csswg-drafts/css-box/Overview.html at 2d6bf178df121110562787ebdc686e5b5211b746 · w3c/csswg-drafts · GitHub

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">

<html>
 <head
  profile="http://dublincore.org/documents/2008/08/04/dc-html/  http://microformats.org/profile/h-card http://microformats.org/profile/hcard http://microformats.org/profile/rel-license "><!-- Note: the h-card profile is for a "microformats 2" vocabulary and
is experimental as of September 2012 -->

  <meta content="text/html; charset=utf-8" http-equiv=content-type>
  <title>CSS basic box model</title>

  <link href="http://purl.org/dc/terms/" rel=schema.dcterms>
  <link href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
   rel=dcterms.rights>
  <meta content="CSS basic box model" name=dcterms.title>
  <meta content=text name=dcterms.type>
  <meta content=2013-10-12 name=dcterms.date>
  <meta content="Bert Bos" name=dcterms.creator>
  <meta content="Anton Prowse" name=dcterms.creator>
  <meta content=W3C name=dcterms.publisher>
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    src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->

   <h1>CSS basic box model</h1>

   <h2 class="no-num no-toc">Editor's Draft 12 October 2013</h2>

   <dl>
    <dt>This version:

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

    <dt>Latest version:

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

    <dt>Previous version:

    <dd><a
     href="http://www.w3.org/TR/2002/WD-css3-box-20021024">http://www.w3.org/TR/2002/WD-css3-box-20021024</a>

    <dt>Feedback:

    <dd><a
     href="mailto:www-style@w3.org?subject=%5Bcss3-box%5D%20feedback">www-style@w3.org</a>
     with subject line “[css3-box] <var>… message topic …</var>” (<a
     href="http://lists.w3.org/Archives/Public/www-style/"
     rel=discussion>archives</a>)

    <dt>Editors:

    <dd class="h-card vcard"><a class="p-name fn u-url url"
     href="http://www.w3.org/People/Bos/" rel=author>Bert Bos</a>, <span
     class="p-org org">W3C</span>, <a class="u-email email"
     href="mailto:bert@w3.org">&lt;bert@w3.org&gt;</a>

    <dd class="h-card vcard"><span class="p-name fn">Anton Prowse</span>,
     <span class="p-org org">Invited Expert</span>, <a class="u-email email"
     href="mailto:prowse@moonhenge.net">&lt;prowse@moonhenge.net&gt;</a>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> © 2013 <a href="http://www.w3.org/"><abbr
    title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a
    href="http://www.csail.mit.edu/"><abbr
    title="Massachusetts Institute of Technology">MIT</abbr></a>, <a
    href="http://www.ercim.eu/"><abbr
    title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
    <a href="http://www.keio.ac.jp/">Keio</a>, <a
    href="http://ev.buaa.edu.cn/">Beihang</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">Abstract</h2>

  <p><abbr title="Cascading Style Sheets">CSS</abbr> describes the rendering
   of documents on various media. When textual documents (e.g., HTML) are
   laid out on visual media (e.g., screen or print), CSS models their layout
   as a collection of rectangular boxes containing words, lines, paragraphs,
   tables, etc., each with properties such as size, color and font.

  <p>This module describes the basic types of boxes: block, list item,
   inline, etc.; and some of their properties, including margins, padding and
   width/height. It defines a layout called a “flow,” in which a series
   of boxes is laid out one after the other, and it defines “floating”
   boxes. Other kinds of layout, such as tables, absolute positioning, ruby
   annotations, grid layouts, columns and page boxes, are described by other
   modules. Also, the layout of text inside a line (including the handling of
   left-to-right and right-to-left scripts) is defined elsewhere.

  <p>In CSS level 3, boxes may contain either horizontal or vertical text.
   Different orientations can be mixed.

  <h2 class="no-toc no-num">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-box%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-box” in the subject, preferably like this:
   “[<!---->css3-box<!---->] <em>…summary of comment…</em>”

  <p>This document was produced by the <a
   href="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
   the <a href="http://www.w3.org/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
   2004 W3C Patent Policy</a>. W3C maintains a <a
   href="http://www.w3.org/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="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
   6 of the W3C Patent Policy</a>.</p>
  <!--end-status-->

  <p>The CSS Working Group plans to ask for W3C Recommendation status for
   this specification only when there are sufficiently many implementations.
   See the section <a href="#exit">“CR exit criteria” </a> for details.

  <p class=mtb>The terminology and the order of the sections is currently in
   flux. This working draft is published to show the features that are being
   discussed in the context of this module, but it is likely that some of the
   names of the features will change.

