csswg-drafts/css-break/Overview.html at 871d456f0504861d57901fa649d0d7b24a0cceeb · 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 profile="http://www.w3.org/2006/03/hcard">
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>
  <title>CSS Fragmentation Module Level 3</title>

  <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 id=x><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 id=css-fragmentation-module>CSS Fragmentation Module Level 3</h1>

   <h2 class="no-num no-toc">Editor's Draft 10 October 2013</h2>

   <dl>
    <dt>This version:</dt>
    <!--<dd><a href="http://dev.w3.org/csswg/css3-break/">http://www.w3.org/TR/2013/WD-css3-break-20131010/</a></dd>-->

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

    <dt>Latest version:

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

    <dt>Editor's Draft:

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

    <dt>Previous version:

    <dd><a
     href="http://www.w3.org/TR/2012/WD-css3-break-20120228/">http://www.w3.org/TR/2012/WD-css3-break-20120228/</a>

    <dt>Editors:

    <dd class=vcard><span class=fn>Rossen Atanassov</span>, <span
     class=org><a class="fn org url"
     href="http://www.microsoft.com/">Microsoft Corporation</a></span>, <span
     class=email>ratan@microsoft.com</span>

    <dd class=vcard><span class=fn><a class=fn
     href="http://fantasai.inkedblade.net/contact">Elika J.
     Etemad</a></span>, <span class=org><a class="fn org url"
     href="http://www.mozilla.org/">Mozilla Corporation</a></span>

    <dt>Issues List:

    <dd><a
     href="http://wiki.csswg.org/spec/css3-break">http://wiki.csswg.org/spec/css3-break</a>

    <dt>Feedback:

    <dd><a
     href="mailto:www-style@w3.org?subject=%5Bcss3-break%5D%20feedback">www-style@w3.org</a>
     with subject line “<kbd>[css3-break] <var>… message topic
     …</var></kbd>” (<a
     href="http://lists.w3.org/Archives/Public/www-style/"
     rel=discussion>archives</a>)
   </dl>
   <!--begin-copyright-->
   <p class=copyright id=copyright-copy-year-2000-w3creg-mit-erci><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" id=abstract> Abstract</h2>

  <p id=this-module-describes-the-fragmentation-> This module describes the
   fragmentation model that partitions a flow into pages. It builds on the
   Page model module and introduces and defines the fragmentation model. It
   adds functionality for pagination, breaking variable fragment size and
   orientation, widows and orphans.

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

  <p id=this-is-a-public-copy-of-the-editors-dra>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 id=the-archived-public-mailing-list-www-sty>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-break%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-break” in the subject, preferably like this:
   “[<!---->css3-break<!---->] <em>…summary of comment…</em>”

  <p id=this-document-was-produced-by-the-css-wo>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 id=this-document-was-produced-by-a-group-op>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 following features are at risk: …</p>-->

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

  <ul class=toc>
   <li id=introduction-1.1.-module-interactions-1.><a href="#intro"><span
    class=secno>1. </span> Introduction</a>
    <ul class=toc>
     <li id=module-interactions-><a href="#placement"><span class=secno>1.1.
      </span> Module Interactions</a>

     <li id=values-><a href="#values"><span class=secno>1.2. </span>
      Values</a>
    </ul>

   <li id=fragmentation-model-and-terminology-><a
    href="#fragmentation-model"><span class=secno>2. </span> Fragmentation
    Model and Terminology</a>

   <li id=controlling-breaks-3.1.-breaks-between-b><a
    href="#breaking-controls"><span class=secno>3. </span> Controlling
    Breaks</a>
    <ul class=toc>
     <li id=breaks-between-boxes-the-break-before-an><a
      href="#break-between"><span class=secno>3.1. </span> Breaks Between
      Boxes: the ‘<code class=property>break-before</code>’ and ‘<code
      class=property>break-after</code>’ properties</a>
      <ul class=toc>
       <li class=no-num id=generic-break-values-><a
        href="#generic-break-values"> Generic Break Values</a>

       <li class=no-num id=page-break-values-><a href="#page-break-values">
        Page Break Values</a>

       <li class=no-num id=column-break-values-><a
        href="#column-break-values"> Column Break Values</a>

       <li class=no-num id=region-break-values-><a
        href="#region-break-values"> Region Break Values</a>
      </ul>

     <li id=breaks-within-boxes-the-break-inside-pro><a
      href="#break-within"><span class=secno>3.2. </span> Breaks Within
      Boxes: the ‘<code class=property>break-inside</code>’ property</a>

