csswg-drafts/css3-box/Overview.html at 2fec492f68d8ec32a50ba52ca345fa7af8ad90e0 · w3c/csswg-drafts · GitHub

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

<html>
 <head
  profile="http://www.w3.org/2006/03/hcard   http://microformats.org/wiki/rel-license">
  <title>CSS basic box model</title>

  <style type="text/css">
  ol.stack p {margin-top: 0}
</style>
  <link href="../default.css" rel=stylesheet title=Default type="text/css">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
  title=Default 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 basic box model</h1>

   <h2 class="no-num no-toc" id=longstatus-date-26-sep-2006>Editor's Draft 16
    November 2011</h2>

   <dl>
    <dt>This version:

    <dd><a href="http://www.w3.org/TR/2011/ED-css3-box-20111116/">
     http://www.w3.org/TR/2011/ED-css3-box-20111116</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>Editors:

    <dd class=vcard><span class=fn>Bert Bos</span> (<span
     class=org>W3C</span>) <span class=email>bert@w3.org</span>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2011 <a
    href="http://www.w3.org/"><acronym
    title="World Wide Web Consortium">W3C</acronym></a><sup>&reg;</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><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
1DDA
 can be mixed.

  <h2 class="no-toc no-num" id=status-of-this-document>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
   &#8220;css3-box&#8221; in the subject, preferably like this:
   &#8220;[<!---->css3-box<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

  <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>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>This module should eventually replace corresponding parts of
   the revised CSS level 2 specification <a href="#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>

  <h2 class="no-num no-toc" id=table-of-contents>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="#aspect-ratios-of-replaced-elements"><span class=secno>5.
    </span>Aspect ratios of replaced elements</a>

   <li><a href="#basic-flow-layout"><span class=secno>6. </span>Basic flow
    layout</a>

   <li><a href="#types-of-boxes"><span class=secno>7. </span>Types of
    boxes</a>
    <ul class=toc>
     <li><a href="#display"><span class=secno>7.1. </span>The display
      property</a>

     <li><a href="#mixing-the-box-model-with-other-formatti"><span
      class=secno>7.2. </span>Mixing the box model with other formatting
      models</a>

     <li><a href="#block-level-boxes-containing-blocks-flow"><span
      class=secno>7.3. </span>Block-level boxes, containing blocks, flows and
      anonymous boxes</a>

     <li><a href="#run-in-boxes"><span class=secno>7.4. </span>Run-in
      boxes</a>

     <li><a href="#compact-boxes"><span class=secno>7.5. </span>Compact
      boxes</a>
    </ul>

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

   <li><a href="#margins"><span class=secno>9. </span>Margins</a>
    <ul class=toc>
     <li><a href="#the-margin-properties"><span class=secno>9.1. </span>The
      margin properties</a>

     <li><a href="#collapsing-margins"><span class=secno>9.2.
      </span>Collapsing margins</a>
    </ul>

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

   <li><a href="#min-max"><span class=secno>11. </span>The min-width,
    max-width, min-height and max-height properties</a>

   <li><a href="#intrinsic"><span class=secno>12. </span>Definition of
    intrinsic sizes &ndash; high quality</a>

   <li><a href="#intrinsic-low"><span class=secno>13. </span>Definition of
    intrinsic sizes &ndash; low quality</a>

   <li><a href="#Calculating"><span class=secno>14. </span>Calculating
    widths, heights and margins</a>
    <ul class=toc>
     <li><a href="#inline-non-replaced"><span class=secno>14.1.
      </span>Inline, non-replaced elements</a>

     <li><a href="#inline-replaced"><span class=secno>14.2. </span>Inline or
      floating, replaced elements</a>

     <li><a href="#blockwidth"><span class=secno>14.3. </span>Block-level,
      non-replaced elements in normal flow when &lsquo;<code
      class=property>overflow</code>&rsquo; computes to &lsquo;<code
      class=css>visible</code>&rsquo;</a>

