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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><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 28 March 2012</h2>
<dl>
<dt>This version:</dt>
<!-- <dd><a href="http://www.w3.org/TR/2012/ED-css3-break-20120328/">http://www.w3.org/TR/2012/WD-css3-break-20120328/</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>None
<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>Discussion:
<dd><a
href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
with subject line "<code>[css3-break] …message topic…</code>"
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2012 <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>), 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 <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-properties"><span class=secno>3.1. </span> Forced
breaks and keep-together: the ‘<code
class=property>break-before</code>’, ‘<code
class=property>break-after</code>’, ‘<code
class=property>break-inside</code>’ properties</a>
<li><a href="#widows-orphans"><span class=secno>3.2. </span> Breaks
inside elements: ‘<code class=property>orphans</code>’,
‘<code class=property>widows</code>’</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 Fragmenters</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>
Decorating Box Breaks</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="#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="#fragmenter"><i>fragmenters</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 <a href="#CSS3COLOR"
rel=biblioentry>[CSS3COLOR]<!--{{CSS3COLOR}}--></a>, when combined with
this module, expands the definition of the <color> value type as
used in this specification.
<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=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>, such as 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>.
<dt><dfn id=fragmenter>fragmenter</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>. Fragmenters can be pre-defined, or generated as needed.
When breakable content overflows a fragmenter 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="#fragmenter"><i>fragmenters</i></a>. A
given fragmentation context can only have one block flow direction across
all its fragmenters.
<dt><dfn id=fragmentation0>fragmentation</dfn>
<dd> The process of splitting a content flow across the <a
href="#fragmenter"><i>fragmenters</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="#fragmenter"><i>fragmenter</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.
</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="#fragmenter"><i>fragmenter</i></a> and causes the
remaining content to be laid out in the next <a
href="#fragmenter"><i>fragmenter</i></a>, in some cases causing a new
fragmenter 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 fragmenter F effectively splits the fragmenter into two
fragmenters (F<sub>1</sub> and F<sub>2</sub>). The only difference is that
the type of break between the two pieces F<sub>1</sub> and F<sub>2</sub>
is the <a href="#break-types">type of break</a> created by the
fragmentation context that split F, not the type of break normally created
by F's own fragmentation context.
<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 ‘<code
class=property>box-decoration-break</code>’ 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
fragmented flow. Five properties indicate where the user agent may or
should break the content flow. In the case of pagination, 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.
<h3 id=break-properties><span class=secno>3.1. </span> Forced breaks and
keep-together: the ‘<a href="#break-before"><code
class=property>break-before</code></a>’, ‘<a
href="#break-after"><code class=property>break-after</code></a>’,
‘<a href="#break-inside"><code
class=property>break-inside</code></a>’ properties</h3>
<table class=propdef summary="property definition">
<tbody>
<tr>
<td><em>Name:</em>
<td><dfn id=break-before>break-before</dfn>
<tr>
<td><em>Value:</em>
<td>auto | always | left | right | page | column | region | avoid |
avoid-page | avoid-column | avoid-region
<tr>
<td><em>Initial:</em>
<td>auto
<tr>
<td><em>Applies to:</em>
<td>block-level elements, table row groups, table rows (but see prose)
<tr>
<td><em>Inherited:</em>
<td>no
<tr>
<td><em>Percentages:</em>
<td>N/A
<tr>
<td><em>Media:</em>
<td>visual
<tr>
<td><em>Computed value:</em>
<td>specified value
</table>
<table class=propdef summary="property definition">
<tbody>
<tr>
<td><em>Name:</em>
<td><dfn id=break-after>break-after</dfn>
<tr>
<td><em>Value:</em>
<td>auto | always | left | right | page | column | region | avoid |
avoid-page | avoid-column | avoid-region
<tr>
<td><em>Initial:</em>
<td>auto
<tr>
<td><em>Applies to:</em>
<td>block-level elements, table row groups, table rows (but see prose)
<tr>
<td><em>Inherited:</em>
<td>no
<tr>
<td><em>Percentages:</em>
<td>N/A
<tr>
<td><em>Media:</em>
<td>visual
<tr>
<td><em>Computed value:</em>
<td>specified value
</table>
<table class=propdef summary="property definition">
<tbody>
<tr>
<td><em>Name:</em>
<td><dfn id=break-inside>break-inside</dfn>
<tr>
<td><em>Value:</em>
<td>auto | avoid | avoid-page | avoid-column | avoid-region
<tr>
<td><em>Initial:</em>
<td>auto
<tr>
<td><em>Applies to:</em>
<td>block-level elements, block containers, table row groups, table rows
(but see prose)
<tr>
<td><em>Inherited:</em>
<td>no
<tr>
<td><em>Percentages:</em>
<td>N/A
<tr>
<td><em>Media:</em>
<td>visual
<tr>
<td><em>Computed value:</em>
<td>specified value
</table>
<p> These properties describe page/column/region break behavior
before/after/inside the generated box. These values have the following
meaning:
<dl>
<dt> auto
<dd> Neither force nor forbid a break before/after/inside the principle
box.
