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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html dir=ltr lang=en>
<head profile="http://www.w3.org/2006/03/hcard"><meta content="IE=edge"
http-equiv=X-UA-Compatible>
<meta content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Regions Module</title>
<link href="http://dev.w3.org/csswg/default.css" rel=stylesheet
type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=Stylesheet
type="text/css">
<link href=alternate-spec-style.css id=st rel=stylesheet title="additional
spec styles" type="text/css">
<script src=style-toggle.js type="text/javascript">
</script>
<script src="http://test.csswg.org/harness/annotate.js#CSS3-REGIONS_DEV"
type="text/javascript">
</script>
<body>
<div class=head id=div-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--></p>
<h1 id=css-regions-module>CSS Regions Module</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 13 July 2011</h2>
<dl>
<dt>This version:
<dd><a
href="http://www.w3.org/TR/2011/ED-css3-regions-20110713/">http://www.w3.org/csswg/css3-regions</a>
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-regions">http://www.w3.org/csswg/css3-regions</a>
<dt>Previous version:
<dd>none
<dt>Editors:
<dd class=vcard><span class=fn>Vincent Hardy</span>, <span
class=org>Adobe Systems, Inc.</span>, <span
class=email>vhardy@adobe.com</span>
<dd class=vcard><span class=fn>Alex Mogilevsky</span>, <span
class=org>Microsoft</span>, <span
class=email>alexmog@microsoft.com</span>
<dt>Authors and Former Editors:
<dd class=vcard><span class=fn>Stephen Zilles</span>, <span
class=org>Adobe Systems, Inc.</span>, <span
class=email>szilles@adobe.com</span>
<dd class=vcard><span class=fn>Alexandru Chiculita</span>, <span
class=org>Adobe Systems, Inc.</span>, <span
class=email>achicu@adobe.com</span>
<dd class=vcard><span class=fn>Andrei Bucur</span>, <span class=org>Adobe
Systems, Inc.</span>, <span class=email>abucur@adobe.com</span>
<dd class=vcard><span class=fn>Mihnea Ovidenie</span>, <span
class=org>Adobe Systems, Inc.</span>, <span
class=email>mihnea@adobe.com</span>
<dd class=vcard><span class=fn>Peter Sorotokin</span>, <span
class=org>Adobe Systems, Inc.</span>, <span
class=email>psorotok@adobe.com</span>
<dd class=vcard><span class=fn>Virgil Palanciuc</span>, <span
class=org>Adobe Systems, Inc.</span>, <span
class=email>virgilp@adobe.com</span>
<dd class=vcard><span class=fn>Arno Gourdol</span>, <span class=org>Adobe
Systems, Inc.</span>, <span class=email>agourdol@adobe.com</span>
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2011 <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>The CSS Regions module allows content to flow across multiple areas
called regions. The regions do not necessarily follow the document order.
The CSS Regions module provides an advanced content flow mechanism, which
can be combined with positioning schemes as defined by other CSS modules
such as the Multi-Column Module <a href="#CSS3COL"
rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a> or the Grid Layout Module
<a href="#CSS3-GRID-LAYOUT"
rel=biblioentry>[CSS3-GRID-LAYOUT]<!--{{CSS3-GRID-LAYOUT}}--></a> to
position the regions where content flows.
<h2 class="no-num no-toc" id=status-of-this-document>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-regions%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-regions” in the subject, preferably like this:
“[<!---->css3-regions<!---->] <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-->
<h2 class="no-num no-toc" id=table-of-contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
<ul class=toc>
<li><a href="#named-flows-and-regions"><span class=secno>1.1.