  <p>This module should eventually replace corresponding parts of the revised
   CSS level 2 specification <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. But this is an early draft
   and any differences to level 2 are most likely unintentional, unless they
   concern new features, such as vertical text or float displacement.
   <em>Please help us improve the next draft by pointing out such
   differences.</em>

  <p>Some issues that affect multiple sections in this document:

  <div class=issue>
   <p>Horizontal centering (more precisely: centering in the inline
    direction). The draft contains several suggestions:

   <ol>
    <li>Setting ‘<a href="#margin"><code
     class=property>margin</code></a>’ to ‘<a href="#fill"><code
     class=css>fill</code></a>’. ‘<a href="#fill"><code
     class=css>Fill</code></a>’ is similar to ‘<code
     class=css>auto</code>’, but allowed to be negative:
     <pre>P {margin: 1em fill}</pre>

    <li>Setting a property ‘<a href="#alignment"><code
     class=property>alignment</code></a>’ on the element to center:
     <pre>P {alignment: center}</pre>

    <li>Setting a property ‘<a href="#child-align"><code
     class=property>child-align</code></a>’ on the parent of the element to
     center:
     <pre>DIV {child-align: middle}</pre>

    <li>Rely on grid templates <a href="#ref-CSS3LAYOUT"
     rel=biblioentry>[CSS3LAYOUT]<!--{{CSS3LAYOUT}}--></a>. You set a
     template on the parent or on the element itself. One or the other may be
     easier. E.g., to horizontally center the single child of a <a
     href="#horizontal">horizontal</a> DIV):
     <pre>DIV {grid: ". @ ." }</pre>

     <p>To horizontally center a P with a template on the P itself requires
      that the width of the P isn't set with the ‘<a href="#width"><code
      class=property>width</code></a>’ property, but with the ‘<code
      class=property>grid-columns</code>’ property (or the ‘<code
      class=property>grid</code>’ shorthand). In the example below the
      width is set to ‘<code class=property>fit-content</code>’:

     <pre>P {grid: * fit-content *  ".  @ ."}</pre>

     <p>To vertically center the single child of a <a
      href="#vertical.">vertical</a> DIV:

     <pre>DIV {grid: "@"}
DIV:slot(@) {vertical-align: middle}</pre>

     <p>To vertically center a P inside a vertical parent by means of a
      template on the P itself requires that the height of the P isn't set
      with ‘<a href="#height"><code class=property>height</code></a>’,
      but with ‘<code class=property>grid-rows</code>’ (or the ‘<code
      class=property>grid</code>’ shorthand). Below, the height is set to
      10em:

     <pre>P {grid: "." *
         "@" 10em
         "." * }</pre>

     <p>See also <a href="/Style/CSS/Tracker/actions/18">ACTION-18</a>
      (especially <a
      ideas at end of Bert's comment</a> ), which is continued in <a
      href="/Style/CSS/Tracker/issues/53">ISSUE-53.</a>
   </ol>
  </div>

  <div class=issue id=contain-floats>
   <p>Ensuring that the bottom padding (more precisely: the foot padding)
    starts after any preceding and descendant floats.

   <p>The main use case is to make sure an element is at least as high as all
    its floating descendants. I.e., at the end of the element, clear the
    floats that are descendants of the element. That may or may not also
    clear any floats that were declared before the element. Some possible
    approaches:

   <ol>
    <li>Set ‘<a href="#overflow1"><code
     class=property>overflow</code></a>’ to anything else than ‘<code
     class=property>visible</code>’ (and ‘<a href="#height"><code
     class=property>height</code></a>’ to ‘<code
     class=property>auto</code>’). This requires no new property, but has
     the side effect of influencing the scrolling mechanism. It also doesn't
     clear any floats that occurred before the element and doesn't
     distinguish left, right or other floats. [2004-02-29 ftf]

    <li>A new property ‘<a href="#clear-after"><code
     class=property>clear-after</code></a>’.

    <li>Use the value ‘<code class=css>shrink-wrap</code>’ (or some other
     name, e.g., ‘<code class=css>contain</code>’) on ‘<a
     href="#height"><code class=property>height</code></a>’ (and ‘<a
     href="#width"><code class=property>width</code></a>’, in case of
     vertical text) to make an element into a <a href="#flow-root"><em>flow
     root.</em></a> This requires no new property, only a new value, but
     doesn't clear any floats before the element and doesn't distinguish
     left, right or other floats. [The idea to add the functionality to ‘<a
     href="#height"><code class=property>height</code></a>’ is due to IanH
     2004-02-29]

    <li>The same, but only allow the new keyword on the ‘<a
     href="#min-height"><code class=property>min-height</code></a>’ and
     ‘<a href="#min-width"><code class=property>min-width</code></a>’
     properties. [The idea to add the functionality to ‘<a
     href="#min-height"><code class=property>min-height</code></a>’ is due
     to Fantasai, 2012-09]

    <li>A new value ‘<code class=css>container</code>’ on ‘<a
     href="#display0"><code class=property>display</code></a>’. This
     doesn't allow a ‘<code class=css>list-item</code>’ to clear its
     floats.
   </ol>
  </div>

  <div class=issue>
   <p>Vertical centering and alignment (more precisely: in the block flow
    direction).