     <li><a href="#other-block-level-non-replaced-elements-"><span
      class=secno>14.4. </span>Other block-level, non-replaced elements in
      normal flow</a>

     <li><a href="#inline-block-or-floating-non-replaced-el"><span
      class=secno>14.5. </span>&lsquo;<code
      class=property>Inline-block</code>&rsquo; or floating, non-replaced
      elements</a>

     <li><a href="#abs-non-replaced-width"><span class=secno>14.6.
      </span>Absolutely positioned, non-replaced elements</a>

     <li><a href="#abs-replaced"><span class=secno>14.7. </span>Absolutely
      positioned, replaced elements</a>

     <li><a href="#block-level-replaced-elements-in-normal-"><span
      class=secno>14.8. </span>Block-level, replaced elements in normal
      flow</a>

     <li><a href="#floating-non-replaced-elements"><span class=secno>14.9.
      </span>Floating, non-replaced elements</a>

     <li><a href="#root-height"><span class=secno>14.10. </span>Auto heights
      for flow roots</a>
    </ul>

   <li><a href="#the-float-property"><span class=secno>15. </span>The float
    property</a>
    <ul class=toc>
     <li><a href="#introduction-to-floats"><span class=secno>15.1.
      </span>Introduction to floats</a>

     <li><a href="#rules-for-positioning-floats"><span class=secno>15.2.
      </span>Rules for positioning floats</a>
    </ul>

   <li><a href="#the-clear-property"><span class=secno>16. </span>The clear
    property</a>

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

   <li><a href="#overflow0"><span class=secno>18. </span>Overflow</a>
    <ul class=toc>
     <li><a href="#overflow"><span class=secno>18.1. </span>The overflow,
      overflow-x and overflow-y properties</a>
    </ul>

   <li><a href="#the-visibility-property"><span class=secno>19. </span>The
    visibility property</a>

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

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

   <li><a href="#stacking-contexts"><span class=secno>22. </span>Stacking
    contexts</a>
    <ul class=toc>
     <li><a href="#definitions"><span class=secno>22.1.
      </span>Definitions</a>

     <li><a href="#painting-order"><span class=secno>22.2. </span>Painting
      order</a>

     <li><a href="#notes"><span class=secno>22.3. </span>Notes</a>

     <li><a href="#the-ltnumbergt-value"><span class=secno>22.4. </span>The
      &lt;number&gt; value</a>

     <li><a href="#auto"><span class=secno>22.5. </span>The auto value</a>

     <li><a href="#complex-size"><span class=secno>22.6. </span>Algorithm for
      shrink-wrapping replaced elements with complex width/height</a>
    </ul>

   <li><a href="#the-float-displace-property"><span class=secno>23.
    </span>The &lsquo;<code class=property>float-displace</code>&rsquo;
    property</a>

   <li><a href="#the-indent-edge-reset-property"><span class=secno>24.
    </span>The &lsquo;<code class=property>indent-edge-reset</code>&rsquo;
    property</a>

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

   <li><a href="#exit"><span class=secno>26. </span>CR exit criteria</a>

   <li><a href="#acknowledgments"><span class=secno>27.
    </span>Acknowledgments</a>

   <li class=no-num><a href="#references">References</a>

   <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
    &ldquo;Example.&rdquo; Examples are not normative.
  </div>

  <p class=note>Notes look like this and normally start with the word
   &ldquo;Note.&rdquo; 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 &ldquo;Inherited&rdquo; and &ldquo;Initial&rdquo;
   rows in the table are used by the Cascading and Inheritance module <a
   href="#CSS3CASCADE"
   rel=biblioentry>[CSS3CASCADE]<!--{{CSS3CASCADE}}--></a> and
   &ldquo;Media&rdquo; by the Media Queries specification <a href="#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="#CSS3-WRITING-MODES"
    rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>
    &ndash; switch between vertical and horizontal text, properties <dfn
    id=block-flow>block-flow</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="#CSS3SYN"
    rel=biblioentry>[CSS3SYN]<!
62B4
--{{!CSS3SYN}}--></a> &ndash; syntax of
    properties and values

