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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 Controls 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 Controls Module Level 3</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 24 January
2012</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2012/ED-css3-break-20120124/">http://www.w3.org/csswg/css3-break/</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-break/">http://www.w3.org/csswg/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.com/">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/"><acronym
title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym
title="Massachusetts Institute of Technology">MIT</acronym></a>, <a
href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>,
<a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p> 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="#page-breaks"><span class=secno>2. </span>Page Breaks</a>
<ul class=toc>
<li><a href="#pg-br-before-after"><span class=secno>2.1. </span> Break
before/after elements: ‘<code
class=property>page-break-before</code>’, ‘<code
class=property>page-break-after</code>’, ‘<code
class=property>page-break-inside</code>’ </a>
<li><a href="#using-named-pages"><span class=secno>2.2. </span>Using
named pages: ‘<code class=property>page</code>’</a>
<li><a href="#breaks-inside"><span class=secno>2.3. </span>Breaks inside
elements: ‘<code class=property>orphans</code>’,
‘<code class=property>widows</code>’</a>
<li><a href="#allowed-pg-brk"><span class=secno>2.4. </span>Allowed page
breaks</a>
<li><a href="#forced-pg-brk"><span class=secno>2.5. </span>Forced page
breaks</a>
<li><a href="#best-pg-brk"><span class=secno>2.6. </span>"Best" page
breaks</a>
</ul>
<li><a href="#conformance"><span class=secno>3. </span>Conformance</a>
<ul class=toc>
<li><a href="#conventions"><span class=secno>3.1. </span>Document
E0EA
Conventions</a>
<li><a href="#conformance-classes"><span class=secno>3.2.
</span>Conformance Classes</a>
<li><a href="#partial"><span class=secno>3.3. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>3.4. </span> Experimental
Implementations</a>
<li><a href="#testing"><span class=secno>3.5. </span> Non-Experimental
Implementations</a>
<li><a href="#cr-exit-criteria"><span class=secno>3.6. </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> Paged media (e.g., paper, transparencies, photo album pages, pages
displayed on computer screens as printed output simulations) differ from
<a href="/TR/CSS2/media.html#continuous-media-group"> continuous media</a>
in that the content of the document is split into one or more discrete
static display surfaces. To handle pages, CSS3 Paged Media describes how:
<ul>
<li><a href="#page-breaks">page breaks</a> are created and avoided;
<li>the page properties such as size, orientation, margins, border, and
padding are specified;
<li>headers and footers are established within the page margins;
<li>content such as page counters are placed in the headers and footers;
and
<li>orphans and widows can be controlled.
</ul>
<p> This module defines a <a href="#page-model">page model</a> that
specifies how a document is formatted within a rectangular area, called
the <a href="#page-box-page-rule"> page box</a>, that has finite width and
height.
<p> Although CSS3 does not specify how user agents transfer page boxes to
sheets, it does include certain mechanisms for telling user agents about
the intended page sheet <a href="#page-size">size and orientation</a>. In
the general case, CSS3 assumes that one page box will be transferred to
one surface of similar size.
<p> 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 value, but for readability it has not been
listed explicitly.
<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=page-breaks><span class=secno>2. </span>Page Breaks</h2>
<p> The following sections explain page breaking in CSS3 paged media. Five
properties indicate where the user agent may or should break pages, and on
what page (left or right) the subsequent content should resume. Each page
break ends layout in the current <a href="#page-box">page box</a> and
causes remaining pieces of the document tree to be laid out in a new page
box.
<p> When a page break splits a box, the box's bottom margins, borders, and
padding have no visual effect where the split occurs; the box's background
and left and right margins, border, and padding extend to the bottom of
the page, through an ensuing blank page if one exists, and onto the top of
the subsequent page.
<h3 id=pg-br-before-after><span class=secno>2.1. </span> Break before/after
elements: ‘<a href="#page-break-before"><code
class=property>page-break-before</code></a>’, ‘<a
href="#page-break-after"><code
class=property>page-break-after</code></a>’, ‘<a
href="#page-break-inside"><code
class=property>page-break-inside</code></a>’</h3>
<table class=propdef summary="property definition">
<tbody>
<tr>
<th>Name:
<td><dfn id=page-break-before>page-break-before</dfn>
<tr>
<th>Value:
<td>auto | always | avoid | left | right
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>block-level elements
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>paged
<tr>
<th>Computed value:
<td>specified value
</table>
<table class=propdef summary="property definition">
<tbody>
<tr>
<th>Name:
<td><dfn id=page-break-after>page-break-after</dfn>
<tr>
<th>Value:
<td>auto | always | avoid | left | right
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>block-level boxes, table row groups, table rows (but see prose)
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>paged
<tr>
<th>Computed value:
<td>specified value
</table>
<table class=propdef summary="property definition">
<tbody>
<tr>
<th>Name:
<td><dfn id=page-break-inside>page-break-inside</dfn>
<tr>
<th>Value:
<td>auto | avoid
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>block-level boxes, table row groups, table rows, and table cells
(but see prose)
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>paged
<tr>
<th>Computed value:
<td>specified value
</table>
<p>Values for these properties have the following meanings:
<dl>
<dt>auto
<dd> Neither force nor forbid a page break before / after / inside the
generated box.