</span>Named flows and regions</a>
<li><a href="#region-styling"><span class=secno>1.2. </span>Regions
Styling</a>
</ul>
<li><a href="#css-regions-model"><span class=secno>2. </span>CSS Regions
Model</a>
<ul class=toc>
<li><a href="#named-flows"><span class=secno>2.1. </span>Named flows</a>
<li><a href="#regions"><span class=secno>2.2. </span>Regions</a>
<li><a href="#flow-breaking-rules"><span class=secno>2.3. </span>Flow
breaking rules</a>
<ul class=toc>
<li><a
href="#breaking-a-named-flow-and-determining-the-current-flow-region"><span
class=secno>2.3.1. </span> Breaking a named flow and determining the
current flow region</a>
<li><a href="#allowed-region-breaks"><span class=secno>2.3.2.
</span>Allowed region breaks</a>
<li><a href="#forced-region-breaks"><span class=secno>2.3.3.
</span>Forced region breaks</a>
<li><a href="#best-region-breaks"><span class=secno>2.3.4.
</span>"Best" region breaks</a>
</ul>
<li><a href="#the-visual-formatting-model-and-flows"><span
class=secno>2.4. </span>The Visual Formatting Model and Flows</a>
<ul class=toc>
<li><a href="#normal-flow"><span class=secno>2.4.1. </span>Normal
flow</a>
<li><a href="#positioned-flow"><span class=secno>2.4.2.
</span>Positioned flow</a>
<li><a href="#column-flow"><span class=secno>2.4.3. </span>Column
flow</a>
<li><a href="#generated-flow"><span class=secno>2.4.4.
</span>Generated flow</a>
</ul>
<li><a href="#relation-to-document-events"><span class=secno>2.5.
</span>Relation to document events</a>
</ul>
<li><a href="#properties-and-rules"><span class=secno>3. </span>Properties
and Rules</a>
<ul class=toc>
<li><a href="#the-flow-property"><span class=secno>3.1. </span>The
‘<code class=property>flow</code>’ property</a>
<li><a href="#the-content-property"><span class=secno>3.2. </span>The
‘<code class=property>content</code>’ property</a>
<li><a href="#the-content-order-property"><span class=secno>3.3.
</span>The ‘<code class=property>content-order</code>’
property</a>
<li><a
href="#region-flow-break-properties-break-before-break-after-break-inside"><span
class=secno>3.4. </span>Region flow break properties: ‘<code
class=property>break-before</code>’, ‘<code
class=property>break-after</code>’, ‘<code
class=property>break-inside</code>’</a>
<li><a href="#the-region-overflow-property"><span class=secno>3.5.
</span>The region-overflow property</a>
<li><a href="#the-at-region-style-rule"><span class=secno>3.6.
</span>The @region rule</a>
</ul>
<li><a href="#cssom_view_and_css_regions"><span class=secno>4.
</span>CSSOM View and CSS Regions</a>
<ul class=toc>
<li><a href="#the-namedflow-interface"><span class=secno>4.1. </span>The
NamedFlow interface</a>
<li><a href="#extension-to-the-element-interface"><span class=secno>4.2.
</span>Extension to the Element interface</a>
<li><a href="#region-flow-layout-events"><span class=secno>4.3.
</span>Region Flow Layout Events</a>
</ul>
<li><a href="#integration-with-other-specifications"><span class=secno>5.
</span>Integration with other specifications</a>
<ul class=toc>
<li><a href="#region-requirements"><span class=secno>5.1. </span>Region
requirements</a>
<li><a href="#integration-examples"><span class=secno>5.2.
</span>Integration Examples</a>
<ul class=toc>
<li><a href="#css-grid-layout"><span class=secno>5.2.1. </span>CSS
Grid Layout</a>
<li><a href="#css-multi-column-layout"><span class=secno>5.2.2.
</span>CSS Multi-Column Layout</a>
<li><a href="#css-visual-formatting"><span class=secno>5.2.3.
</span>CSS Visual Formatting</a>
</ul>
</ul>
<li><a href="#relation-to-other-specifications"><span class=secno>6.
</span>Relation to other specifications</a>
<li><a href="#conformance"><span class=secno>7. </span>Conformance</a>
<li><a href="#changes"><span class=secno>8. </span>Changes</a>
<ul class=toc>
<li><a href="#changes_from_June_09_2011"><span class=secno>8.1.