   <li><cite>CSS Values and Units Module</cite> <a href="#CSS3VAL"
    rel=biblioentry>[CSS3VAL]<!--{{!CSS3VAL}}--></a> &ndash; definitions of
    value types <dfn id=ltnumbergt><var>&lt;number&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="#CSS3TEXT"
    rel=biblioentry>[CSS3TEXT]<!--{{!CSS3TEXT}}--></a> &ndash; inline layout
    and definition of <dfn id=line-box.>line box.</dfn>

   <li><cite>CSS Generated and Replaced Content module</cite> <a
    href="#CSS3GENCON" rel=biblioentry>[CSS3GENCON]<!--{{!CSS3GENCON}}--></a>
    &ndash; 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="#CSS3BG"
    rel=biblioentry>[CSS3BG]<!--{{!CSS3BG}}--></a> &ndash; defines <dfn
    id=ltbg-positiongt><var>&lt;bg-position&gt;</var></dfn>

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

   <li><cite>CSS 2D Transforms</cite> <a href="#CSS3-2D-TRANSFORMS"
    rel=biblioentry>[CSS3-2D-TRANSFORMS]<!--{{!CSS3-2D-TRANSFORMS}}--></a>
    &ndash; 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 (&lsquo;<code class=css>::paragraph</code>&rsquo;).
   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="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, because it is generalized to
   apply also to vertical text.

  <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>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 <a href="#block-level"><em>block-level</em></a>
   boxes, <a href="#line-box."><em title="line box">line boxes</em></a> and
   <a href="#inline-level"><em title=inline-level>inline-level
   boxes.</em></a> A block-level box is like a paragraph. A line box is like
   a line of text. Inline-level boxes are like words inside a line.

  <p>When the specification says a box <var>A</var> <dfn
   id=contains>contains</dfn> a box <var>B</var>, then <var>B</var> is a box
   generated by an element that is a descendant of the element that generated
   <var>A</var>. Typically, box <var>B</var> is geometrically contained in
   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, 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="Schematic representation of rel" 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 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 &ldquo;generated content,&rdquo; viz., the
    list bullet, which is not present in the source document.

   <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, 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.
  </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>
   (defined below), which is inserted because the layout rules of CSS require
   a box to contain a certain kind of other box, but the boxes generated by
   the child elements are not of that kind. The anonymous box is defined to
   belong to the parent.

  <p class=note>Note: This is mainly important for <span class=index
   id=wysiwyg-document-editors>WYSIWYG document editors</span>: if the user
   selects a box on the screen in order to set a property, the editor knows
   which element to set the property on.

  <p>Properties are set on elements and influence how the element is turned
   into boxes, but in this specification we refer interchangeably to
   &ldquo;the <var>P</var> property of an element&rdquo; and &ldquo;the
   <var>P</var> property of a box&rdquo; (both of which actually mean
   &ldquo;<em>the value</em> of property <var>P</var> of&hellip;&rdquo;),
   unless it is important to distinguish the box and the element, e.g.,
   because the element has several boxes and they don't all have the same
   value for the property.

  <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 are also subject to <a
   href="#collapse."><em title=collapse>collapsing.</em></a>

  <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
   &ldquo;absent&rdquo; on some side of the box, that actually 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 &lsquo;<code class=property>border-radius</code>&rsquo; <a
   href="#CSS3BG" rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>.

  <p>A box or element is <dfn id=vertical>vertical</dfn> if its &lsquo;<a
   href="#block-flow"><code class=property>block-flow</code></a>&rsquo;
   property is &lsquo;<code class=css>lr</code>&rsquo; or &lsquo;<code
   class=css>rl</code>&rsquo;, otherwise it is <dfn
   id=horizontal.>horizontal.</dfn> The <dfn id=writing-mode>writing
   mode</dfn> of a box or element is the pair of values of its &lsquo;<a
   href="#block-flow"><code class=property>block-flow</code></a>&rsquo; and
   &lsquo;<a href="#direction"><code
   class=property>direction</code></a>&rsquo; (&lsquo;<code
   class=css>ltr</code>&rsquo; or &lsquo;<code class=css>rtl</code>&rsquo;).
   I.e., there are eight writing modes: four horizontal ones and four
   vertical ones.