<dt>always
<dd> Always force a page break before / after the generated box.
<dt>avoid
<dd> Avoid a page break before / after / inside the generated box.
<dt>left
<dd> Force one or two page breaks before (after) the generated box so that
the next page is formatted as a left page.
<dt>right
<dd> Force one or two page breaks before (after) the generated box so that
the next page is formatted as a right page.
</dl>
<p> A potential page break location is typically under the influence of the
parent element's ‘<a href="#page-break-inside"><code
class=property>page-break-inside</code></a>’ property, the ‘<a
href="#page-break-after"><code
class=property>page-break-after</code></a>’ property of the
preceding element, and the ‘<a href="#page-break-before"><code
class=property>page-break-before</code></a>’ property of the
following element. When these properties have values other than
‘<code class=css>auto</code>’, the values ‘<code
class=css>always</code>’, ‘<code class=css>left</code>’,
and ‘<code class=css>right</code>’ take precedence over
‘<code class=property>avoid</code>’. See the section on <a
href="#allowed-pg-brk">allowed page breaks</a> for the exact rules on how
these properties affect page 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="#page-break-inside"><code
class=property>page-break-inside</code></a>’—table cells of
block-level tables in the normal flow of the root element. User agents
should also apply these properties to floated boxes whose containing block
is in the normal flow of the root element. User agents may also apply
these properties to other boxes.
<h3 id=using-named-pages><span class=secno>2.2. </span>Using named pages:
‘<a href="#page"><code class=property>page</code></a>’</h3>
<table class=propdef summary="property definition">
<tbody>
<tr>
<th>Name:
<td><dfn id=page>page</dfn>
<tr>
<th>Value:
<td>auto | <identifier>
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>block-level elements
<tr>
<th>Inherited:
<td>no (but see prose)
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>paged
<tr>
<th>Computed value:
<td>specified value
</table>
<p> The &
B678
lsquo;<a href="#page"><code class=property>page</code></a>’
property is used to specify a particular type of page where an element
should be displayed.
<div class=example>
<p>This example will put all tables on a right-hand side landscape page
(named "rotated"):</p>
<pre>
@page rotated { size: landscape }
table { page: rotated; page-break-before: right }
</pre>
</div>
<p> The ‘<a href="#page"><code class=property>page</code></a>’
property works as follows: If a block box with inline content has a
‘<a href="#page"><code class=property>page</code></a>’
property that is different from the preceding block box with inline
content, then one or two page breaks are inserted between them, and the
boxes after the break are rendered on a page box of the named type. See <a
href="#forced-pg-brk">"Forced page breaks" below</a>.
<p> The ‘<a href="#page"><code class=property>page</code></a>’
property does not inherit. However, if the ‘<a href="#page"><code
class=property>page</code></a>’ value on an element is ‘<code
class=css>auto</code>’, then it is treated as having the same name
as its nearest ancestor with a non-auto value. When indicated on the root
element, the effective name is the empty string.
<p> Because a previous version of this specification indicated that the
‘<a href="#page"><code class=property>page</code></a>’
property is inherited, an implementation that inherits the ‘<a
href="#page"><code class=property>page</code></a>’ property and
treats ‘<code class=css>auto</code>’ as always naming the
empty string remains conformant to CSS3 Paged Media. Therefore authors
should not explicitly specify the ‘<code
class=css>auto</code>’ value on a descendant of an element with a
non-‘<code class=css>auto</code>’ ‘<a href="#page"><code
class=property>page</code></a>’ value as the resulting behavior will
be unpredictable.
<p> Page names are case-sensitive identifiers. However the ‘<code
class=css>auto</code>’ value, being a CSS keyword, is <a
href="http://www.w3.org/TR/CSS21/syndata.html#characters">case-insensitive</a>.