   <p>The use cases are centering a single child vertically in its parent,
    centering all content of an element vertically (e.g., all text on a
    slide) and aligning only the last child (or the last <var>n</var>
    children) against the bottom of the parent.

   <p>Possible solutions include:

   <ol>
    <li>Extending ‘<code class=property>vertical-align</code>’ to apply
     to block elements:
     <pre>DIV {vertical-align: middle}</pre>

     <p>This centers or aligns all content, it cannot top-align the first
      child and bottom-align the last.

    <li>Allowing ‘<code class=property>vertical-align</code>’ on a new
     type of box called ‘<code class=css>container</code>’. (The main
     function of ‘<code class=css>container</code>’ is to contain its
     floating descendants.) This wouldn't allow a ‘<code
     class=css>list-item</code>’, a ‘<code
     class=css>table-caption</code>’ or an ‘<code
     class=property>inline-block</code>’‘<code class=css> to center its
     content.</code>’)

    <li>Setting ‘<a href="#margin"><code
     class=property>margin</code></a>’ to ‘<a href="#fill"><code
     class=css>fill</code></a>’:
     <pre>P {margin-top: fill}</pre>

     <p>This works to center an element in its parent, but not to center the
      content of an element with only text.

    <li>Relying on grid templates <a href="#ref-CSS3LAYOUT"
     rel=biblioentry>[CSS3LAYOUT]<!--{{CSS3LAYOUT}}--></a>. If there are
     enough elements, that could work:
     <pre>
&lt;STYLE>
 BODY {grid: "a . b"} /* 3 columns */
 SECTION.a {flow: a; grid: "@" "." "z"} /* 3 rows */
 SECTION.b {flow: b; grid: "@" "." "z"} /* 3 rows */
 P.note {flow: z}
&lt;/STYLE>
&lt;SECTION CLASS=a>
 &lt;H1>...&lt;/H1>
 &lt;P>...
 &lt;P CLASS=note>...
&lt;/SECTION>
&lt;SECTION CLASS=b>
 &lt;H1 ID=b>...&lt;/H1>
 &lt;P>...
 &lt;P CLASS=note>...
&lt;/SECTION>
</pre>

     <p>And assuming ‘<code class=property>vertical-align</code>’ applies
      to slots, then that also vertically centers the contents of an element.
      (This centers horizontally if the element is <a
      href="#vertical.">vertical,</a> because ‘<code
      class=property>vertical-align</code>’ is relative to the writing
      mode.)

     <pre>P {grid: "@"}
P:slot(@) {vertical-align: center}</pre>

     <p>But it is not enough to bottom-align the two paragraphs with class
      “note” in this example:

     <pre>
&lt;STYLE>
 BODY {grid: "a . b"}
 H1#a, H1#a ~ P {flow: a}
 H1#b, H1#b ~ P {flow: b}
 P.note {/* align to bottom?? */}
&lt;/STYLE>
&lt;H1 ID=>...&lt;/H1>
&lt;P>...
&lt;P CLASS=note>...
&lt;H1 ID=b>...&lt;/H1>
&lt;P>...
&lt;P CLASS=note>...
</pre>

    <li>
     <p>Use centering in the inline direction after switching the parent to
      an orthogonal block flow direction. E.g., if the document is <a
      href="#horizontal">horizontal</a> and we want to vertically center the
      single child of a DIV:

     <pre>DIV {writing-mode: vertical-rl}
DIV > * {writing-mode: horizontal; width: 100%;
  margin-top: auto; margin-bottom: auto}</pre>

     <p>This cannot top-align the first child and bottom-align the second.

    <li>
     <p>Vertically centering a single element could also be modeled as a
      float with an offset. A draft of <a href="#ref-CSS-PAGE-FLOATS"
      rel=biblioentry>[CSS-PAGE-FLOATS]<!--{{CSS-PAGE-FLOATS}}--></a> from
      2013 suggests the following to center a P vertically inside its parent
      DIV:

     <pre>DIV {height: 15em;
  colums: 1}            /* Make the DIV act like a "page" */
P {float: top left;     /* Place the P in the corner of the DIV... */
  float-offset: 0 50%}  /* ... and add 50% offset vertically */</pre>

     <p>Floats like this also allow to put all children at the top and the
      last child against the bottom. But only one child: trying to push
      multiple children down to the bottom results in their order being
      reversed.
   </ol>
  </div>

  <h2 class="no-num no-toc">Table of contents</h2>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#how-to-read-this-specification"><span class=secno>1.
    </span>How to read this specification</a>

   <li><a href="#dependencies-on-other-modules"><span class=secno>2.
    </span>Dependencies on other modules</a>

   <li><a href="#introduction-amp-definitions"><span class=secno>3.
    </span>Introduction &amp; definitions</a>