  <p>This specification sometimes refers to abstract edges <dfn
   id=head>head,</dfn> <dfn id=end>end,</dfn> <dfn id=tail>tail</dfn> and
   <dfn id=start>start,</dfn> which are relative to the writing mode of a
   box. They map to top, right, bottom and left as follows:

  <table class=equiv-table>
   <thead>
    <tr>
     <th>Value of &lsquo;<a href="#block-flow"><code
      class=property>block-flow</code></a>&rsquo;

     <th>Value of &lsquo;<a href="#direction"><code
      class=property>direction</code></a>&rsquo;

     <th>Meaning of “head”

     <th>Meaning of “end”

     <th>Meaning of “tail”

     <th>Meaning of “start”

   <tbody>
    <tr>
     <th>&lsquo;<code class=css>tb</code>&rsquo;

     <th>&lsquo;<code class=css>ltr</code>&rsquo;

     <td>top

     <td>right

     <td>bottom

     <td>left

    <tr>
     <th>&lsquo;<code class=css>tb</code>&rsquo;

     <th>&lsquo;<code class=css>rtl</code>&rsquo;

     <td>top

     <td>left

     <td>bottom

     <td>right

    <tr>
     <th>&lsquo;<code class=css>bt</code>&rsquo;

     <th>&lsquo;<code class=css>ltr</code>&rsquo;

     <td>bottom

     <td>right

     <td>top

     <td>left

    <tr>
     <th>&lsquo;<code class=css>bt</code>&rsquo;

     <th>&lsquo;<code class=css>rtl</code>&rsquo;

     <td>bottom

     <td>left

     <td>top

     <td>right

    <tr>
     <th>&lsquo;<code class=css>rl</code>&rsquo;

     <th>&lsquo;<code class=css>ltr</code>&rsquo;

     <td>right

     <td>bottom

     <td>left

     <td>top

    <tr>
     <th>&lsquo;<code class=css>rl</code>&rsquo;

     <th>&lsquo;<code class=css>rtl</code>&rsquo;

     <td>right

     <td>top

     <td>left

     <td>bottom

    <tr>
     <th>&lsquo;<code class=css>lr</code>&rsquo;

     <th>&lsquo;<code class=css>rtl</code>&rsquo;

     <td>left

     <td>bottom

     <td>right

     <td>top

    <tr>
     <th>&lsquo;<code class=css>lr</code>&rsquo;

     <th>&lsquo;<code class=css>ltr</code>&rsquo;

     <td>left

     <td>top

     <td>right

     <td>bottom
  </table>

  <p>For example, the “head padding” by default refers to the &lsquo;<a
   href="#padding-top"><code class=property>padding-top</code></a>&rsquo; and
   the “end border” is by default the &lsquo;<code
   class=property>border-right</code>&rsquo;.

  <p>Similarly, the specification sometimes refers to the <dfn id=inline
   title="inline dimension">inline</dfn> and <dfn id=block-flow0
   title="block flow dimension">block flow</dfn> dimensions of a box, instead
   of width and height, as follows:

  <table class=equiv-table>
   <tbody>
    <tr>
     <th>Value of &lsquo;<a href="#block-flow"><code
      class=property>block-flow</code></a>&rsquo;

     <th>Meaning of “inline dimension”

     <th>Meaning of “block flow dimension”

    <tr>
     <td>tb

     <td>width

     <td>height

    <tr>
     <td>rl

     <td>height

     <td>width

    <tr>
     <td>lr

     <td>height

     <td>width
  </table>

  <p>These correspond to the dimensions in the <a
   href="#inline-base-direction."><em>inline base direction</em></a> and the
   <a href="#block-flow-direction"><em>block flow direction,</em></a>
   respectively.