<dt> always
<dd> Always force a break before/after the principle box.
<dt> avoid
<dd> Avoid a break before/after/inside the principle box.
<dt> left
<dd> Force one or two page breaks before/after the principle box so that
the next page is formatted as a left page.
<dt> right
<dd> Force one or two page breaks before/after the principle box so that
the next page is formatted as a right page.
<dt> page
<dd> Always force a page break before/after the principle box.
<dt> column
<dd> Always force a column break before/after the principle box.
<dt> region
<dd> Always force a region break before/after the principle box.
<dt> avoid-page
<dd> Avoid a page break before/after/inside the principle box.
<dt> avoid-column
<dd> Avoid a column break before/after/inside the principle box.
<dt> avoid-region
<dd> Avoid a region break before/after/inside the principle box.
</dl>
<p> A potential page/column/region break location is typically under the
influence of the parent element'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. When these properties have values other
than ‘<code class=css>auto</code>’, the <dfn
id=forced-break-values>forced break values</dfn> (‘<code
class=css>always</code>’, ‘<code class=css>left</code>’,
‘<code class=css>right</code>’, ‘<code
class=css>page</code>’, ‘<code class=css>column</code>’
and ‘<code class=css>region</code>’) take precedence over 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>’). See the section on <a
href="#breaking-rules">rules for breaking</a> for the exact rules on how
these properties affect forced breaks.
<p> User Agents must apply these properties to block-level boxes and to
table rows, table row groups, and—in the case of ‘<a
href="#break-inside"><code
class=property>break-inside</code></a>’—block containers in
the normal flow of the root fragmented element. 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.
<h3 id=widows-orphans><span class=secno>3.2. </span> Breaks inside
elements: ‘<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>
<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>
<table class=propdef summary="property definition">
<tbody>
<tr>
<th>Name:
<td><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.
<h2 id=breaking-rules><span class=secno>4. </span> Rules for Breaking</h2>
<p> A fragmented flow may be broken across fragmenters 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.
<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>When the block flow direction of the siblings' parent is parallel to
that of the fragmentation context
<dd> block-level boxes, table row group boxes, table row boxes,
multi-column column row boxes
<dt>When the block flow direction of the siblings' parent is
perpendicular to that of the fragmentation context
<dd> 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 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 child
and its parent, a ‘<a href="#break-before"><code
class=property>break-before</code></a>’ value on a first-child box
is propagated to its parent. Likewise a ‘<a
href="#break-after"><code class=property>break-after</code></a>’
value on a last-child box is propagated to its parent.
<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.
<p> The UA is not required to fragment the contents of scrollable elements
e.g. those with ‘<code class=property>overflow</code>’ set to
‘<code class=css>auto</code>’ or ‘<code
class=css>scroll</code>’, and may instead either graphically slice
their contents as necessary to fragment the element or treat the element
as unbreakable and overflow the fragmenter. In such cases it must treat
the element as having ‘<code class=css>break-inside:
avoid</code>’.
<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 fragmenters 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>. <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 ‘<code
class=css>right</code>’ or ‘<code class=css>left</code>’
values to position ensuing content on a right- or left-facing page. When
‘<code class=css>left</code>’ and ‘<code
class=css>right</code>’ are both 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>
<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 fragmenters 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):
<ul>
<li>Break as few times as possible.