   <li><a href="#the-viewport-and-the-canvas"><span class=secno>4. </span>The
    viewport and the canvas</a>

   <li><a href="#flows"><span class=secno>5. </span>Flows</a>

   <li><a href="#types-of-boxes"><span class=secno>6. </span>Types of
    boxes</a>
    <ul class=toc>
     <li><a href="#display"><span class=secno>6.1. </span>The display
      property</a>

     <li><a href="#block-level-boxes-containing-blocks-flow"><span
      class=secno>6.2. </span>Block-level boxes, containing blocks, flows and
      anonymous boxes</a>

     <li><a href="#run-in-boxes"><span class=secno>6.3. </span>Run-in
      boxes</a>

     <li><a href="#compact-boxes"><span class=secno>6.4. </span>Compact
      boxes</a>

     <li><a href="#container-box"><span class=secno>6.5. </span>Container
      box</a>

     <li><a href="#mixing-the-box-model-with-other-formatti"><span
      class=secno>6.6. </span>Mixing the box model with other formatting
      models</a>
    </ul>

   <li><a href="#the-padding-properties"><span class=secno>7. </span>The
    padding properties</a>

   <li><a href="#the-margin-properties"><span class=secno>8. </span>The
    margin properties</a>

   <li><a href="#the-width-and-height-properties"><span class=secno>9.
    </span>The width and height properties</a>

   <li><a href="#min-max"><span class=secno>10. </span>The min-width,
    max-width, min-height and max-height properties</a>

   <li><a href="#intrinsic"><span class=secno>11. </span>Definition of
    min-content/max-content</a>

   <li><a href="#aspect-ratios-of-replaced-elements"><span class=secno>12.
    </span>Aspect ratios of replaced elements</a>

   <li><a href="#block-level-formatting"><span class=secno>13.
    </span>Block-level formatting</a>
    <ul class=toc>
     <li><a href="#collapsing-margins"><span class=secno>13.1.
      </span>Collapsing margins</a>
    </ul>

   <li><a href="#inline-level-formatting"><span class=secno>14.
    </span>Inline-level formatting</a>

   <li><a href="#Calculating"><span class=secno>15. </span>Calculating
    widths, heights and margins</a>
    <ul class=toc>
     <li><a href="#inline-non-replaced"><span class=secno>15.1.
      </span>Inline, non-replaced elements</a>

     <li><a href="#inline-replaced"><span class=secno>15.2. </span>Inline or
      floating, replaced elements</a>

     <li><a href="#blockwidth"><span class=secno>15.3. </span>Block-level,
      non-replaced elements in normal flow when ‘<code
      class=property>overflow</code>’ computes to ‘<code
      class=css>visible</code>’</a>

     <li><a href="#other-block-level-non-replaced-elements-"><span
      class=secno>15.4. </span>Other block-level, non-replaced elements in
      normal flow</a>

     <li><a href="#inline-block-or-floating-non-replaced-el"><span
      class=secno>15.5. </span>‘<code class=property>Inline-block</code>’
      or floating, non-replaced elements</a>

     <li><a href="#abs-non-replaced-width"><span class=secno>15.6.
      </span>Absolutely positioned, non-replaced elements</a>

     <li><a href="#abs-replaced"><span class=secno>15.7. </span>Absolutely
      positioned, replaced elements</a>

     <li><a href="#block-level-replaced-elements-in-normal-"><span
      class=secno>15.8. </span>Block-level, replaced elements in normal
      flow</a>

     <li><a href="#floating-non-replaced-elements"><span class=secno>15.9.
      </span>Floating, non-replaced elements</a>

     <li><a href="#root-height"><span class=secno>15.10. </span>Auto heights
      for flow roots</a>
    </ul>

   <li><a href="#the-float-property"><span class=secno>16. </span>The float
    property</a>
    <ul class=toc>
     <li><a href="#introduction-to-floats"><span class=secno>16.1.
      </span>Introduction to floats</a>

     <li><a href="#rules-for-positioning-floats"><span class=secno>16.2.
      </span>Rules for positioning floats</a>
    </ul>

   <li><a href="#the-clear-property"><span class=secno>17. </span>The clear
    property</a>

   <li><a href="#the-clear-after-property"><span class=secno>18. </span>The
    clear-after property</a>

   <li><a href="#overflow0"><span class=secno>19. </span>Overflow</a>
    <ul class=toc>
     <li><a href="#overflow"><span class=secno>19.1. </span>The overflow,
      overflow-x and overflow-y properties</a>
    </ul>

   <li><a href="#the-visibility-property"><span class=secno>20. </span>The
    visibility property</a>

   <li><a href="#the-alignment-property"><span class=secno>21. </span>The
    alignment property</a>

   <li><a href="#the-child-align-property"><span class=secno>22. </span>The
    child-align property</a>