<div class=example>
<p> In this example, the two tables are rendered on landscape pages
(indeed, on the same page, if they fit). The page type "narrow" is used
for the <p> after the second table, as the page properties for the
table element are no longer in effect:</p>
<pre>
@page narrow { size: 9cm 18cm }
@page rotated { size: landscape }
div { page: narrow }
table { page: rotated }</pre>
<p> with this document:</p>
<pre>
<div>
<table>...</table>
<table>...</table>
<p>This text is rendered on a 'narrow' page</p>
</div></pre>
</div>
<h3 id=breaks-inside><span class=secno>2.3. </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-level elements
<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-level elements
<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 element that must be left at the bottom of
a page. The ‘<a href="#widows"><code
class=property>widows</code></a>’ property specifies the minimum
number of line boxes of a block element that must be left at the top of a
page. Examples of how they are used to control page breaks are given
below.
<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=allowed-pg-brk><span class=secno>2.4. </span>Allowed page breaks</h3>
<p>In the normal flow, page breaks may occur at the following places:
<ol>
<li id=brk-btw-blocks> In the vertical margin between sibling block boxes
(or rows in a table). When an unforced page break occurs here, both the
adjoining ‘<code class=property>margin-top</code>’ and
‘<code class=property>margin-bottom</code>’ are set to zero.
<li id=brk-btw-lines>Between line boxes inside a block box.
<li id=brk-end-block> 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.
</ol>
<p>These breaks are subject to the following rules:
<ul>
<li> <strong>Rule A:</strong> Breaking at <a
href="#brk-btw-blocks">(1)</a> is allowed only if the ‘<a
href="#page-break-after"><code
class=property>page-break-after</code></a>’ and ‘<a
href="#page-break-before"><code
class=property>page-break-before</code></a>’ properties of all the
elements generating boxes that meet at this margin allow it, which is
when at least one of them has the value ‘<code
class=css>always</code>’, ‘<code
class=css>left</code>’, or ‘<code
class=css>right</code>’, or when all of them are ‘<code
class=css>auto</code>’.
<li> <strong>Rule B:</strong> However, if all of them are ‘<code
class=css>auto</code>’ and a common ancestor of all the elements
has a ‘<a href="#page-break-inside"><code
class=property>page-break-inside</code></a>’ value of ‘<code
class=css>avoid</code>’, then breaking here is not allowed.
<li> <strong>Rule C:</strong> Breaking at <a href="#brk-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.
<li> <strong>Rule D:</strong> In addition, breaking at <a
href="#brk-btw-blocks">(2)</a> or <a href="#brk-end-block">(3)</a> is
allowed only if the ‘<a href="#page-break-inside"><code
class=property>page-break-inside</code></a>’ property of all
ancestors is ‘<code class=css>auto</code>’.
</ul>
<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=property>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=forced-pg-brk><span class=secno>2.5. </span>Forced page breaks</h3>
<p> A page break must occur at <a href="#brk-btw-blocks">(1)</a> if, among
the'page-break-after' and ‘<a href="#page-break-before"><code
class=property>page-break-before</code></a>’ properties of all the
elements generating boxes that meet at this margin, there is at least one
with the value ‘<code class=css>always</code>’, ‘<code
class=css>left</code>’, or ‘<code
class=css>right</code>’.
<p> When multiple ‘<a href="#page-break-before"><code
class=property>page-break-before</code></a>’ and/or ‘<a
href="#page-break-after"><code
8026
class=property>page-break-after</code></a>’ properties with values
of "always", "left" or "right" apply at a margin, only one of them should
generate page breaks. That is, no content-empty pages are generated by
page-break properties, except for at most one content-empty page as may be
required by ‘<code class=property>right</code>’ and
‘<code class=property>left</code>’ values to position ensuing
content on a right- or left-facing page.
<p> A page break must also occur at <a href="#brk-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 ‘<a href="#page"><code
class=property>page</code></a>’.
<p> When a forced page break occurs at <a href="#brk-btw-blocks">(1)</a>,
the used values of any adjoining ‘<code
class=property>margin-bottom</code>’ are set to zero.
<h3 id=best-pg-brk><span class=secno>2.6. </span>"Best" page breaks</h3>
<p> CSS3 does <em>not</em> define which of a set of allowed page breaks
must be used; except as defined earlier in the section, CSS3 does not
forbid a user agent to break at every possible break point, or to not
break at all. But CSS3 does recommend 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 pages that don't end with a forced break appear to have about
the same height.
<li>Avoid breaking inside a replaced element.
</ul>
<div class=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 part of the
paragrap must not violate the ‘<a href="#widows"><code
class=property>widows</code></a>’ constraint, and so the second
part must contain at least two line boxes; likewise the first part must
contain at least four line boxes.
<li> If the block contains 23 line boxes or more, the first part should
contain 20 lines and the second part the remaining lines. (But if any
part of the block is placed on the current page, that part must contain
at least four line boxes and the second part 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 may NOT be split (that
would violate the orphan constraint), so it must move as a block to the
next page.
