property><a
   href="#line-box-contain">line-box-contain</a></code>’, 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 ‘<code class=property><a
   href="#text-height">text-height</a></code>’ and ‘<code
   class=property><a href="#line-height">line-height</a></code>’ property
   values specified for their parent.

  <p>Depending on the value of the ‘<code class=property><a
   href="#line-stacking-strategy">line-stacking-strategy</a></code>’, the
   ‘<code class=property><a href="#line-height">line-height</a></code>’
   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>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>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: ‘<code
   class=property>none</code>’.

  <table class=propdef id=name-62>
   <tbody>
    <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:

  <dl>
   <dt><strong>normal</strong>

   <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
    id=ltnumbergt 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 ‘<code
    class=property>normal</code>’ 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.

   <dt><span class=index-inst id=ltlengthgt0 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>


   <dd>The box height is set to this length. Negative values are illegal. The
    length is the computed value.

   <dt><span class=index-inst id=ltnumbergt0 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>


   <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.

   <dt><span class=index-inst id=ltpercentagegt0 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>


   <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.

   <dt>none

   <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 (‘<code class=property>none</code>’).
    For block-level elements, equivalent to ‘<code
    class=property>normal</code>’ (and the computed value is ‘<code
    class=property>normal</code>’). <span class=issue>This definition
    should be briefer, and should refer to the next section.</span>
  </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 <span class=property>‘<code
   class=property><a href="#line-height">line-height</a></code>’</span>
   value according to the largest font size (when appropriate).

  <p>Note that replaced elements have a <span class=property>‘<code
   class=property><a href="#font-size">font-size</a></code>’</span> and a
   <span class=property>‘<code class=property><a
   href="#line-height">line-height</a></code>’</span> property, even if
   they are not used directly to determine the extended block-progression
   dimension of the box: ‘<code class=property>em</code>’ and ‘<code
   class=property>ex</code>’ values are relative to values of ‘<code
   class=property><a href="#font-size">font-size</a></code>’ and percentage
   values for <span class=property>‘<code class=property><a
   href="#vertical-align">vertical-align</a></code>’</span> 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>When the <span class=property>‘<code class=property><a
   href="#line-height">line-height</a></code>’</span> 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.

  <h4 id=LineStacking><span class=secno>3.4.2 </span>Line Stacking: the <span
   class=property>‘<code class=property><a
   href="#line-box-contain">line-box-contain</a></code>’</span> property</h4>

  <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 || glyphs || 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 ‘<code
      class=property>inherit</code>’ and ‘<code
      class=property>initial</code>’)
  </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 ‘<code class=property><a
    href="#line-height">line-height</a></code>’ is not ‘<code
    class=property>none</code>’ 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 ‘<code class=property><a
    href="#line-height">line-height</a></code>’ is not ‘<code
    class=property>none</code>’ 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 ‘<code
    class=property>linefeed-treatment</code>’, ‘<code
    class=property>white-space-treatment</code>’, and ‘<code
    class=property>all-space-treatment</code>’ properties. <span
    class=issue>[LDB: This isn't quite right. What about zwnj, etc.?</span>

   <dt>glyphs

   <dd>The block progression dimension of all the glyph bounding boxes of
    glyphs in the line box must fit within the line box.

   <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
    ‘<code class=property><a href="#line-height">line-height</a></code>’
    is not ‘<code class=property>none</code>’ must fit within the line
    box. <span class=issue>Should this split into ‘<code
    class=property>inline-border</code>’ and ‘<code
    class=property>inline-margin</code>’?</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
   ‘<code class=property><a
   href="#vertical-align">vertical-align</a></code>’ 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 ‘<code
   class=css>block inline replaced</code>’ but the backwards-compatible
   HTML model is similar to (but not exactly) ‘<code class=css>font