   <li><a href="#stacking-contexts"><span class=secno>23. </span>Stacking
    contexts</a>
    <ul class=toc>
     <li><a href="#definitions"><span class=secno>23.1.
      </span>Definitions</a>

     <li><a href="#painting-order"><span class=secno>23.2. </span>Painting
      order</a>

     <li><a href="#notes"><span class=secno>23.3. </span>Notes</a>

     <li><a href="#the-ltnumbergt-value"><span class=secno>23.4. </span>The
      &lt;number&gt; value</a>

     <li><a href="#auto"><span class=secno>23.5. </span>The auto value</a>

     <li><a href="#complex-size"><span class=secno>23.6. </span>Algorithm for
      shrink-wrapping replaced elements with complex width/height</a>
    </ul>

   <li><a href="#the-float-displace-and-indent-edge-reset"><span
    class=secno>24. </span>The ‘<code
    class=property>float-displace</code>’ and ‘<code
    class=property>indent-edge-reset</code>’ properties
    [alternative 1]</a>
    <ul class=toc>
     <li><a href="#the-float-displace-property"><span class=secno>24.1.
      </span>The ‘<code class=property>float-displace</code>’
      property</a>

     <li><a href="#the-indent-edge-reset-property"><span class=secno>24.2.
      </span>The ‘<code class=property>indent-edge-reset</code>’
      property</a>
    </ul>

   <li class=no-num><a href="#the-float-displace-property-alternative2">The
    ‘float-displace’ property [alternative 2]</a>

   <li class=no-num><a href="#the-float-displace-property-alternative3">The
    ‘float-displace’ property [alternative 3]</a>

   <li><a href="#conformance"><span class=secno>25. </span>Conformance</a>

   <li><a href="#acknowledgments"><span class=secno>26.
    </span>Acknowledgments</a>

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

     <li class=no-num><a href="#informative-references">Informative
      references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>

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

  <h2 id=how-to-read-this-specification><span class=secno>1. </span>How to
   read this specification</h2>

  <p>All sections are normative, unless stated otherwise.

  <div class=example>
   <p>Examples look like this and normally start with the word “Example.”
    Examples are not normative.
  </div>

  <p class=note>Notes look like this and normally start with the word
   “Note.” Notes are not normative.

  <p class=issue>Editorial notes look like this. They will be removed before
   the document becomes Candidate Recommendation.

  <p>Each property is defined in part in the text and in part by a table that
   groups together a number of facts about the property, including a regular
   expression to restrict its syntax. See <span class=issue>[where?]</span>
   for the meaning. The “Inherited” and “Initial” rows in the table
   are used by the Cascading and Inheritance module <a
   href="#ref-CSS3CASCADE"
   rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a> and “Media” by
   the Media Queries specification <a href="#ref-MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{MEDIAQ}}--></a>.

  <p>The specification may refer to the <a href="#used-value."><em>used
   value</em></a> and the <a href="#computed-value"><em>computed
   value</em></a> of a property. Unless stated explicitly, the short form
   “value” means the computed value.

  <h2 id=dependencies-on-other-modules><span class=secno>2.
   </span>Dependencies on other modules</h2>

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

  <ul>
   <li><cite>CSS Writing Modes Module</cite> <a
    href="#ref-CSS3-WRITING-MODES"
    rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>
    – switch between vertical and horizontal text, properties <dfn
    id=writing-mode>writing-mode</dfn> and <dfn id=direction>direction</dfn>,
    definitions of the terms <dfn id=block-flow-direction>block flow
    direction</dfn> and <dfn id=inline-base-direction.>inline base
    direction.</dfn>

   <li><cite>CSS Syntax Module</cite> <a href="#ref-CSS3SYN"
    rel=biblioentry>[CSS3SYN]<!--{{!CSS3SYN}}--></a> – syntax of properties
    and values

   <li><cite>CSS Values and Units Module</cite> <a href="#ref-CSS3VAL"
    rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a> – definitions of value
    types <dfn id=ltnumbergt><var>&lt;number&gt;</var></dfn>, <dfn
    id=ltpercentagegt><var>&lt;percentage&gt;</var></dfn>, <dfn
    id=ltanglegt><var>&lt;angle&gt;</var></dfn> and <dfn
    id=ltlengthgt><var>&lt;length&gt;</var></dfn>; defines the terms <dfn
    id=computed-value>computed value</dfn> and <dfn id=used-value.>used
    value.</dfn>

   <li><cite>CSS Text Module</cite> <a href="#ref-CSS3TEXT"
    rel=biblioentry>[CSS3TEXT]<!--{{!CSS3TEXT}}--></a> – inline layout and
    definition of <dfn id=line-box.>line box.</dfn>

   <li><cite>CSS Generated and Replaced Content module</cite> <a
    href="#ref-CSS3GENCON"
    rel=biblioentry>[CSS3GENCON]<!--{{!CSS3GENCON}}--></a> – defines the
    terms <dfn id=replaced-element>replaced element</dfn> and <dfn
    id=intrinsic0>intrinsic</dfn> size (and intrinsic ratio).