</span>Changes from June 09<sup>th</sup> 2011 version</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=introduction><span class=secno>1. </span>Introduction</h2>
<p><em>This section is non-normative.</em>
<p>Capturing the complex layouts of a typical magazine, newspaper, or
textbook requires capabilities beyond those available in existing CSS
modules. This is the purpose of the CSS Regions module.
<p>
<p>The CSS Regions module can be seen as an extension of the concept of
multi-column elements. With CSS Multi-column layout <a href="#CSS3COL"
rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>, columns share the same
dimensions and define column boxes organized in rows. Content flows from
one column to the next.
<p>
<p>The multi-column model is an example of flowing content from one area to
another, where the areas are the multi-column element's column boxes and
the flow is made of the multi-column element's children.
<p>
<p>However, for more complex layouts, content needs to flow from one area
of the page to the next without limitation of the areas' sizes and
positions. These arbitrary areas are the target of specific content flows.
In this document these areas are called regions, and the content flows are
called named flows. Regions are based on the rectangular geometry of the
CSS box model. Elements in a named flow are taken out of the normal visual
formatting and rendered in a chain of regions.
<h3 id=named-flows-and-regions><span class=secno>1.1. </span>Named flows
and regions</h3>
<p>Consider the layout illustrated in figure 1.
<div class=figure> <img alt="multiple chained regions which should receive
content from a flow" src="images/regions-intro.svg" width=400>
<p class=caption>Layout requiring sophisticated content flow</p>
</div>
<p>The designer's intent is to position an image in region ‘<code
class=property>A</code>’ and to flow an article's text from region
‘<code class=css>1</code>’, to region ‘<code
class=css>2</code>’, ‘<code class=css>3</code>’ and
‘<code class=css>4</code>’.
<p>The following code snippet shows the content to flow between the regions
1, 2, 3 and 4.
<pre>
<div <em>id="article"</em>>
<h1>Introduction</h1>
<p>This is an example ...</p>
<h1>More Details</h1>
<p>This illustrates ...</p>
<p>Then, the example ...</p>
<p>Finally, this ...</p>
</div>
</pre>
<p>CSS layout facilities can position and size regions as needed. However,
there is no existing mechanism to associate the content with the regions
so that content flows as intended. Figure 2 shows an example of the
intended visual rendering of the content.
<div class=figure> <img alt="Sample rendering showing a single thread of
text flowing through a chain of regions"
src="images/regions-intro-rendering.png" width=450>
<p class=caption>Sample rendering for desired layout</p>
</div>
<p>Since the CSS Regions module is independent of the layout of regions and
the mechanism used to create them, the following assumes there is a CSS
selector for the regions and, for the purpose of the example, the
selectors for regions 1, 2, 3 and 4 show as ‘<code
class=css><region1_sel></code>’, ‘<code
class=css><region2_sel></code>’, ‘<code
class=css><region3_sel></code>’ and ‘<code
class=css><region4_sel></code>’ respectively. Such a selector
could be an id selector (e.g., ‘<code
class=css>#region_1</code>’) or a grid cell selector (e.g.,
‘<code class=css>#myGrid::grid-cell([cell-name])</code>’) if
using the CSS Grid Layout module, for example.
<div class=example>
<pre>
<style>
#article {
<strong>flow: article_flow;</strong>
}
<region1_sel>, <region2_sel>,
<region3_sel>, <region4_sel> {
<strong>content: from-flow(article_flow);</strong>
}
</style>
</pre>
</div>
<p>The above stylesheet directs the ‘<code
class=css>#article</code>’ element to a named flow called
‘<code class=css>article_flow</code>’ by setting the ‘<a
href="#flow2"><code class=property>flow</code></a>’ property. Then,
content is "poured" from that named flow into the desired regions by
setting the regions' ‘<a href="#content-from"><code
class=property>content</code></a>’ property to ‘<code
class=css>from-flow(article_flow)</code>’.