     <li id=breaks-between-lines-orphans-widows-><a
      href="#widows-orphans"><span class=secno>3.3. </span> Breaks Between
      Lines: ‘<code class=property>orphans</code>’, ‘<code
      class=property>widows</code>’</a>

     <li id=page-break-aliases-the-page-break-before><a
      href="#page-break-properties"><span class=secno>3.4. </span> Page Break
      Aliases: the ‘<code class=property>page-break-before</code>’,
      ‘<code class=property>page-break-after</code>’, and ‘<code
      class=property>page-break-inside</code>’ properties</a>
    </ul>

   <li id=rules-for-breaking-4.1.-possible-break-p><a
    href="#breaking-rules"><span class=secno>4. </span> Rules for
    Breaking</a>
    <ul class=toc>
     <li id=possible-break-points-><a href="#possible-breaks"><span
      class=secno>4.1. </span> Possible Break Points</a>

     <li id=types-of-breaks-><a href="#break-types"><span class=secno>4.2.
      </span> Types of Breaks</a>

     <li id=forced-breaks-><a href="#forced-breaks"><span class=secno>4.3.
      </span> Forced Breaks</a>

     <li id=unforced-breaks-><a href="#unforced-breaks"><span
      class=secno>4.4. </span> Unforced Breaks</a>

     <li id=optimizing-unforced-breaks-><a href="#best-breaks"><span
      class=secno>4.5. </span> Optimizing Unforced Breaks</a>
    </ul>

   <li id=box-model-for-breaking-5.1.-breaking-int><a
    href="#breaking-boxes"><span class=secno>5. </span> Box Model for
    Breaking</a>
    <ul class=toc>
     <li id=breaking-into-varying-size-fragmentainer><a
      href="#varying-size-boxes"><span class=secno>5.1. </span> Breaking into
      Varying-size Fragmentainers</a>

     <li id=margins-at-breaks-><a href="#break-margins"><span
      class=secno>5.2. </span> Margins at Breaks</a>

     <li id=splitting-boxes-at-breaks-><a href="#break-decoration"><span
      class=secno>5.3. </span> Splitting Boxes at Breaks</a>

     <li id=transforms-positioning-and-pagination-><a
      href="#transforms"><span class=secno>5.4. </span> Transforms,
      Positioning, and Pagination</a>
    </ul>

   <li id=conformance-6.1.-document-conventions-6.><a
    href="#conformance"><span class=secno>6. </span> Conformance</a>
    <ul class=toc>
     <li id=document-conventions-><a href="#conventions"><span
      class=secno>6.1. </span> Document Conventions</a>

     <li id=conformance-classes-><a href="#conformance-classes"><span
      class=secno>6.2. </span>Conformance Classes</a>

     <li id=partial-implementations-><a href="#partial"><span
      class=secno>6.3. </span> Partial Implementations</a>

     <li id=experimental-implementations-><a href="#experimental"><span
      class=secno>6.4. </span> Experimental Implementations</a>

     <li id=non-experimental-implementations-><a href="#testing"><span
      class=secno>6.5. </span> Non-Experimental Implementations</a>

     <li id=cr-exit-criteria-><a href="#cr-exit-criteria"><span
      class=secno>6.6. </span> CR Exit Criteria</a>
    </ul>

   <li class=no-num id=changes-><a href="#changes"> Changes</a>

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

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

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

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

   <li class=no-num id=property-index0><a href="#property-index">Property
    index</a>
  </ul>
  <!--end-toc-->

  <h2 id=intro><span class=secno>1. </span> Introduction</h2>

  <p id=this-section-is-not-normative.><em>This section is not
   normative.</em>

  <p id=in-paged-media-e.g.-paper-transparencies> In paged media (e.g.,
   paper, transparencies, photo album pages, pages displayed on computer
   screens as printed output simulations), as opposed to <a
   href="http://www.w3.org/TR/CSS2/media.html#continuous-media-group">
   continuous media</a>, the content of the document is split into one or
   more discrete display surfaces. In order to avoid awkward breaks (such as
   halfway through a line of text), the layout engine must be able to shift
   around content that would fall across the page break. This process is
   called <dfn id=pagination>pagination</dfn>.