</ul>
</div>
<!-- End section "Page Breaks" -->
<h2 id=conformance><span class=secno>3. </span>Conformance</h2>
<h3 id=conventions><span class=secno>3.1. </span>Document Conventions</h3>
<p> Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “must”,
“must NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“should”, “should NOT”, “RECOMMENDED”, “may”, and
“OPTIONAL” in the normative parts of this document are to be
interpreted as described in RFC 2119. However, for readability, these
words do not appear in all uppercase letters in this specification.
<p> All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>
<p> Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
<code>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p> Informative notes begin with the word “Note” and are set apart from
the normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.
<h3 id=conformance-classes><span class=secno>3.2. </span>Conformance
Classes</h3>
<p>Conformance to CSS Fragmentation is defined for three conformance
classes:
<dl>
<dt><dfn id=style-sheet title="style sheet!!as conformance class">style
sheet</dfn>
<dd> A <a href="http://www.w3.org/TR/CSS21/conform.html#style-sheet">CSS
style sheet</a>.
<dt><dfn id=renderer>renderer</dfn>
<dd> A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that interprets the semantics of a style sheet and renders documents that
use them.
<dt><dfn id=authoring-tool>authoring tool</dfn>
<dd> A <a href="http://www.w3.org/TR/CSS21/conform.html#user-agent">UA</a>
that writes a style sheet.
</dl>
<p> A style sheet is conformant to CSS Fragmentation if all of its
declarations that use properties defined in this module have values that
are valid according to the generic CSS grammar and the individual grammars
of each property as given in this module.
<p> A renderer is conformant to CSS Fragmentation if, in addition to
interpreting the style sheet as defined by the appropriate specifications,
it supports all the features defined by CSS Fragmentation by parsing them
correctly and rendering the document accordingly. However, the inability
of a UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not required to
render color on a monochrome monitor.)
<p> An authoring tool is conformant to CSS Fragmentation if it writes style
sheets that are syntactically correct according to the generic CSS grammar
and the individual grammars of each feature in this module, and meet all
other conformance requirements of style sheets as described in this
module.
<h3 id=partial><span class=secno>3.3. </span> Partial Implementations</h3>
<p> So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers <strong>must</strong> treat as
invalid (and <a href="http://www.w3.org/TR/CSS21/conform.html#ignore">
ignore as appropriate</a>) any at-rules, properties, property values,
keywords, and other syntactic constructs for which they have no usable
level of support. In particular, user agents <strong>must not</strong>
selectively ignore unsupported component values and honor supported values
in a single multi-value property declaration: if any value is considered
invalid (as unsupported values must be), CSS requires that the entire
declaration be ignored.
<h3 id=experimental><span class=secno>3.4. </span> Experimental
Implementations</h3>
<p> To avoid clashes with future CSS features, the CSS2.1 specification
reserves a <a
href="http://www.w3.org/TR/CSS21/syndata.html#vendor-keywords">prefixed
syntax</a> for proprietary and experimental extensions to CSS.
<p> Prior to a specification reaching the Candidate Recommendation stage in
the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations use a
vendor-prefixed syntax for such features, including those in W3C Working
Drafts. This avoids incompatibilities with future changes in the draft.
<h3 id=testing><span class=secno>3.5. </span> Non-Experimental
Implementations</h3>
<p> Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they can
demonstrate to be correctly implemented according to spec.
<p> To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental CSS
renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before releasing
an unprefixed implementation of any CSS features. Testcases submitted to
W3C are subject to review and correction by the CSS Working Group.
<p> Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group's website at <a
href="http://www.w3.org/Style/CSS/Test/">
http://www.w3.org/Style/CSS/Test/</a>. Questions should be directed to the
<a href="http://lists.w3.org/Archives/Public/public-css-testsuite">
public-css-testsuite@w3.org</a> mailing list.
<h3 id=cr-exit-criteria><span class=secno>3.6. </span> CR Exit Criteria</h3>
<p> For this specification to be advanced to Proposed Recommendation, there
must be at least two independent, interoperable implementations of each
feature. Each feature may be implemented by a different set of products,
there is no requirement that all features be implemented by a single
product. For the purposes of this criterion, we define the following
terms:
<dl>
<dt>independent
<dd> each implementation must be developed by a different party and cannot
share, reuse, or derive from code used by another qualifying
implementation. Sections of code that have no bearing on the
implementation of this specification are exempt from this requirement.
<dt>interoperable
<dd> passing the respective test case(s) in the official CSS test suite,
or, if the implementation is not a Web browser, an equivalent test. Every
relevant test in the test suite should have an equivalent test created if
such a user agent (UA) is to be used to claim interoperability. In
addition if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those equivalent