<h3 id=region-styling><span class=secno>1.2. </span>Regions Styling</h3>
<p>Region styling allows content to be styled depending on the region it
flows into. It is a form of context-based styling, similar to <a
href="#CSSMEDIAQUERIES">Media Queries</a> <a href="#MEDIAQ"
rel=biblioentry>[MEDIAQ]<!--{{MEDIAQ}}--></a> which enable or disable
selectors depending on the rendering context. With region styling,
additional selectors may apply depending on the region into which content
flows.
<p>In our example, the designer wants to make text flowing into region 1
larger, bold and dark blue. In addition, <code
class=html><h1></code> headers should be run-ins and crimson.
<p>This design can be expressed with region styling as shown below.
<div class=example>
<pre>
<style>
/*
* Default article styling.
*/
#article {
color:#777;
text-align: justify;
}
#article h1 {
border-left: 1px solid #777;
padding-left: 2ex;
}
/*
* Additional styling to apply to content when it falls into
* region_1
*/
<strong>@region <region1_sel> {</strong>
#article::region-lines {
font-weight: bold;
color: #0C3D5F;
font-size: larger;
}
#article h1 {
color: crimson;
display: run-in;
border: none;
padding: 0px;
}
<strong>}</strong>
</style>
</pre>
</div>
<p>The ‘<code class=css>@region</code>’ rule for region 1
limits its selectors to content flowing into region 1. The following
figure shows how the rendering changes if we do not increase the font size
nor make it bold for content flowing into region 1. As more content can be
fitted, more content is subject to the contextual selectors, resulting in
more dark blue text into region 1.
<p>The ‘<code class=css>::region-lines</code>’ pseudo-element
is describe
133E
d later in this specification.
<div class=figure> <img alt="Illustrate how changing region styling affects
the flow of content." src="images/region-styling.png" width=450>
<p class=caption>Different rendering with a different region styling</p>
</div>
<h2 id=css-regions-model><span class=secno>2. </span>CSS Regions Model</h2>
<h3 id=named-flows><span class=secno>2.1. </span>Named flows</h3>
<p>In the <a href="http://www.w3.org/TR/CSS21/visuren.html">CSS formatting
model</a>, elements can be in the <a href="#normal-flow-def"><em>normal
flow</em></a> or out of the <a href="#normal-flow-def"><em>(normal)
flow</em></a>. Boxes generated by elements in the normal flow are subject
to their container box's normal layout scheme. Boxes generated by elements
out of the normal flow are subject to a different layout scheme. For
example, absolutely positioned elements are subject to absolute
positioning into their <a
href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">containing
block</a>. This can be described by saying that the absolutely positioned
elements are part of a special flow (called <a href="#positioned-flow0"
title=positioned-flow>positioned flow</a>) which is subject to a special
layout by its container box (i.e., its container box positions it into the
containing block's box).
<p>
<p>In both cases, there is a notion of flow containing a sequence of
elements and there is a notion of <a
href="http://www.w3.org/Style/Group/css2-src/visuren.html#block-boxes">(block)
container box</a> into which the elements flow.
<p>
<p>The CSS Regions module generalizes the concept of flow by adding the
concept of a <em>named flow</em>. This module lets authors explicitly
place elements into a <em>named flow</em>.
<p>
<p>With this model, all elements are moved to a flow as part of the visual
formatting. That flow may be the normal flow, a named flow or a positioned
flow, for example. A flow gets formatted visually when associated with one
or several elements' container box(es). When an flow is associated with
container boxes, the boxes generated by the flow's elements are laid out
according to the container box's layout scheme and the flow is subject to
the <a href="#flow-breaking-rules">‘<code class=css>flow breaking
rules</code>’</a> described below.
<p>A container's layout scheme is the strategy used by a container to
position the boxes generated by its children and itself. Examples are the
normal layout (block and inline formatting) (<a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>), table layout (<a
href="#CSS21" rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>), the
multi-column layout (<a href="#CSS3COL"
rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>) or the grid layout (<a
href="#CSS3-GRID-LAYOUT"
rel=biblioentry>[CSS3-GRID-LAYOUT]<!--{{CSS3-GRID-LAYOUT}}--></a>).