  <p id=in-css-in-addition-to-paged-media-certai> In CSS, in addition to
   paged media, certain layout features such as <a
   href="http://www.w3.org/TR/css3-regions/">regions</a> <a
   href="#CSS3-REGIONS"
   rel=biblioentry>[CSS3-REGIONS]<!--{{CSS3-REGIONS}}--></a> and <a
   href="http://www.w3.org/TR/css3-multicol/">multi-column layout</a> <a
   href="#CSS3COL" rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a> create a
   similarly fragmented environment. The generic term for breaking content
   across containers is <dfn id=fragmentation>fragmentation</dfn>. This
   module explains how content breaks across <a
   href="#fragmentation-container"><i>fragmentation containers</i></a> (<a
   href="#fragmentainer"><i>fragmentainers</i></a>) such as pages and columns
   and how such breaks can be <a href="#breaking-controls">controlled by the
   author</a>.

  <h3 id=placement><span class=secno>1.1. </span> Module Interactions</h3>

  <p id=this-module-replaces-and-extends-the-pag> This module replaces and
   extends the pagination controls defined in <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{CSS21}}--></a> <a
   href="http://www.w3.org/TR/CSS21/page.html#page-breaks">section 13.3</a>
   and in <a href="#CSS3PAGE"
   rel=biblioentry>[CSS3PAGE]<!--{{CSS3PAGE}}--></a>.</p>
  <!-- End section "Modules Interactions" -->

  <h3 id=values><span class=secno>1.2. </span> Values</h3>

  <p id=this-specification-follows-the-css-prope> This specification follows
   the <a href="http://www.w3.org/TR/CSS21/about.html#property-defs"> CSS
   property definition conventions</a> from <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
   this specification are defined in CSS Level 2 Revision 1 <a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS modules may expand
   the definitions of these value types: for example [[CSS-VALUES]], when
   combined with this module, adds the ‘<code class=css>initial</code>’
   value to the properties defined here.

  <p id=in-addition-to-the-property-specific-val> In addition to the
   property-specific values listed in their definitions, all properties
   defined in this specification also accept the <a
   href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">
   inherit</a> keyword as their property value. For readability it has not
   been repeated explicitly.

  <h2 id=fragmentation-model><span class=secno>2. </span> Fragmentation Model
   and Terminology</h2>

  <dl>
   <dt><dfn id=fragmentation-container>fragmentation container</dfn> (<dfn
    id=fragmentainer>fragmentainer</dfn>)

   <dd> A box—such as a page box, column box, or region—that contains a
    portion (or all) of a <a href="#fragmented-flow"><i>fragmented
    flow</i></a>. Fragmentainers can be pre-defined, or generated as needed.
    When breakable content overflows a fragmentainer in the block dimension,
    instead of overflowing it breaks into the next container in its <a
    href="#fragmentation-context"><i>fragmentation context</i></a>.

   <dt><dfn id=fragmentation-context>fragmentation context</dfn>

   <dd> An ordered series of <a
    href="#fragmentainer"><i>fragmentainers</i></a>, such as created by a <a
    href="http://www.w3.org/TR/css3-multicol/">multi-column element</a>, a
    chain of <a href="http://www.w3.org/TR/css3-regions">CSS regions</a>, or
    a <a href="http://www.w3.org/TR/css3-page/">paged media display</a>. A
    given fragmentation context can only have one block flow direction across
    all its <a href="#fragmentainer"><i>fragmentainers</i></a>.

   <dt><dfn id=fragmented-flow>fragmented flow</dfn>

   <dd> A content flow that is being laid out in a <a
    href="#fragmentation-context"><i>fragmentation context</i></a>. The <a
    href="#fragmented-flow"><i>fragmented flow</i></a> consists of the
    content of a (possibly anonymous) box called the <dfn
    id=fragmenation-root>fragmenation root</dfn>.

   <dt><dfn id=fragmentation-direction>fragmentation direction</dfn>

   <dd> The block flow direction of the <a
    href="#fragmentation-context"><i>fragmentation context</i></a>, i.e. the
    direction in which content is fragmented. (In this level of CSS, content
    only fragments in one dimension.)

   <dt><dfn id=fragmentation0>fragmentation</dfn>

   <dd> The process of splitting a content flow across the <a
    href="#fragmentainer"><i>fragmentainers</i></a> that form a <a
    href="#fragmentation-context"><i>fragmentation context</i></a>.

   <dt><dfn id=box-fragment>box fragment</dfn> or <dfn
    id=fragment>fragment</dfn>

   <dd> The portion of a box that belongs to exactly one <a
    href="#fragmentainer"><i>fragmentainer</i></a>. A box in continuous flow
    always consists of only one fragment. A box in a fragmented flow consists
    of one or more fragments.