<li>Make all fragmenters that don't end with a forced break appear to be
equally filled with content.
<li>Avoid breaking inside a replaced element.
</ul>
<div class=example id=widows-orphans-example>
<p> Suppose, for example, that the style sheet contains ‘<code
class=css>orphans : 4</code>’, ‘<code class=css>widows :
2</code>’, and there are 20 lines (line boxes) available at the
bottom of the current page, and the next block in normal flow is
considered for placement:</p>
<ul>
<li> If the block contains 20 line boxes or fewer, it should be placed on
the current page.
<li> If the block contains 21 or 22 line boxes, the second fragment of
the paragraph must not violate the ‘<a href="#widows"><code
class=property>widows</code></a>’ constraint, and so the second
fragment must contain at least two line boxes; likewise the first
fragment must contain at least four line boxes.
<li> If the block contains 23 line boxes or more, the first fragment
should contain 20 lines and the second fragment the remaining lines. But
if any fragment of the block is placed on the current page, that
fragment must contain at least four line boxes and the second fragment
at least two line boxes.
</ul>
<p> Now suppose that ‘<a href="#orphans"><code
class=property>orphans</code></a>’ is ‘<code
class=css>10</code>’, ‘<a href="#widows"><code
class=property>widows</code></a>’ is ‘<code
class=css>20</code>’, and there are 8 lines available at the bottom
of the current page:</p>
<ul>
<li> If the block contains 8 lines or fewer, it should be placed on the
current page.
<li> If the block contains 9 lines or more, it must NOT be split (that
would violate the ‘<a href="#orphans"><code
class=property>orphans</code></a>’ constraint), so it must move as
a block to the next page.
</ul>
</div>
<h2 id=breaking-boxes><span class=secno>5. </span> Box Model for Breaking</h2>
<p class=note> The sizing terminology used in this section is defined in
Appendix D of <a href="#CSS3-WRITING-MODES"
rel=biblioentry>[CSS3-WRITING-MODES]<!--{{CSS3-WRITING-MODES}}--></a>.
<h3 id=varying-size-boxes><span class=secno>5.1. </span> Breaking into
Varying-size Fragmenters</h3>
<p> When a flow is fragmented into varying-size fragmenters, the following
rules are observed for adapting layout:
<ul>
<li> Intrinsic sizes are calculated and maintained across the entire
element. Where an initial containing block size is needed to resolve an
intrinsic size, assume the size of the first fragmenter defining a
pagination context.
<li> Layout is performed per-fragmenter, with each fragmenter continuing
progress from the breakpoint on the previous, but recalculating sizes and
positions using its own size as if the entire element were fragmented
across fragmenters of this size. Progress is measured in percentages (not
absolute lengths) of used/available space and in amount of used/remaining
content. However, when laying out scrollable elements and certain
replaced elements the UA may instead maintain a consistent measure and
resolved extent across fragmenters.
<li><strong class=issue>Option A:</strong> Fragments of boxes that began
on a previous fragmenter must start at the top of the new fragmenter. If
this results in multiple boxes side-by-side that would otherwise be
staggered (if they were not continuations) in order to fit, the elements'
widths are reduced (on that fragmenter only) in proportion to their
original widths until they fit. However they are not reduced past their
min-content width or in case the width copmutes to a non-percent length
value.
<div class=figure>
<pre>First page
<!-- -->+----------------------------------+
<!-- -->|########### ......... ############|
<!-- -->|# # ..... # #|
<!-- -->|# left # ....... # right #|
<!-- -->|# float # ......... # float #|
<!-- -->|# # ........ # #|
<!-- -->|# # ......... # #|
<!-- -->|# # ........ # #|
<!-- -->|#,,,,,,,,,# ......... #,,,,,,,,,,#|
<!-- -->+----------------------------------+
<!-- -->
<!-- -->Second page in case the 'width' computes to 'auto' or 'percent'
<!-- -->
<!-- -->+-----------------+
<!-- -->|#``````##```````#|
<!-- -->|# left ## right #|
<!-- -->|# float## float #|
<!-- -->|# cont.## cont. #|
<!-- -->|#################|
<!-- -->|.................|
<!-- -->|............... |
<!-- -->|.................|
<!-- -->|.... |