<h3 id=regions><span class=secno>2.2. </span>Regions</h3>
<p class=index id=region title=region>A region is an element that generates
a <a href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">block
container box</a> and has an associated <em>named flow</em> (see the
‘<a href="#content-from"><code
class=property>content</code></a>’ property).
<p>A <span title=region>region</span>‘<code class=css>s intrinsic
width is zero.</code>
<div class=issue> There was a recent discussion on intrinsic sizing of
regions which <a
href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0629.html">has
not been resolved</a>.</div>
<h3 id=flow-breaking-rules><span class=secno>2.3. </span>Flow breaking
rules</h3>
<p>Breaking a named flow across multiple regions is similar to breaking a
document’s content across multiple pages. One important difference
is that page boxes are generated based on the available content whereas
regions are a predefined set of recipients for the named flow content.
<h4 id=breaking-a-named-flow-and-determining-the-current-flow-region><span
class=secno>2.3.1. </span> Breaking a named flow and determining the
current flow region</h4>
<p>The ‘<a href="#content-order"><code
class=property>content-order</code></a>’ property defines how <span
title=region>regions</span> are organized in to a <span
title=region-chain>region chain</span>.
<p>Each region in turn consumes content from its associated <span
title=named-flow>named flow</span>. This means that the named flow content
is positioned in the <dfn id=current-region title=current-region>current
region</dfn> until a region break occurs, at which point the <a
href="#current-region" title=current-region>current region</a> becomes the
next one in the <span title=region-chain>region chain</span>. If there is
no more <span>region</span> in the region chain and there is still content
in the flow, the positioning of the remaining content is controlled by the
‘<a href="#region-overflow"><code
class=property>region-overflow</code></a>’ property on the last
<span>region</span> in the chain.
<p>The following sections define where region breaks may happen when
positioning a named flow across <span title=region>regions</span> in a
<span title=region-chain>region chain</span>.
<div class=issue>
<p>The sections on "Allowed region breaks", "Forced region breaks" and
""Best" region breaks" are adapted from the <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> specification. There is an
updated version of these paragraphs in <a href="#CSS3PAGE"
rel=biblioentry>[CSS3PAGE]<!--{{CSS3PAGE}}--></a> as well.</p>
<p>In addition, the CSS Multi-column Layout Module <a href="#CSS3COL"
rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a> defines unified breaking
properties (which this specification extends).</p>
<p>It seems that we should:</p>
<ol>
<li>Define a common set of rules for breaking content that accounts for
pages, columns and regions.
<li>Define the properties for breaking content in a single specification
as well.
<li>Specify how column, page and region breaks interact.
</ol>
</div>
<h4 id=allowed-region-breaks><span class=secno>2.3.2. </span>Allowed region
breaks</h4>
<p>In a named flow, region breaks can occur at the following places:
<ol>
<li>In the vertical margin between block-level boxes. When an unforced
region break occurs here, the <a href="cascade.html#used-value">used
values</a> of the relevant <a class=noxref
href="box.html#propdef-margin-top"><span
class=propinst-margin-top>‘<code
class=property>margin-top</code>’</span></a> and <a class=noxref
href="box.html#propdef-margin-bottom"><span
class=propinst-margin-bottom>‘<code
class=property>margin-bottom</code>’</span></a> properties are set
to ‘<code class=css>0</code>’. When a forced region break
occurs here, the used value of the relevant <a class=noxref
href="box.html#propdef-margin-bottom"><span
class=propinst-margin-bottom>‘<code
class=property>margin-bottom</code>’</span></a> property is set to
‘<code class=css>0</code>’; the relevant <a class=noxref
href="box.html#propdef-margin-top"><span
class=propinst-margin-top>‘<code
class=property>margin-top</code>’</span></a> used value may either
be set to ‘<code class=css>0</code>’ or retained.