  <p>Finally, the <dfn id=headside>headside,</dfn> <dfn
   id=endside>endside,</dfn> <dfn id=tailside>tailside</dfn> and <dfn
   id=startside>startside</dfn> of a box are defined as the sides that
   correspond to the <a href="#head"><em>head,</em></a> <a
   href="#end"><em>end,</em></a> <a href="#tail"><em>tail</em></a> and <a
   href="#start"><em>start,</em></a> of the <a
   href="#containing-block"><em>containing block</em></a> of the box.

  <div class=example>
   <p>For example, if a box has a &lsquo;<a href="#block-flow"><code
    class=property>block-flow</code></a>&rsquo; of &lsquo;<code
    class=css>tb</code>&rsquo;, then its top content edge acts as its <a
    href="#head"><em>head</em></a> content edge, and the <a
    href="#headside"><em>headside</em></a> edges of its children are
    therefore their top edges, independent of what their own &lsquo;<a
    href="#block-flow"><code class=property>block-flow</code></a>&rsquo; is.

   <div class=figure>
    <p><img
     alt="[Figure: the “head content edge” of     the child box the right edge, but the“headside content edge” is     the top.]"
     src=headside>

    <p class=caption>The second child box is a vertical box and has its
     “head” content edge on the right. But its “headside” content
     edge is at the top.
   </div>
  </div>

  <p class=issue>Check terms. At Cambridge ftf Aug 2008 we chose: Property
   name: block-flow with values tb | lr | rl | bt. Descriptive terms:
   horizontal mode (= tb &amp; bt), vertical mode (= lr &amp; rl)

  <h2 id=the-viewport-and-the-canvas><span class=secno>4. </span>The viewport
   and the canvas</h2>

  <p>User agents for continuous media generally offer users a <dfn
   id=viewport>viewport</dfn> (a window or other viewing area on the screen)
   through which users consult a document. User agents may change the
   document's layout when the viewport is resized (see the <a
   href="#initial-containing-block."><em>initial containing block</em></a>).

  <p>When the viewport is smaller than the area of the <a
   href="#canvas"><em>canvas</em></a> (see below) on which the document is
   rendered, the user agent should offer a scrolling mechanism. There is at
   most one viewport per canvas, but user agents may render to more than one
   canvas (i.e., provide different views of the same document).

  <p>For all media, the term <dfn id=canvas>canvas</dfn> describes the space
   where the formatting structure is rendered. The canvas is infinite for
   each dimension of the space.

  <h2 id=aspect-ratios-of-replaced-elements><span class=secno>5.
   </span>Aspect ratios of replaced elements</h2>

  <p>For the purposes of the width and height calculations below, CSS
   distinguishes four kinds of <a href="#replaced-element"><em class=index
   id=replaced-elements title="replaced element">replaced elements</em></a>:

  <dl>
   <dt>Type 1: replaced elements with both <em class=index
    id=intrinsic-width>intrinsic width</em> and <em class=index
    id=intrinsic-height.>intrinsic height.</em>

   <dd>These are replaced elements that are meant to be displayed with a
    certain fixed factor (the <em>intrinsic ratio</em>) between their width
    and height and also have a default size. Different formats may have
    different ways of specifying the size, e.g., a width and a ratio, or a
    raster and a resolution in dots per inch. Raster images fall in this
    category.

   <dt>Type 2: replaced elements with only an <em>intrinsic ratio</em>

   <dd>These are replaced elements that are meant to be displayed with a
    certain fixed factor between their width and height, but without a
    default size. <span class=index id=svg>SVG</span> images often fall in
    the this category.

   <dt>Type 3: replaced elements with a <dfn id=complex-aspect-ratio>complex
    aspect ratio</dfn>

   <dd>These are replaced elements that do not have a <em>fixed</em> ratio
    between their width and height, but that do have a functional relation
    between them in the sense that for every width there is a unique height.
    HTML documents are examples of this: the width can be chosen freely, but
    at every width there is a definite intrinsic height.