   <li><cite>CSS Backgrounds and Borders Module</cite> <a href="#ref-CSS3BG"
    rel=biblioentry>[CSS3BG]<!--{{!CSS3BG}}--></a> – defines <dfn
    id=ltbg-positiongt><var>&lt;bg-position&gt;</var></dfn>

   <li><cite>CSS Basic User Interface Module</cite> <a href="#ref-CSS3UI"
    rel=biblioentry>[CSS3UI]<!--{{!CSS3UI}}--></a> – defines <dfn
    id=box-sizing>box-sizing</dfn>

   <li><cite>CSS 2D Transforms</cite> <a href="#ref-CSS3-2D-TRANSFORM
3981
S"
    rel=biblioentry>[CSS3-2D-TRANSFORMS]<!--{{!CSS3-2D-TRANSFORMS}}--></a>
    – defines <dfn id=transform>transform</dfn>
  </ul>

  <h2 id=introduction-amp-definitions><span class=secno>3.
   </span>Introduction &amp; definitions</h2>

  <p class=issue>Investigate if it is possible to make a pseudo-element to
   select anonymous boxes (‘<code class=css>::paragraph</code>’). See
   minutes of 10 Aug 2011.

  <p class=note>Note: The model in this specification differs from the model
   described in the CSS level 2 specification <a href="#ref-CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, because it is generalized to
   apply also to vertical text. A document using only features from level 2
   renders exactly as described there, (except possibly for features that are
   undefined in that specification and have become defined since).

  <p>CSS assumes that the document to lay out is modeled as a <span
   class=index id=tree
   title="tree!!of elements|document tree|element tree">tree</span> of <dfn
   id=elements. title=element>elements.</dfn> Each element has an ordered
   list of zero or more child elements, with an optional string of text
   before the list, in-between the children and after the list. Each child
   has one parent, except for the unique element that has no parent, which is
   called the <dfn id=root-element.>root element.</dfn>

  <p class=note>Note that such a tree can be described in XML or SGML, but
   also in other notations and that it is the definition of that notation,
   not CSS, that determines the equivalence between the abstract tree and the
   concrete notation. E.g., SGML often allows white space to be added between
   elements for readability, which does not result in a string of spaces in
   the abstract tree.

  <p>CSS describes how each element and each string of text is laid out by
   transforming the document tree into a set of rectangular boxes, whose
   size, position, and stacking level on the <a
   href="#canvas"><em>canvas</em></a> depend on their properties. We say that
   an element <dfn id=generates title="generated box">generates</dfn> one or
   more boxes. There are numerous different types of boxes. <span
   class=issue>[Or maybe only one? I.e., a block-level box is just a normal
   box that happens to be generated by a block-level element?]</span> A block
   box is like a paragraph; a line box is like a single line of text; and
   inline boxes are like words inside a line.

  <div class=issue>
   <p>Define the <dfn id=box-tree. title="box tree|tree!!of boxes">box
    tree.</dfn> (Is the space between words a kind of box as well, or a
    spacer item? Is the current selection a box? Is there a defined order
    among sibling boxes? If so, are bidi inline boxes ordered in reading
    order or in visual order?)

   <p>[Define the box tree before pagination and then describe how pagination
    can break one box into two?] <!--
<ul>
<li>If the parent element is a list item, the boxes go into the
parent's block box, not the marker box.

<li>If the element is run-in, its boxes go either into a box of its
parent element, or into a box of a later sibling.

<li>The order of sibling boxes is always such that boxes that belong
to elements earlier in the document come before boxes of elements that
come later in the document.

<li>Note that 'position' can put a box into the box of an ancestor
(the one establishing the containing block) rather than a parent.

<li>Note that 'flow' puts boxes into a named flow root.
</ul>
-->
  </div>

  <p>When the specification says a box <var>A</var> <dfn
   id=contains>contains</dfn> a box <var>B</var> it means <var>B</var> is a
   child of <var>A</var> in the <a href="#box-tree."><em>box tree.</em></a>
   Typically, box <var>B</var> is geometrically contained within box
   <var>A</var> as well, but that need not be the case, because of, e.g.,
   overflow or negative margins.

  <p class=note>Note: The precise rules are below and in other modules, but
   in summary, <span class=issue>[write something that helps convey the idea
   of a flow, e.g.:]</span> a block-level box contains either other
   block-level boxes (e.g., a section containing paragraphs, or a table
   containing rows), or it contains line boxes (e.g., a paragraph containing
   lines of text). A line box contains inline-level boxes (e.g., a line with
   words in different styles). An inline-level box may contain either text
   interspersed with more inline-level boxes, or it may contain a block-level
   box (e.g., a small table that is rendered inline).