<li>Between <a href="visuren.html#line-box">line boxes</a> inside a <a
href="visuren.html#block-boxes">block container</a> box.
<li>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) if 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 (1) is allowed only if the
‘<a href="#break-after"><code
class=property>break-after</code></a>’ ‘<a
href="#break-before"><code class=property>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=property>always</code>’, ‘<code
class=property>left</code>’, or ‘<code
class=property>right</code>’, or when all of them are ‘<code
class=property>auto</code>’.
<li><strong>Rule B:</strong> However, if all of them are ‘<code
class=property>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=property>avoid-region</code>’, then breaking here is not
allowed.
<li><strong>Rule C:</strong> Breaking at (2) is allowed only if the number
of <a href="visuren.html#line-box">line boxes</a> between the break and
the start of the enclosing block box is the value of ‘<code
class=property>orphans</code>’ or more, and the number of line
boxes between the break and the end of the box is the value of
‘<code class=property>widows</code>’ or more.
<li><strong>Rule D:</strong> In addition, breaking at (2) or (3) is
allowed only if the ‘<a href="#break-inside"><code
class=property>break-inside</code></a>’ property of the element and
all its ancestors is ‘<code class=property>auto</code>’.
</ul>
<p>If the above does not provide enough break points to keep content from
overflowing the region boxes, then rules A, B and D are dropped in order
to find additional breakpoints.
<p>If that still does not lead to sufficient break points, rule C is
dropped as well, to find still more break points.
<h4 id=forced-region-breaks><span class=secno>2.3.3. </span>Forced region
breaks</h4>
<p>A region break <em>must</em> occur at (1) if, among the ‘<a
href="#break-after"><code class=property>break-after</code></a>’ and
‘<a href="#break-before"><code
class=property>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=property>always</code>’,
‘<code class=property>left</code>’, or ‘<code
class=property>right</code>’.
<h4 id=best-region-breaks><span class=secno>2.3.4. </span>"Best" region
breaks</h4>
<p>CSS 2.1 does <em>not</em> define which of a set of allowed region
breaks must be used; CSS 2.1 does not forbid a user agent from
breaking at every possible break point, or not to break at all. But
CSS 2.1 does recommend that user agents observe the following
heuristics (while recognizing that they are sometimes contradictory):
<ul>
<li>Break as few times as possible.
<li>Avoid breaking inside a replaced element.
</ul>
<div class=example>
<p style="display:none">Example(s):</p>
<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 (<a
href="visuren.html#line-box">line boxes</a>) available at the bottom of
the current region:</p>
<ul>
<li>If a paragraph at the end of the current region contains 20 lines or
fewer, it should be placed on the current region.
<li>If the paragraph contains 21 or 22 lines, the second part of the
paragraph must not violate the <a class=noxref
href="page.html#propdef-widows"><span class=propinst-widows>‘<code
class=property>widows</code>’</span></a> constraint, and so the
second part must contain exactly two lines
<li>If the paragraph contains 23 lines or more, the first part should
contain 20 lines and the second part the remaining lines.
</ul>
<p>Now suppose that <a class=noxref href="page.html#propdef-orphans"><span
class=propinst-orphans>‘<code
class=property>orphans</code>’</span></a> is ‘<code
class=css>10</code>’, <a class=noxref
href="page.html#propdef-widows"><span class=propinst-widows>‘<code
class=property>widows</code>’</span></a> is ‘<code
class=css>20</code>’, and there are 8 lines available at the bottom
of the current region:</p>
<ul>
<li>If a paragraph at the end of the current region contains 8 lines or
fewer, it should be placed on the current region.
<li>If the paragraph contains 9 lines or more, it cannot be split (that
would violate the orphan constraint), so it should move as a block to
the next region.
</ul>
</div>
<h3 id=the-visual-formatting-model-and-flows><span class=secno>2.4.