   <dt><dfn id=remaining-fragmentainer-extent>remaining fragmentainer
    extent</dfn>

   <dd> The remaining extent in the <a
    href="#fragmentainer"><i>fragmentainer</i></a> available to a given
    element, i.e. between the end of preceding content in <a
    href="#fragmentainer"><i>fragmentainer</i></a> and the edge of the <a
    href="#fragmentainer"><i>fragmentainer</i></a>.
  </dl>

  <p id=each-fragmentation-break-hereafter-break> Each <dfn
   id=fragmentation-break>fragmentation break</dfn> (hereafter, <dfn
   id=break>break</dfn>) ends layout of the fragmented box in the current <a
   href="#fragmentainer"><i>fragmentainer</i></a> and causes the remaining
   content to be laid out in the next <a
   href="#fragmentainer"><i>fragmentainer</i></a>, in some cases causing a
   new <a href="#fragmentainer"><i>fragmentainer</i></a> to be generated to
   hold the deferred content. When multiple flows are laid out in parallel,
   fragmentation is performed independently in each flow.

  <p id=breaking-a-fragmentainer-f-effectively-s> Breaking a <i>fragmentainer
   <var>F</var> effectively splits the <a
   href="#fragmentainer"><i>fragmentainer</i></a> into two <a
   href="#fragmentainer"><i>fragmentainers</i></a> (<var>F<sub>1</sub></var>
   and <var>F<sub>2</sub></var>). The only difference is that the type of
   break between the two pieces <var>F<sub>1</sub></var> and
   <var>F<sub>2</sub></var> is the <a href="#break-types">type of break</a>
   created by the <a href="#fragmentation-context"><i>fragmentation
   context</i></a> that split <var>F</var>, not the type of break normally
   created by <var>F</var>‘<code class=css>s own <a
   href="#fragmentation-context"><i>fragmentation context</i></a>.
   </code></i>

  <p class=note id=breaking-inline-content-into-lines-is-an> Breaking inline
   content into lines is another form of fragmentation, and similarly creates
   box fragments when it breaks <a
   href="http://www.w3.org/TR/CSS21/visuren.html#inline-boxes">inline
   boxes</a> across <a
   href="http://www.w3.org/TR/CSS21/visuren.html#line-box">line boxes</a>.
   However, inline breaking is not covered here; see <a href="#CSS3TEXT"
   rel=biblioentry>[CSS3TEXT]<!--{{CSS3TEXT}}--></a> and the
   ’box-decoration-break' property in <a href="#CSS3BG"
   rel=biblioentry>[CSS3BG]<!--{{CSS3BG}}--></a>.

  <h2 id=breaking-controls><span class=secno>3. </span> Controlling Breaks</h2>

  <p id=the-following-sections-explain-how-break> The following sections
   explain how breaks are controlled in a <a
   href="#fragmented-flow"><i>fragmented flow</i></a>. A page/column/region
   break opportunity between two boxes is under the influence of the
   containing block's ‘<a href="#break-inside"><code
   class=property>break-inside</code></a>’ property, the ‘<a
   href="#break-after"><code class=property>break-after</code></a>’
   property of the preceding element, and the ‘<a
   href="#break-before"><code class=property>break-before</code></a>’
   property of the following element. A page/column/region break opportunity
   between line boxes is under the influence of the containing block's ‘<a
   href="#break-inside"><code class=property>break-inside</code></a>’,
   ‘<a href="#widows"><code class=property>widows</code></a>’, and ‘<a
   href="#orphans"><code class=property>orphans</code></a>’ properties. A
   fragmentation break can be allowed, forced, or discouraged depending on
   the values of these properties. In the case of forced page breaks, the
   author can also specify on which page (<a
   href="http://www.w3.org/TR/css3-page/#left-right-first">left or right</a>)
   the subsequent content should resume.

  <p id=see-the-section-on-rules-for-breaking-fo> See the section on <a
   href="#breaking-rules">rules for breaking</a> for the exact rules on how
   these properties affect fragmentation.

  <h3 id=break-between><span class=secno>3.1. </span> Breaks Between Boxes:
   the ‘<a href="#break-before"><code
   class=property>break-before</code></a>’ and ‘<a
   href="#break-after"><code class=property>break-after</code></a>’
   properties</h3>

  <table class=propdef summary="property definition">
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=break-before>break-before</dfn>, <dfn
      id=break-after>break-after</dfn>

    <tr>
     <th>Value:

     <td>auto | avoid | always | any | avoid-page | page | left | right |
      recto | verso | avoid-column | column | avoid-region | region

    <tr>
     <th>Initial:

     <td>auto

    <tr>
     <th>Applies to:

     <td>block-level elements, table row groups, table rows (but see prose)

    <tr>
     <th>Inherited:

     &l
6109
t;td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>specified value
  </table>