   <dt>Type 4: replaced elements without an intrinsic ratio

   <dd>These are replaced elements that have no relation between their width
    and height. Certain <span class=index id=svg0>SVG</span> images are
    examples of this.
  </dl>

  <p>For <span class=index id=raster-images>raster images</span> without
   reliable resolution information, a size of 1 px unit per image source
   pixel must be assumed. These images thus are of type 1.

  <div class=example>
   <p>E.g., in the section on <a href="#inline-replaced">inline replaced
    elements,</a> if the replaced element is an HTML document and the height
    is specified as &lsquo;<code class=css>auto</code>&rsquo;, e.g.:

   <pre>
... &lt;object data="example.html"
      style="width: 30em; height: auto">&lt;/object>...
</pre>

   <p>then the used height will be 150px, which is unlikely to be the real
    height of the example.html document. But if the height is specified as
    &lsquo;<code class=css>complex</code>&rsquo;, e.g.:

   <pre>
... &lt;object data="example.html"
      style="width: 30em; height: complex">&lt;/object>...
</pre>

   <p>then the height will be the height the example.html document normally
    has when displayed on its own with the given width. This enables almost
    seamless integration of external text in a document, without a scrollbar
    or similar. (The external text is still displayed with its own style
    sheet.)
  </div>

  <div class=example>
   <p>An (external) mathematical formula in MathML is an example of replaced
    content with an intrinsic width and height. Assume the file m.mml
    contains a formula, then the HTML fragment

   <pre>
... derive &lt;img src="m.mml"
alt="that the sum of p(i) for i greater than 0 equals N">
for the case...
</pre>

   <p>renders the formula at its intrinsic size.
  </div>

  <p class=issue>The mathematical formula would however benefit from a way to
   negotiate available space against intrinsic size, or even negotiate
   available space against the number of boxes, so that the formula could be
   broken in two or more boxes and occupy space on two or more lines (similar
   to how lines of text are broken or words are hyphenated). But an easier
   solution might be to extend HTML with native support for math and CSS with
   math boxes, so that replaced elements remain single boxes.

  <h2 id=basic-flow-layout><span class=secno>6. </span>Basic flow layout</h2>

  <p>As described in the introduction, elements give rise to boxes and those
   boxes are laid out on a <a href="#canvas"><em>canvas.</em></a> Different
   kinds of boxes are laid out differently. This section describes the layout
   of one <a href="#flow"><em>flow</em></a> of boxes.

  <p>The approximate model for the layout of a flow of boxes is that sibling
   boxes are laid out one after the other in one long series with margins
   between them and parent boxes tightly wrap the series of child boxes. The
   &lsquo;<a href="#block-flow"><code
   class=property>block-flow</code></a>&rsquo; property of the parent
   determines if that series grows down (&lsquo;<code
   class=css>tb</code>&rsquo;), to the left (&lsquo;<code
   class=css>rl</code>&rsquo;), to the right (&lsquo;<code
   class=css>lr</code>&rsquo;) or up (&lsquo;<code
   class=css>bt</code>&rsquo;). Although the rules below depend on terms that
   are only defined further down, the rules are given at the outset, to
   provide at least an approximate model of how boxes are positioned relative
   to one another.

  <p>The following rules define the position of <a
   href="#block-level"><em>block-level</em></a> boxes relative to the box
   that is their <a href="#flow-root"><em>flow root.</em></a> Other sections
   and other modules describe how other boxes are laid out. E.g., floating
   boxes are described further down in this module and absolutely positioned
   boxes are described in the Absolute Positioning module <a href="#CSS3POS"
   rel=biblioentry>[CSS3POS]<!--{{CSS3POS}}--></a>.

