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<!-- bikeshed, update to latest WM wording -->
<!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=2015-01-20 name=dcterms.date>
<meta content="Rossen Atanassov" name=dcterms.creator>
<meta content="Microsoft Corporation" name=dcterms.creator>
<meta
content="<a class="fn" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>"
name=dcterms.creator>
<meta content="Mozilla Corporation" name=dcterms.creator>
<meta content=W3C name=dcterms.publisher>
<meta content="http://dev.w3.org/csswg/css3-break/"
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">
<style type=css>
.css::before, .css::after, .property::before, .property::after { content: none !important;}
</style>
<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=subtitle>Breaking the Web, one fragment at a
time</h2>
<h2 class="no-num no-toc">Editor's Draft 20 January 2015</h2>
<dl>
<dt>This version:
<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=%5Bcss-break%5D%20feedback">www-style@w3.org</a>
with subject line “<kbd>[css-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><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2015 <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
402E
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, columns, or regions. 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="http://www.w3.org/Style/CSS/members">CSS Working Group</a> (part of
the <a href="http://www.w3.org/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February
2004 W3C Patent Policy</a>. W3C maintains a <a
href="http://www.w3.org/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
6 of the W3C Patent Policy</a>.</p>
<!--end-status-->
<!--<p>The following features are at risk: …</p>-->
<p> See the <a href="#changes">Changes section</a> for significant changes
since the previous draft.
<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>
<ul class=toc>
<li><a href="#parallel-flows"><span class=secno>2.1. </span> Parallel
Fragmentation Flows</a>
<li><a href="#nested-flows"><span class=secno>2.2. </span> Nested
Fragmentation Flows</a>
</ul>
<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> Adjoining
Margins at Breaks</a>
<li><a href="#box-splitting"><span class=secno>5.3. </span> Splitting
Boxes</a>
<li><a href="#break-decoration"><span class=secno>5.4. </span>
Fragmented Borders and Backgrounds: the ‘<code
class=property>box-decoration-break</code>’ property</a>
<ul class=toc>
<li><a href="#joining-boxes"><span class=secno>5.4.1. </span> Joining
Boxes for ‘<code class=css>slice</code>’</a>
</ul>
<li><a href="#transforms"><span class=secno>5.5. </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"><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="#ref-CSS-REGIONS-1"
rel=biblioentry>[CSS-REGIONS-1]<!--{{CSS-REGIONS-1}}--></a> and <a
href="http://www.w3.org/TR/css3-multicol/">multi-column layout</a> <a
href="#ref-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="#ref-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="#ref-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="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 <a
href="#ref-CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>. Other CSS
modules may expand the definitions of these value types: for example <a
href="#ref-CSS3VAL" rel=biblioentry>[CSS3VAL]<!--{{CSS3VAL}}--></a>, 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>. (Descendants
of the <a href="#fragmentation-root"><i>fragmentation root</i></a> may
have other block flow directions, but fragmentation proceeds according to
the block flow direction applied to the <a
href="#fragmentation-root"><i>fragmentation root</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=fragmentation-root>fragmentation 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.
<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="#ref-CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>/<a href="#ref-CSS-TEXT-3"
rel=biblioentry>[CSS-TEXT-3]<!--{{CSS-TEXT-3}}--></a>.
<h3 id=parallel-flows><span class=secno>2.1. </span> Parallel Fragmentation
Flows</h3>
<p> When multiple flows are laid out parallel to each other, fragmentation
is performed independently in each flow. For example, if an element is
floated, then a forced break inside the float will not affect the content
wrapping outside the float (except insofar as it may increase the height
of the float).
<p> The following are examples of parallel flows whose contents will
fragment independently:
<ul>
<li>The contents of a float vs. the content wrapping outside the float.
<li>The contents of a float vs. the contents of an adjacent float.
<li>The contents of each table cell in a single table row.
<li>The contents of each grid item in a single grid row.
<li>The contents of each flex item in a flex layout row.
<li>The contents of absolutely-positioned elements that cover the same
range of their containing block's fragmentation context.
</ul>
<p> Content overflowing the content edge of a fixed-size box is considered
parallel to the content after the fixed-size box and follows the normal
fragmentation rules. Although overflowing content doesn't affect the size
of the <a href="#fragmentation-root"><i>fragmentation root</i></a> box, it
does increase the length of the <a href="#fragmented-flow"><i>fragmented
flow</i></a>, spilling into or generating additional <a
href="#fragmentainer"><i>fragmentainers</i></a> as necessary.
<h3 id=nested-flows><span class=secno>2.2. </span> Nested Fragmentation
Flows</h3>
<p> Breaking a <a href="#fragmentainer"><i>fragmentainer</i></a>
<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>’s own <a
href="#fragmentation-context"><i>fragmentation context</i></a>.