  <p id=these-properties-specify-pagecolumnregio> These properties specify
   page/column/region break behavior before/after the generated box. The <dfn
   id=forced-break-values>forced break values</dfn> ‘<code
   class=css>always</code>’, ‘<code class=css>any</code>’, ‘<code
   class=css>left</code>’, ‘<code class=css>right</code>’, ‘<code
   class=css>recto</code>’, ‘<code class=css>verso</code>’, ‘<code
   class=css>page</code>’, ‘<code class=css>column</code>’ and ‘<code
   class=css>region</code>’ create a <a href="#forced-breaks">forced
   break</a> in the flow while the <dfn id=avoid-break-values>avoid break
   values</dfn> ‘<code class=css>avoid</code>’, ‘<code
   class=css>avoid-page</code>’, ‘<code class=css>avoid-column</code>’
   and ‘<code class=css>avoid-region</code>’ indicate that content should
   be kept together.

  <p id=since-breaks-are-only-allowed-between-si> Since breaks are only
   allowed between siblings, not between a box and its container (see <a
   href="#possible-breaks">Possible Break Points</a>), a ‘<a
   href="#break-before"><code class=property>break-before</code></a>’ value
   on a first-child box is propagated to its container. Likewise a ‘<a
   href="#break-after"><code class=property>break-after</code></a>’ value
   on a last-child box is propagated to its container. (Conflicting values <a
   href="#forced-breaks">combine</a> as defined below.)

  <p id=values-for-break-before-and-break-after-> Values for ‘<a
   href="#break-before"><code class=property>break-before</code></a>’ and
   ‘<a href="#break-after"><code class=property>break-after</code></a>’
   are defined in the sub-sections below. User Agents must apply these
   properties to boxes in the normal flow of the <i>fragmentation root</i>.
   User agents should also apply these properties to floated boxes whose
   containing block is in the normal flow of the root fragmented element.
   User agents may also apply these properties to other boxes.

  <h4 class=no-num id=generic-break-values> Generic Break Values</h4>

  <p id=these-values-have-an-effect-regardless-o> These values have an effect
   regardless of the type of fragmented context containing the flow.

  <dl>
   <dt>‘<code class=css>auto</code>’

   <dd> Neither force nor forbid a break before/after the principle box.

   <dt>‘<code class=css>avoid</code>’

   <dd> Avoid a break before/after the principle box.

   <dt>‘<code class=css>always</code>’

   <dd> Always force a break (of all possible types, through all
    fragmentation contexts) before/after the principle box.

   <dt>‘<code class=css>any</code>’

   <dd> Force a break of any type, whichever is the type of the deepest
    fragmentation context, before/after the principle box.
  </dl>

  <h4 class=no-num id=page-break-values> Page Break Values</h4>

  <p id=these-values-only-have-an-effect-in-pagi> These values only have an
   effect in paginated contexts; if the flow is not paginated, they have no
   effect.

  <dl>
   <dt>‘<code class=css>avoid-page</code>’

   <dd> Avoid a page break before/after the principle box.

   <dt>‘<code class=css>page</code>’

   <dd> Always force a page break before/after the principle box.

   <dt>‘<code class=css>left</code>’

   <dd> Force one or two page breaks before/after the principle box so that
    the next page is formatted as a left page.

   <dt>‘<code class=css>right</code>’

   <dd> Force one or two page breaks before/after the principle box so that
    the next page is formatted as a right page.

   <dt>‘<code class=css>recto</code>’

   <dd> Force one or two page breaks before/after the principal box so that
    the next page is formatted as either a left page or a right page,
    whichever is second (according to the page progression) in a page spread.

   <dt>‘<code class=css>verso</code>’

   <dd> Force one or two page breaks before/after the principal box so that
    the next page is formatted as either a left page or a right page,
    whichever is first (according to the page progression) in a page spread.
  </dl>

  <h4 class=no-num id=column-break-values> Column Break Values</h4>

  <p id=these-values-only-have-an-effect-in-mult> These values only have an
   effect in multi-column contexts; if the flow is not within a multi-column
   context, they have no effect.

  <dl>
   <dt>‘<code class=css>avoid-column</code>’

   <dd> Avoid a column break before/after the principle box.

   <dt>‘<code class=css>column</code>’

   <dd> Always force a column break before/after the principle box.
  </dl>

  <h4 class=no-num id=region-break-values> Region Break Values</h4>

  <p id=these-values-only-have-an-effect-in-mult0> These values only have an
   effect in multi-region contexts; if the flow is not linked across multiple
   regions, these values have no effect.