  <p>Before applying these rules, the width, height and margins of each box
   must be computed as described in the sections <a
   href="#Calculating">“Calculating widths, heights and margins”</a> and
   <a href="#Collapsing">“Collapsing margins.”</a>

  <ul>
   <li>
    <p>Consider the first box (in document order) of a set of sibling boxes
     that all belong to the same flow. There are four cases:

    <ol>
     <li>
      <p>If none of its margins <a href="#collapse.">collapse</a> with its
       parent's <a href="#head">head</a> margin and the box is not <a
       href="#collapsed-through">collapsed through,</a> then the box is
       placed such that the box's <a href="#headside">headside</a> <a
       href="#margin-edge.">margin edge</a> touches the parent's <a
       href="#head">head</a> <a href="#content-edge.">content edge</a>, the
       <a href="#startside">startside</a> <a href="#margin-edge.">margin
       edge</a> touches the parent's <a href="#start">start</a> <a
       href="#content-edge.">content edge</a> and the <a
       href="#endside">endside</a> <a href="#margin-edge.">margin edge</a>
       touches the parent's <a href="#end">end</a> <a
       href="#content-edge.">content edge.</a>

      <p class=issue>[Add illustration.]

     <li>
      <p>If none of its margins <a href="#collapse.">collapse</a> with its
       parent's <a href="#head">head</a> margin but the box is <a
       href="#collapsed-through">collapsed through,</a> then its <a
       href="#content-edge.">content edge</a> (which is also its <a
       href="#border-edge.">border edge</a> and <a
       href="#padding-edge.">padding edge</a>) is defined as what it would
       have been if the box had a non-zero <a href="#tailside">tailside</a>
       border. (I.e., recompute the margins given that border and then apply
       the previous rule.)

      <p class=note>Note that the box is effectively invisible, but the edges
       have to be defined to position any descendants, such as floating or
       absolutely positioned children.

     <li>
      <p>If exactly one of its margins collapses with its parent's <a
       href="#head">head</a> margin, then the box is placed such that the
       box's <a href="#headside">headside</a> <a href="#border-edge.">border
       edge</a> touches the parent's <a href="#head">head</a> <a
       href="#content-edge.">content edge,</a> the <a
       href="#startside">startside</a> <a href="#margin-edge.">margin
       edge</a> touches the parent's <a href="#start">start</a> <a
       href="#content-edge.">content edge</a> and the <a
       href="#endside">endside</a> <a href="#margin-edge.">margin edge</a>
       touches the parent's <a href="#end">end</a> <a
       href="#content-edge.">content edge.</a>

      <p class=issue>[Add illustration.]

     <li>
      <p>If two of its margins collapse with its parent's <a
       href="#head">head</a> margin, then its position is such that its <a
       href="#content-edge.">content edge</a> (which is also its <a
       href="#border-edge.">border edge</a>) touches its parent's <a
       href="#head">head</a> <a href="#border-edge.">border edge</a> and that
       its <a href="#startside">startside</a> <a href="#margin-edge.">margin
       edge</a> touches its parent's <a href="#start">start</a> <a
       href="#content-edge.">content edge</a> and its <a
       href="#endside">endside</a> <a href="#margin-edge.">margin edge</a>
       touches the parent's <a href="#end">end</a> <a
       href="#content-edge.">content edge.</a>

      <p class=note>Note that the box is effectively invisible, but the edges
       have to be defined to position any descendants.
    </ol>

   <li>
    <p>For a box that has a preceding sibling in the same flow there are two
     cases:

    <ol>
     <li>
      <p>If the box is not <a href="#collapsed-through">collapsed
       through,</a> it is positioned such that its <a
       href="#margin-edge.">margin edge</a> touches the parent's <a
       href="#start">start</a> <a href="#content-edge.">content edge</a> and
       <a href="#end">end</a> <a href="#content-edge.">content edge</a> and
       such that its content edge on the touches the <span
       class=issue>[...]</span>

     <li>
      <p>A box that has a preceding sibling in the same flow and that is <a
       href="#collapsed-through">collapsed through,</a> <span
       class=issue>[...]</span>
    </ol>
  </ul>

  <h2 id=types-of-boxes><span class=secno>7. </span>Types of boxes</h2>