  <div class=figure>
   <p><img
    alt="Each generated box in the rendering   has a link to the element that generated it."
    src=box-intro.png>

   <p class=caption>Relation between four displayed boxes in the rendered
    document (on the right) and the three corresponding elements in the
    source document on the (left).
  </div>

  <div class=example>
   <p>For example, a fragment of HTML such as

   <pre>&lt;ul&gt;
 &lt;li&gt;The first item in the list.
 &lt;li&gt;The second item.
&lt;/ul&gt;</pre>

   <p>may result in <span class=issue>[rewrite the following
    sentence?]</span> one block-level box for the <code>ul</code> element,
    containing two block-level boxes for the two <code>li</code> elements,
    each of which has one line box (i.e., one line of text). Both line boxes
    contain two inline-level boxes: one that contains the list bullet and one
    that contains the text.

   <p>Note how the <code>li</code> is transformed into multiple boxes,
    including one that contains “generated content,” viz., the list
    bullet, which is not present in the source document.

<
C4B3
div id="LC803" class="react-file-line html-div" data-testid="code-cell" data-line-number="803" style="position:relative">   <p>If the document is rendered in a narrow window, it may be that the
    <code>li</code> elements get transformed into even more boxes, because
    the text requires multiple lines. And if the document is rendered on
    paper or in multiple columns, it may be that a page break falls in the
    middle of the <code>ul</code> element, so that it is not transformed into
    a single block-level box, but into two smaller ones, each on a different
    page or column.
  </div>

  <p>Each box belongs to exactly one element. It is either <a
   href="#generates" title="generated box">generated</a> directly by the
   element, or it is an <a href="#anonymous-box"><em>anonymous box,</em></a>
   which has been inserted to ensure that every box and its parent are of
   mutually compatible types as defined by the layout rules of CSS. An
   anonymous box is defined to belong to whichever element generates the
   box's closest element-generated ancestor box. When we need to be precise,
   we say that the anonymous box is <dfn id=induced>induced,</dfn> rather
   than generated, by the element it belongs to.

  <p class=note>For example, an anonymous table wrapper box that is created
   around a table cell because the table cell element doesn't have a table
   element as parent, belongs to the parent element of the table cell, not to
   the table cell itself.

  <p class=note>An anonymous box is never a <em>principal</em> box. Except
   for that, an anonymous box cannot be distinguished by its property values
   and characteristics from a box generated by some hypothetical document
   element. <span class=issue>Remove this note? Do we actually use the
   concept of “principal box” anywhere?</span>

  <p>Properties are set on elements <a href="#ref-CSS3CASCADE"
   rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a> and influence how
   the element is turned into boxes. Every box has the same properties with
   the same values as the element to which it belongs, except for anonymous
   boxes: inheritable properties on anonymous boxes have the same value as
   the element, while non-inheritable properties have the initial value. The
   only property that boxes do not have is the ‘<a href="#display0"><code
   class=property>display</code></a>’ property. <span class=issue>[Is this
   useful? Or is it better to say that boxes are just rectangles with a
   border, a background, a position, a bunch of other boxes inside, and a
   reference to the element they belong to? They don't really have properties
   then and it is only a convenience to say the “the box has a font
   <var>X.</var>”</span>

  <p>Boxes do not have specified values <a href="#ref-CSS3CASCADE"
   rel=biblioentry>[CSS3CASCADE]<!--{{!CSS3CASCADE}}--></a>, but they do have
   computed, used and actual values.

  <p class=note>Boxes are frequently referred to by their property values.
   For example, a “floated box” is a box whose computed value of ‘<a
   href="#float"><code class=property>float</code></a>’ is not ‘<code
   class=css>none</code>’, and a “positioned box” is a box whose
   computed value of ‘<code class=property>position</code>’ is not
   ‘<code class=css>static</code>’. However, terms such as
   <em>block-level box,</em> <a href="#block-container-box"><em>block
   container box</em></a> and <em>table box</em> describe characteristics
   which cannot be deduced solely from a box's property values.

  <div class=figure id=various>
   <p><img
    alt="Diagram of a typical box, showing the   content, padding, border and margin areas"
    src=box.png>

   <p class=caption>The various areas and edges of a typical box
  </div>

  <p>Boxes have padding, a border and margins (see the <a
   href="#various">figure</a>). Different properties determine the thickness
   of each of these (which may be zero). The margins of adjacent boxes are
   also subject to <a href="#collapse."><em
   title=collapse>collapsing</em></a> (i.e., the actual margin between
   adjacent boxes may be less than the sum of the boxes' individual margins).