  <dl>
   <dt>‘<code class=css>avoid-region</code>’

   <dd> Avoid a region break before/after the principle box.

   <dt>‘<code class=css>region</code>’

   <dd> Always force a region break before/after the principle box.
  </dl>

  <h3 id=break-within><span class=secno>3.2. </span> Breaks Within Boxes: the
   ‘<a href="#break-inside"><code class=property>break-inside</code></a>’
   property</h3>

  <table class=propdef summary="property definition">
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=break-inside>break-inside</dfn>

    <tr>
     <th>Value:

     <td>auto | avoid | avoid-page | avoid-column | avoid-region

    <tr>
     <th>Initial:

     <td>auto

    <tr>
     <th>Applies to:

     <td>elements in the normal flow that that establish formatting contexts,
      or are block containers, table row groups, or table rows

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>specified value
  </table>

  <p id=this-property-specifies-pagecolumnregion> This property specifies
   page/column/region break behavior within the generated box. Values have
   the following meanings:

  <dl>
   <dt>‘<code class=css>auto</code>’

   <dd> Impose no additional breaking constraints within the box.

   <dt>‘<code class=css>avoid</code>’

   <dd> Avoid breaks within the box.

   <dt>‘<code class=css>avoid-page</code>’

   <dd> Avoid a page break within the box.

   <dt>‘<code class=css>avoid-column</code>’

   <dd> Avoid a column break within the box.

   <dt>‘<code class=css>avoid-region</code>’

   <dd> Avoid a region break within the box.
  </dl>

  <h3 id=widows-orphans><span class=secno>3.3. </span> Breaks Between Lines:
   ‘<a href="#orphans"><code class=property>orphans</code></a>’, ‘<a
   href="#widows"><code class=property>widows</code></a>’</h3>

  <table class=propdef summary="property definition">
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=orphans>orphans</dfn>, <dfn id=widows>widows</dfn>

    <tr>
     <th>Value:

     <td>&lt;integer&gt;

    <tr>
     <th>Initial:

     <td>2

    <tr>
     <th>Applies to:

     <td>block containers

    <tr>
     <th>Inherited:

     <td>yes

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

    <tr>
     <th>Computed value:

     <td>specified value
  </table>

  <p id=the-orphans-property-specifies-the-minim> The ‘<a
   href="#orphans"><code class=property>orphans</code></a>’ property
   specifies the minimum number of line boxes in a block container that must
   be left in a <a href="#fragment"><i>fragment</i></a> before a fragment
   break. The ‘<a href="#widows"><code class=property>widows</code></a>’
   property specifies the minimum number of line boxes of a block container
   that must be left in a <a href="#fragment"><i>fragment</i></a> after a
   break. Examples of how they are used to control fragmentation breaks are
   given <a href="#widows-orphans-example">below</a>.

  <p id=only-positive-integers-are-allowed-as-va> Only positive integers are
   allowed as values of ‘<a href="#orphans"><code
   class=property>orphans</code></a>’ and ‘<a href="#widows"><code
   class=property>widows</code></a>’. Negative values and zero are invalid
   must cause the declaration to be <a
   href="http://www.w3.org/TR/CSS21/conform.html#ignore">ignored</a>.

  <p id=if-a-block-contains-fewer-lines-than-the> If a block contains fewer
   lines than the value of ‘<a href="#widows"><code
   class=property>widows</code></a>’ or ‘<a href="#orphans"><code
   class=property>orphans</code></a>’, the rule simply becomes that all
   lines in the block must be kept together.

  <h3 id=page-break-properties><span class=secno>3.4. </span> Page Break
   Aliases: the ‘<code class=property>page-break-before</code>’, ‘<code
   class=property>page-break-after</code>’, and ‘<code
   class=property>page-break-inside</code>’ properties</h3>

  <p id=for-compatibility-with-css-level-2-uas-t>For compatibility with <a
   href="http://www.w3.org/TR/CSS21/page.html">CSS Level 2</a>, UAs that
   conform to <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
   must alias the ‘<code class=property>page-break-before</code>’,
   ‘<code class=property>page-break-after</code>’, and ‘<code
   class=property>page-break-inside</code>’ properties to ‘<a
   href="#break-before"><code class=property>break-before</code></a>’,
   ‘<a href="#break-after"><code class=property>break-after</code></a>’,
   and ‘<a href="#break-inside"><code
   class=property>break-inside</code></a>’ by treating the ‘<code
   class=css>page-break-*</code>’ properties as shorthands for the ‘<code
   class=css>break-*</code>’ properties with the following value mappings:

