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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://dublincore.org/documents/2008/08/04/dc-html/ 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="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 Fragmentation Module Level 3" name=dcterms.title>
<meta content=text name=dcterms.type>
<meta content=2013-02-11 name=dcterms.issued>
<meta content="http://dev.w3.org/csswg/css3-break/" name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://www.w3.org/TR/2013/ED-css3-break-20130211/"
name=dcterms.identifier>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1 id=css-fragmentation-module>CSS Fragmentation Module Level 3</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 11 February
2013</h2>
<dl>
<dt>This version:</dt>
<!--<dd><a href="http://www.w3.org/TR/2013/ED-css3-break-20130211/">http://www.w3.org/TR/2013/WD-css3-break-20130211/</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:</dt>
<dd><a href="mailto:www-style@w3.org?subject=%5Bcss-break%5D%20feedback"
>www-style@w3.org</a>
with subject line “<kbd>[css-break]
<var>… message topic …</var></kbd>”
(<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/"
>archives</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" id=abstract> Abstract</h2>
<p> 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>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-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>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the &l
77F3
t;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 following features are at risk: …</p>-->
<h2 class="no-num no-toc" id=contents> Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#intro"><span class=secno>1. </span> Introduction</a>
<ul class=toc>
<li><a href="#placement"><span class=secno>1.1. </span> Module
Interactions</a>
<li><a href="#values"><span class=secno>1.2. </span> Values</a>
</ul>
<li><a href="#fragmentation-model"><span class=secno>2. </span>
Fragmentation Model and Terminology</a>
<li><a href="#breaking-controls"><span class=secno>3. </span> Controlling
Breaks</a>
<ul class=toc>
<li><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><a href="#generic-break-values"> Generic Break
Values</a>
<li class=no-num><a href="#page-break-values"> Page Break Values</a>
<li class=no-num><a href="#column-break-values"> Column Break
Values</a>
<li class=no-num><a href="#region-break-values"> Region Break
Values</a>
</ul>
<li><a href="#break-within"><span class=secno>3.2. </span> Breaks Within
Boxes: the ‘<code class=property>break-inside</code>’ property</a>
<li><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><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><a href="#breaking-rules"><span class=secno>4. </span> Rules for
Breaking</a>
<ul class=toc>
<li><a href="#possible-breaks"><span class=secno>4.1. </span> Possible
Break Points</a>
<li><a href="#break-types"><span class=secno>4.2. </span> Types of
Breaks</a>
<li><a href="#forced-breaks"><span class=secno>4.3. </span> Forced
breaks</a>
<li><a href="#unforced-breaks"><span class=secno>4.4. </span> Unforced
Breaks</a>
<li><a href="#best-breaks"><span class=secno>4.5. </span> Optimizing
Unforced Breaks</a>
</ul>
<li><a href="#breaking-boxes"><span class=secno>5. </span> Box Model for
Breaking</a>
<ul class=toc>
<li><a href="#varying-size-boxes"><span class=secno>5.1. </span>
Breaking into Varying-size Fragmentainers</a>
<li><a href="#break-margins"><span class=secno>5.2. </span> Margins at
Breaks</a>
<li><a href="#break-decoration"><span class=secno>5.3. </span> Splitting
Boxes at Breaks</a>
<li><a href="#transforms"><span class=secno>5.4. </span> Transforms,
Positioning, and Pagination</a>
</ul>
<li><a href="#conformance"><span class=secno>6. </span> Conformance</a>
<ul class=toc>
<li><a href="#conventions"><span class=secno>6.1. </span> Document
Conventions</a>
<li><a href="#conformance-classes"><span class=secno>6.2.
</span>Conformance Classes</a>
<li><a href="#partial"><span class=secno>6.3. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>6.4. </span> Experimental
Implementations</a>
<li><a href="#testing">
8096
;<span class=secno>6.5. </span> Non-Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>6.6. </span> CR Exit
Criteria</a>
</ul>
<li class=no-num><a href="#changes"> Changes</a>
<li class=no-num><a href="#acknowledgments"> 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="#other-references">Other 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=intro><span class=secno>1. </span> Introduction</h2>
<p><em>This section is not normative.</em>
<p> 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> 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> 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> 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 [[CSS3VALUE]], when
combined with this module, adds the ‘<code class=css>initial</code>’
value to the properties defined here.
<p> 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> 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> 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> 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> 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 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> 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:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>specified value
</table>
<p> 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> 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.
<p> Values are defined in the sections below.
<h4 class=no-num id=generic-break-values> Generic Break Values</h4>
<p> 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> 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> 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> 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> 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><integer>
<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> 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> 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> 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>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> 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> 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> 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> Other layout models may add breakpoints to the above
classes.
<p> Since breaks are only allowed between siblings (1), not between a box
and its container, 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.
<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> 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> 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> 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> A forced break then occurs at <a href="#btw-blocks">(1)</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>.
<p> When multiple <a href="#forced-break-values"><i>forced break
values</i></a> apply to a single break point, they combine such 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> 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> When a forced break occurs, it forces ensuing content into the next
fragmenter 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> 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.
<dt>Rule C
<dd> Breaking at <a href="#btw-lines">(2)</a> is allowed only if the
number of line boxes between the break and the start of the enclosing
block box is the value of ‘<a href="#orphans"><code
class=property>orphans</code></a>’ or more, and the number of line
boxes between the break and the end of the box is the value of ‘<a
href="#widows"><code class=property>widows</code></a>’ or more.
<dt>Rule D
<dd> Additionally, breaking at <a href="#btw-blocks">(2)</a> or <a
href="#end-block">(3)</a> is allowed only if the ‘<a
href="#break-inside"><code class=property>break-inside</code></a>’
property of all ancestors is ‘<code class=css>auto</code>’.
</dl>
<p> If the above doesn't provide enough break points to keep content from
overflowing the page boxes, then rules A, B and D are dropped in order to
find additional breakpoints. In this case the UA may use the ‘<code
class=css>avoid</code>’s that are in effect at those points to weigh the
appropriateness of the new breakpoints; however, this specification does
not suggest a precise algorithm.
<p> If that still does not lead to sufficient break points, rule C is
dropped as well, to find still more break points.
<h3 id=best-breaks><span class=secno>4.5. </span> Optimizing Unforced
Breaks</h3>
<p>While CSS3 requires that a fragmented flow must break at allowed break
points in order to avoid overflowing the fragmentainers in its
fragmentation context, it does not define whether content breaks at a
particular <a href="#unforced-breaks">allowed break</a>. However, it is
recommended that user agents observe the following guidelines (while
recognizing that they are sometimes contradictory):