</span>The Visual Formatting Model and Flows</h3>
<p>This section describes the flows to which elements formatted according
to the CSS <a href="http://www.w3.org/TR/CSS21/visuren.html">Visual
Formatting Model</a> and other layout modules (such as Multi-Column Layout
<a href="#CSS3COL" rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>) belong
to.
<p>In particular, this section describes the ‘<a
href="#generated-flow-def"><code class=css>generated
flow</code></a>’ where generated content of pseudo-elements is
placed and several <dfn id=auto-flow title="auto flows">‘<code
class=css>auto flows</code>’</dfn> where children of content
elements are automatically placed.
<h4 id=normal-flow><span class=secno>2.4.1. </span>Normal flow</h4>
<p>In the CSS formatting model, elements are by default placed in the <dfn
id=normal-flow-def>normal flow</dfn> of their container. Also by default,
a container element gets its content from its <a
href="#normal-flow-def"><em>normal flow</em></a>. This means that by
default, a container element will visually format its children elements
and will be the only container associated with its normal flow.
<p>Note that floats and relatively positioned elements, in this model, are
part of the same flow of content and flow into the same container but are
positioned in different ways.
<p>The <a href="#normal-flow-def"><em>normal flow</em></a> is one of the <a
href="#auto-flow"><em>auto flows</em></a>.
<h4 id=positioned-flow><span class=secno>2.4.2. </span>Positioned flow</h4>
<p>An absolutely positioned element is placed into the <dfn
id=positioned-flow0 title=positioned-flow>positioned flow</dfn> of its
container. The container positions this element into its <em>containing
block</em>.
<p>If a container has children in the <a href="#normal-flow-def"><em>normal
flow</em></a> and in the <em>positioned flow</em>, it applies different
positioning schemes to each flow.
<p>The <em>positioned flow</em> is one of the <a href="#auto-flow"><em>auto
flows</em></a>.
<h4 id=column-flow><span class=secno>2.4.3. </span>Column flow</h4>
<p>Children of the multi-column element are placed in the element's <dfn
id=column-flow-def>column flow</dfn>. The column boxes are then implicitly
associated with the element's ‘<a href="#column-flow-def"><code
class=css>column flow</code></a>’. In this model, multiple regions
(column boxes) get their content from a single flow (the element's
‘<a href="#column-flow-def"><code class=css>column
flow</code></a>’, which is the content of the element).
<p>The <a href="#column-flow-def"><em>column flow</em></a> is one of the <a
href="#auto-flow"><em>auto flows</em></a>.
<div class=issue> Need to add a <dfn id=grid-cell-flow>grid cell flow</dfn>
section.</div>
<div class=issue> Need to add examples for the different flow types.</div>
<h4 id=generated-flow><span class=secno>2.4.4. </span>Generated flow</h4>
<p>In the CSS formatting model, when the ‘<code
class=css>::before</code>’ or ‘<code
class=css>::after</code>’ pseudo-elements have their content
property set to one of ‘<code
class=css><string></code>’, ‘<code
class=css><uri></code>’, ‘<code
class=css><counter></code>’, ‘<code
class=css>attr(<identifier>)</code>’, ‘<code
class=css>open-quote</code>’, ‘<code
class=css>close-quote</code>’, ‘<code
class=css>no-open-quote</code>’ or ‘<code
class=css>no-close-quote</code>’, they create a <dfn
id=generated-flow-def>generated flow</dfn> which they format visually.
<h3 id=relation-to-document-events><span class=secno>2.5. </span>Relation
to document events</h3>
<p><em>This section is non-normative.</em>
<p>The CSS Region module does not alter in any way the normal processing of
events in the document tree. In particular, if an event occurs on an
element that is part of a named flow, the <a
href="http://www.w3.org/TR/DOM-Level-3-Events/#dom-event-architecture">event's
bubble and capture phases</a> happen following the document tree order.