  <table class=data>
   <thead>
    <tr>
     <th>Shorthand (‘<code class=css>page-break-*</code>’) Values

     <th>Longhand (‘<code class=css>break-*</code>’) Values

   <tbody>
    <tr>
     <td>‘<code class=css>auto | left | right | avoid</code>’

     <td>‘<code class=css>auto | left | right | avoid</code>’

    <tr>
     <td>‘<code class=css>always</code>’

     <td>‘<code class=css>page</code>’
  </table>

  <h2 id=breaking-rules><span class=secno>4. </span> Rules for Breaking</h2>

  <p id=a-fragmented-flow-may-be-broken-across-f> A fragmented flow may be
   broken across fragmentainers at a number of <a
   href="#possible-breaks">possible break points</a>. In the case of <a
   href="#forced-breaks">forced breaks</a>, the UA is required to break the
   flow at that point. In the case of <a href="#unforced-breaks">unforced
   breaks</a>, the UA has to choose among the possible breaks that are
   allowed.

  <p id=some-content-is-not-fragmentable-for-exa> Some content is not
   fragmentable, for example many types of <a
   href="http://www.w3.org/TR/CSS21/conform.html#replaced-element">replaced
   elements</a> <a href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>
   (such as images or video), scrollable elements, or a single line of text
   content. Such content is considered <dfn id=monolithic>monolithic</dfn>:
   it contains no possible break points. In addition to any content which is
   not fragmentable, UAs may consider as <a
   href="#monolithic"><i>monolithic</i></a> any elements with ‘<code
   class=property>overflow</code>’ set to ‘<code class=css>auto</code>’
   or ‘<code class=css>scroll</code>’ and any elements with ‘<code
   class=css>overflow: hidden</code>’ and a non-‘<code
   class=css>auto</code>’ <a
   href="http://www.w3.org/TR/css3-writing-modes/#extent">logical height</a>
   (and no specified maximum logical height).

  <h3 id=possible-breaks><span class=secno>4.1. </span> Possible Break Points</h3>

  <p id=fragmentation-splits-boxes-in-the-block-> Fragmentation splits boxes
   in the block flow dimension. In block-and-inline flow, breaks may occur at
   the following places:

  <dl>
   <dt id=btw-blocks>Class 1

   <dd> Between sibling boxes of the following types:
    <dl>
     <dt>Block-parallel Fragmentation

     <dd> When the block flow direction of the siblings' containing block is
      parallel to that of the fragmentation context: block-level boxes, table
      row group boxes, table row boxes, multi-column column row boxes

     <dt>Block-perpendicular Fragmentation

     <dd> When the block flow direction of the siblings' containing block is
      perpendicular to that of the fragmentation context: table column group
      boxes, table column boxes, multi-column column boxes
    </dl>

   <dt id=btw-lines>Class 2

   <dd> Between line boxes inside a block container box.

   <dt id=end-block>Class 3

   <dd> Between the content edge of a block container box and the outer edges
    of its child content (margin edges of block-level children or line box
    edges for inline-level children) <em>if</em> there is a (non-zero) gap
    between them.
  </dl>

  <p class=note id=other-layout-models-may-add-breakpoints-> Other layout
   models may add breakpoints to the above classes.

  <p id=monolithic-breaking> The UA is not required to fragment the contents
   of <a href="#monolithic"><i>monolithic</i></a> elements, and may instead
   either slice the element's graphical representation as necessary to
   fragment it or treat its box as unbreakable and overflow the
   fragmentainer. In both cases it must treat the element as having ‘<code
   class=css>break-inside: avoid</code>’, i.e. only slice or overflow at
   the fragmentainer edge if there are no <a href="#possible-breaks">possible
   break points</a> on the fragmentainer.

  <p id=when-paginating-if-there-are-no-possible> When paginating, if there
   are no possible break points below the top of the page, and not all the
   content fits, the UA may break anywhere in order to avoid losing content
   off the edge of the page.