  <p>Each box has a <dfn id=content-area>content area</dfn> (a.k.a. <dfn
   id=content-box>content box</dfn>). The rectangle that bounds this area is
   the <dfn id=content-edge.>content edge.</dfn> Around the content area is
   the <dfn id=padding-area>padding area</dfn> and its outside bounds are
   called the <dfn id=padding-edge.>padding edge.</dfn> The padding area and
   content area together form the <dfn id=padding-box.>padding box.</dfn>
   Outside the padding is the <dfn id=border-area>border area</dfn> and the
   outside boundary of that area is the <dfn id=border-edge.>border
   edge.</dfn> The border area, padding area and content area together form
   the <dfn id=border-box.>border box.</dfn> Finally, outside the border is
   the <dfn id=margin-area>margin area</dfn> and its outer edge is the <dfn
   id=margin-edge.>margin edge.</dfn>

  <p>When the specification says that the padding or border is <dfn id=absent
   title="absent|padding!!absent|border!!absent">“absent”</dfn> on some
   side of the box, that means that its thickness is zero.

  <p><a href="#line-box."><em title="line box">Line boxes</em></a> cannot
   have any padding, border or margin, and therefore their margin edge,
   border edge, padding edge and content edge all coincide.

  <p class=note>Note that the margin, unlike the border and padding, may have
   a negative thickness. That is one way to make adjacent boxes overlap each
   other.

  <p class=note>Note that the edges always form rectangles, even if there is
   a ‘<code class=property>border-radius</code>’ <a href="#ref-CSS3BG"
   rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a> or a shape <a
   href="#ref-CSS3-EXCLUSIONS"
   rel=biblioentry>[CSS3-EXCLUSIONS]<!--{{CSS3-EXCLUSIONS}}--></a>.

  <p>We say that a box or element is <dfn id=horizontal>horizontal</dfn> if
   its ‘<a href="#writing-mode"><code
   class=property>writing-mode</code></a>’ property is ‘<code
   class=css>horizontal-tb</code>’, otherwise it is <dfn
   id=vertical.>vertical.</dfn>

  <div class=note>
   <p>Note that there are theoretically eight possible orientations for text,
    but CSS only defines six:

   <table class=equiv-table style="table-layout: fixed; width: 100%">
    <thead>
     <tr>
      <th>

      <th>‘<code class=css>writing-mode: horizontal-tb</code>’

      <th>‘<code class=css>writing-mode: vertical-rl</code>’

      <th>‘<code class=css>writing-mode: vertical-lr</code>’

    <tbody>
     <tr>
      <th>‘<code class=css>direction: ltr</code>’

      <td>
       <p>Text is written from left to right and paragraphs grow downwards

       <p><img alt="" class=small src=horizontal.png>

      <td>
       <p>Text is written top to bottom and paragraphs grow to the left

       <p><img alt="" class=small src=vertical.png>

      <td>
       <p>Text is written top to bottom and paragraphs grow to the right

       <p><img alt="" class=small src=vertical-lr.png>

     <tr>
      <th>‘<code class=css>direction: rtl</code>’

      <td>
       <p>Text is written from right to left and paragraphs grow downwards

       <p><img alt="" class=small src=horizontal-rtl.png>

      <td>
       <p>Text is written bottom to top and paragraphs grow to the left

       <p><img alt="" class=small src=vertical-rtl.png>

      <td>
       <p>Text is written bottom to top and paragraphs grow to the right

       <p><img alt="" class=small src=vertical-lr-rtl.png>
   </table>

   <p>There is no “horizontal-bt.” More information about ‘<a
    href="#writing-mode"><code class=property>writing-mode</code></a>’ and
    ‘<a href="#direction"><code class=property>direction</code></a>’ can
    be found in <a href="#ref-CSS3-WRITING-MODES"
    rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>.
  </div>

  <p class=mtb>Calculations of the size and position of boxes are usually
   very similar for horizontal and vertical boxes, except that the horizontal
   and vertical directions are interchanged. To avoid many almost identical
   definitions, this specification therefore often uses abstract terms
   instead of top, right, bottom, left, width and height: four terms (head
   edge, foot edge, start edge and end edge) for the four edges of a box,
   four terms (A edge, B edge, C edge and D edge) for the edges of the
   children of a box and two terms (block dimension and inline dimension) for
   the width and height of a box.

  <p class=issue>The names head and foot are under discussion. There is an
   issue in <a href="#ref-CSS3-WRITING-MODES"
   rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>
   that suggests some possible names: “head,” “before,”
   “before-block,” “foot,” “tail,” “after” and
   “after-block.”

  <p>The <dfn id=head>head,</dfn> <dfn id=foot>foot,</dfn> <dfn
   id=start>start</dfn> and <dfn id=end>end</dfn> edge of a box refer to the
   top, right, bottom and left edges, depending on the ‘<a
   href="#writing-mode"><code class=property>writing-mode</code></a>’ and
   ‘<a href="#direction"><code class=property>direction</code></a>’
   properties of that box, as follows:

  <table class=equiv-table>
   <thead>
    <tr>
     <th rowspan=2>Value of<br>