<h2 id=properties-and-rules><span class=secno>3. </span>Properties and
Rules</h2>
<p>The main CSS Regions module properties are the ‘flow’ and ‘<a
href="#content-from"><code class=property>content</code></a>’
properties. The ‘flow’ property places an element into a specific
named flow or lets the <a href=auto-flows>automatic flow assignment</a>
take place. The ‘content’ property binds a region with a flow. When
multiple regions are bound to the same flow, the ‘<a
href="#content-order"><code class=property>content-order</code></a>’
property determines the order in which content flows into the sequence of
regions. This sequences of regions is called a <dfn id=region-chain>region
chain</dfn>.
<p>
<p>The way in which the content is broken into segments that fit in a
region can be controlled by the “break” properties. Finally, styling
that is unique to the region can be specified by ‘region-style’ rules.
<h3 id=the-flow-property><span class=secno>3.1. </span>The ‘<a
href="#flow2"><code class=property>flow</code></a>’ property</h3>
<p>The ‘flow’ property places an element into a named flow or
automatically places it in the appropriate flow (see the <a
href="#formatting-model-considerations">Visual Formatting Model and
Flows</a> section). Elements that belong to the same flow are laid out in
the regions associated with that flow.
<table class=propdef summary="flow property definition">
<tbody>
<tr>
<th>Name:
<td><dfn id=flow2>flow</dfn>
<tr>
<th>Value:
<td><ident>
<tr>
<th>Initial:
<td>auto
<tr>
<th>Applies to:
<td>any element
<tr>
<th>Inherited:
<td>no
<tr>
<th>Percentages:
<td>N/A
<tr>
<th>Media:
<td>visual
<tr>
<th>Computed value:
<td>as specified
</table>
<dl>
<dt>auto
<dd>The element is automatically moved to the appropriate flow according
to the <a href="#formatting_model_considerations">Visual Formatting Model
and Flows</a> definitions.
<dt><a
href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier"><ident></a>
<dd>The element is placed (appended) to the flow with that name. The
element is said to have a <dfn id=specified-flow>specified flow</dfn>.
The values ‘<code class=property>none</code>’, ‘<code
class=property>inherit</code>’, ‘<code
class=property>default</code>’ and ‘<code
class=property>initial</code>’ and invalid flow names.
</dl>
<p>A named flow needs to be associated with one or more regions for its
elements to be visually formatted. If no region is associated with a given
named flow, the elements in the named flow are not rendered: they do not
generate boxes and are not displayed.
<p>The children of an element with a specified flow may themselves have a
specified flow.
<p>If an element has a the same specified flow value as one of its
ancestors, it is moved out of its parent's normal flow and becomes a
sibling of it's ancestor for the purpose of layout in the regions taking
content from this flow.
<p>The ‘<a href="#flow2"><code class=property>flow</code></a>’
property does not affect the CSS cascade and inheritance for the elements
on which it is specified. The flow property affects the visual formatting
of elements associated to a named flow and of regions getting their
content from a named flow.
<p>The containing block for absolutely positioned descendants of an element
with a specified flow is the region into which the element is rendered.
<p>All the elements participating inside a named flow are rendered as
children of an anonymous block that spans across all the regions
associated with the specified named flow. The elements flowed inside the
same named flow are taken in document order.
<p><span title=region>Regions</span> create a new stacking context, but
inherit the floats that are
1DDA
already defined by the parent context.
<div class=example>
<p>In the following example, the inline content coming from the <a
href="#content-from"><code class=html>content</code></a> <span
title=named-flow>named flow</span> wraps around the <code
class=html>#float</code> box.</p>
<pre>
<style>
#float {
float: left;
width: 100px;
height: 300px;
}
#region1, #region2 {
width: 200px;
height: 200px;
content: from-flow(body_text);
}
#content {
flow: body_text;
}
</style>
<div id="float"></div>
<div id="region1"></div>
<div id="region2"></div>
<div id="content"></div>
</pre>
</div>
<p> For an <code class=html><iframe></code>, an <code
class=html><object></code> or a <code
class=html><embed></code> element, the ‘<a href="#flow2"><code
class=property>flow</code></a>’ property has a different behavior.