  <h3 id=break-types><span class=secno>4.2. </span> Types of Breaks</h3>

  <p id=there-are-different-types-of-breaks-in-c> There are different types
   of breaks in CSS, defined based on the type of fragmentainers they span:

  <dl>
   <dt><dfn id=page-break>page break</dfn>

   <dd> A break between two <a
    href="http://www.w3.org/TR/css3-page/#page-box">page boxes</a>. <a
    href="#CSS3PAGE" rel=biblioentry>[CSS3PAGE]<!--{{!CSS3PAGE}}--></a>

   <dt><dfn id=spread-break>spread break</dfn>

   <dd> A break between two page boxes that are not associated with <a
    href="http://www.w3.org/TR/css3-page/#facing-pages">facing pages</a>. A
    spread break is always also a page break. <a href="#CSS3PAGE"
    rel=biblioentry>[CSS3PAGE]<!--{{!CSS3PAGE}}--></a>

   <dt><dfn id=column-break>column break</dfn>

   <dd> A break between two <a
    href="http://www.w3.org/TR/css3-multicol/#column-box">column boxes</a>.
    Note that if the column boxes are on different pages, then the break is
    also a <a href="#page-break"><i>page break</i></a>. Similarly, if the
    column boxes are in different regions, then the break is also a <a
    href="#region-break"><i>region break</i></a>. <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a>

   <dt><dfn id=region-break>region break</dfn>

   <dd> A break between two <a
    href="http://www.w3.org/TR/css3-regions/#regions">regions</a>. Note that
    if the region boxes are on different pages, then the break is also a <a
    href="#page-break"><i>page break</i></a>. <a href="#CSS3-REGIONS"
    rel=biblioentry>[CSS3-REGIONS]<!--{{!CSS3-REGIONS}}--></a>
  </dl>

  <p class=note id=a-fifth-type-of-break-is-the-line-break-> A fifth type of
   break is the <dfn id=line-break>line break</dfn>, which is a break between
   two <a href="http://www.w3.org/TR/CSS21/visuren.html#line-box">line
   boxes</a>. These are not covered in this specification; see <a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.

  <h3 id=forced-breaks><span class=secno>4.3. </span> Forced Breaks</h3>

  <p id=a-forced-break-occurs-at-a-class-1-break> A forced break occurs at a
   <a href="#btw-blocks">Class 1 break point</a> if, among the ‘<a
   href="#break-after"><code class=property>break-after</code></a>’
   properties specified on or propagated to the earlier sibling box and the
   ‘<a href="#break-before"><code class=property>break-before</code></a>’
   properties specified on or propagated to the later sibling box there is at
   least one with a <i>forced break value</i>. (Thus a <i>forced break
   value</i> effectively overrides any <i>avoid break value</i> that also
   applies at that break point.)

  <p id=when-multiple-forced-break-values-apply-> When multiple <a
   href="#forced-break-values"><i>forced break values</i></a> apply to a
   single break point, they combine such that all types of break are honored
   and no content-empty page boxes are generated, except for at most one
   content-empty page as may be required by the <a
   href="#forced-break-values"><i>forced break values</i></a> to position
   ensuing content on a specific type of page. When ‘<code
   class=css>left</code>’, ‘<code class=css>right</code>’, ‘<code
   class=css>recto</code>’, and/or ‘<code class=css>verso</code>’ are
   combined, the value specified on the latest element in the flow wins.

  <p class=note id=a-page-break-must-also-occur-at-1-if-the> A page break
   must also occur at <a href="#btw-blocks">(1)</a> if the last line box
   above this margin and the first one below it do not have the same value
   for ‘<code class=property>page</code>’. See <a href="#CSS3PAGE"
   rel=biblioentry>[CSS3PAGE]<!--{{!CSS3PAGE}}--></a>

  <p id=when-a-forced-break-occurs-it-forces-ens> When a forced break occurs,
   it forces ensuing content into the next fragmentainer of the type
   associated with the break, breaking through as many fragmentation contexts
   as necessary until the specified break types are all satisfied. If the
   forced break is not contained within a matching type of fragmentation
   context, then the forced break has no effect.

  <h3 id=unforced-breaks><span class=secno>4.4. </span> Unforced Breaks</h3>

  <p id=while-breaking-controls-can-force-breaks> While <a
   href="#breaking-controls">breaking controls</a> can force breaks, they can
   also discourage them. The following rules control whether breaking at a <a
   href="#possible-breaks">possible break point</a> is allowed:

  <dl>
   <dt>Rule A

   <dd> A fragmented flow may break at <a href="#btw-blocks">(1)</a> only if
    all the ‘<a href="#break-after"><code
    class=property>break-after</code></a>’ and ‘<a
    href="#break-before"><code class=property>break-before</code></a>’
    values applicable to this break point allow it, which is when at least
    one of them forces a break or when all of them are ‘<code
    class=css>auto</code>’.

   <dt>Rule B

   <dd> However, if all of them are ‘<code class=css>auto</code>’ and a
    common ancestor of all the elements has a ‘<a
    href="#break-inside"><code class=property>break-inside</code></a>’
    value of ‘<code class=css>avoid</code>’, then breaking here is not
    allowed.