<div class=example> For example, if a region box is broken at a page
boundary, then the content of the region will be affected by a page break
at that point (but not by a region break). Note that when a multi-column
element breaks across pages, it generates a new row of columns on the next
page for the rest of its content, so that a page break within a
multi-column element is always both a page break and a column break.</div>
<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 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. A forced break overrides any break
restrictions acting at that break point. 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> ‘<a href="#always"><code class=css>always</code></a>’,
‘<a href="#any"><code class=css>any</code></a>’, ‘<a
href="#left"><code class=css>left</code></a>’, ‘<a href="#right"><code
class=css>right</code></a>’, ‘<a href="#recto"><code
class=css>recto</code></a>’, ‘<a href="#verso"><code
class=css>verso</code></a>’, ‘<a href="#page"><code
class=css>page</code></a>’, ‘<a href="#column"><code
class=css>column</code></a>’ and ‘<a href="#region"><code
class=css>region</code></a>’ create a <a href="#forced-breaks">forced
break</a> in the flow while the <dfn id=avoid-break-values>avoid break
values</dfn> ‘<a href="#avoid0"><code class=css>avoid</code></a>’,
‘<a href="#avoid-page0"><code class=css>avoid-page</code></a>’, ‘<a
href="#avoid-column0"><code class=css>avoid-column</code></a>’ and ‘<a
href="#avoid-region0"><code class=css>avoid-region</code></a>’ indicate
that content should be kept together.
<p> 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.) This propagation
stops before it breaks through the nearest matching fragmentation context.
<p> 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 <a
href="#fragmentation-root"><i>fragmentation root</i></a>. 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> These values have an effect regardless of the type of fragmented
context containing the flow.
<dl>
<dt><dfn id=auto>‘<code class=css>auto</code>’</dfn>
<dd> Neither force nor forbid a break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=avoid>‘<code class=css>avoid</code>’</dfn>
<dd> Avoid a break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=always>‘<code class=css>always</code>’</dfn>
<dd> Always force a break (of all possible types, through all
fragmentation contexts) before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=any>‘<code class=css>any</code>’</dfn>
<dd> Force a break of any type, whichever is the type of the deepest
fragmentation context, before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
</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><dfn id=avoid-page>‘<code class=css>avoid-page</code>’</dfn>
<dd> Avoid a page break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=page>‘<code class=css>page</code>’</dfn>
<dd> Always force a page break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=left>‘<code class=css>left</code>’</dfn>
<dd> Force one or two page breaks before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a> so that the next page is formatted as a left page.
<dt><dfn id=right>‘<code class=css>right</code>’</dfn>
<dd> Force one or two page breaks before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a> so that the next page is formatted as a right page.
<dt><dfn id=recto>‘<code class=css>recto</code>’</dfn>
<dd> Force one or two page breaks before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a> 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><dfn id=verso>‘<code class=css>verso</code>’</dfn>
<dd> Force one or two page breaks before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a> 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><dfn id=avoid-column>‘<code class=css>avoid-column</code>’</dfn>
<dd> Avoid a column break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=column>‘<code class=css>column</code>’</dfn>
<dd> Always force a column break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
</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><dfn id=avoid-region>‘<code class=css>avoid-region</code>’</dfn>
<dd> Avoid a region break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
<dt><dfn id=region>‘<code class=css>region</code>’</dfn>
<dd> Always force a region break before/after the <a
href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">principal
box</a>.
</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
element's principal box. Values have the following meanings:
<dl>
<dt><dfn id=auto0>‘<code class=css>auto</code>’</dfn>
<dd> Impose no additional breaking constraints within the box.
<dt><dfn id=avoid0>‘<code class=css>avoid</code>’</dfn>
<dd> Avoid breaks within the box.
<dt><dfn id=avoid-page0>‘<code class=css>avoid-page</code>’</dfn>
<dd> Avoid a page break within the box.
<dt><dfn id=avoid-column0>‘<code class=css>avoid-column</code>’</dfn>
<dd> Avoid a column break within the box.
<dt><dfn id=avoid-region0>‘<code class=css>avoid-region</code>’</dfn>
<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>
<em>before</em> a fragmentation 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> <em>after</em> 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 and 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="#ref-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>‘<a href="#always"><code class=css>always</code></a>’
<td>‘<a href="#page"><code class=css>page</code></a>’
</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="#ref-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 ‘<a
href="#auto0"><code class=css>auto</code></a>’ or ‘<code
class=css>scroll</code>’ and any elements with ‘<code
class=css>overflow: hidden</code>’ and a non-‘<a href="#auto0"><code
class=css>auto</code></a>’ <a
href="http://www.w3.org/TR/css3-writing-modes/#extent">logical height</a>
(and no specified maximum logical height).
<p> To guarantee progress, fragmentainers are assumend to have a minimum
extent of 1px regardless of their used size.
<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 A
<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: <a
href="http://www.w3.org/TR/2011/REC-CSS2-20110607/visuren.html#positioning-scheme">in-flow</a>
block-level boxes, a float and an immediately-adjacent in-flow or
floated box, 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 B
<dd> Between line boxes inside a block container box.
<dt id=end-block>Class C
<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. <!-- XXX add example -->
</dl>
<p class=note> There is no inherent prioritization among these classes of
break points. However, individual break points may be prioritized or
de-prioritized by using the <a href="#breaking-controls">breaking
controls</a>.
<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. In this case 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 other <a
href="#possible-breaks">possible break points</a> in the fragmentainer.
<p> When fragmenting, if there are no possible break points below the top
of the fragmentainer, and not all the content fits, the UA may break
anywhere in order to avoid losing content off the edge of the
fragmentainer.
<p class=note> Other layout models may add breakpoints to the above
classes. For example, <a href="#ref-CSS3-FLEXBOX"
rel=biblioentry>[CSS3-FLEXBOX]<!--{{CSS3-FLEXBOX}}--></a> adds certain
points within a flex formatting context to classes A and C.
<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>