drafts: css3-regions/Overview.html@09b933bd65e2 (annotated)

css3-regions/Overview.html@09b933bd65e2 (annotated)

css3-regions/Overview.html

Fri, 12 Oct 2012 15:30:09 -0700

author: Alan Stearns <stearns@adobe.com>
date: Fri, 12 Oct 2012 15:30:09 -0700
changeset 6847: 09b933bd65e2
parent 6841: d99a9a222d1d
child 6903: a11e43b48600
permissions: -rwxr-xr-x

[css-regions] defer named flows from external resources

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   <title>CSS Regions Module Level 3</title>
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    <h1 id=css-regions-module>CSS Regions Module Level 3</h1>
    <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 12 October
 </h2>
    <dl>
     <dt>This version:
     <dd><a
      href="http://www.w3.org/TR/2012/ED-css3-regions-20121012/">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/TR/css3-regions/</a>
     <dt>Previous version:
     <dd><a
      href="http://www.w3.org/TR/2011/WD-css3-regions-20111129/">http://www.w3.org/TR/2011/WD-css3-regions-20111129/</a>
     <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>
     <dd class=vcard><span class=fn>Alan Stearns</span>, <span class=org>Adobe
      Systems, Inc.</span>, <span class=email>stearns@adobe.com</span>
     <dt>Issues List:
     <dd><a
      href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&amp;component=Regions&amp;resolution=---">Bugzilla
      Bugs for CSS regions</a>
     <dt>Discussion:
     <dd><a
      href="http://lists.w3.org/Archives/Public/www-style/">www-style@w3.org</a>
      with subject line "<code>[css3-regions] message topic</code>"
    </dl>
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    <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>,
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     use</a> rules apply.</p>
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   </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 are not necessarily contiguous in 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>
   <p class="big note"><span class=note-prefix>Note </span>This document uses
    an experimental style sheet. We welcome your feedback on the styles at <a
    href="mailto:site-comments@w3.org">site-comments@w3.org</a>.</p>
   <!--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-->
   <p>This draft is related to the drafts about Multi-column Layout <a
    href="#CSS3COL" rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>, Grid
    Layout <a href="#CSS3GRID"
    rel=biblioentry>[CSS3GRID]<!--{{CSS3GRID}}--></a>, Flexible Box Layout <a
    href="#CSS3-FLEXBOX"
    rel=biblioentry>[CSS3-FLEXBOX]<!--{{CSS3-FLEXBOX}}--></a>, and Template
    Layout <a href="#CSS3LAYOUT"
    rel=biblioentry>[CSS3LAYOUT]<!--{{CSS3LAYOUT}}--></a>.
   <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 region chains</a>
      <li><a href="#region-styling"><span class=secno>1.2. </span>CSS Region
       names and styling</a>
     </ul>
    <li><a href="#css-regions-concepts"><span class=secno>2. </span>CSS
     regions concepts</a>
     <ul class=toc>
      <li><a href="#regions"><span class=secno>2.1. </span>Regions</a>
      <li><a href="#region-chain"><span class=secno>2.2. </span>Region
       chain</a>
      <li><a href="#named-flow"><span class=secno>2.3. </span>Named flows</a>
      <li><a href="#regions-flow-breaking-rules"><span class=secno>2.4.
       </span>Regions flow breaking rules</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-into-property"><span class=secno>3.1. </span>The
       ‘<code class=property>flow-into</code>’ property</a>
      <li><a href="#the-flow-from-property"><span class=secno>3.2. </span>The
       ‘<code class=property>flow-from</code>’ property</a>
       <ul class=toc>
        <li><a href="#circular-dependencies"><span class=secno>3.2.1.
         </span>Cycle Detection</a>
        <li><a href="#fragmenting-regions"><span class=secno>3.2.2.
         </span>Nested fragmentation contexts</a>
       </ul>
      <li><a href="#region-flow-break"><span class=secno>3.3. </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-fragment-property"><span class=secno>3.4.
       </span>The region-fragment property</a>
      <li><a href="#the-at-region-style-rule"><span class=secno>3.5.
       </span>The @region rule</a>
     </ul>
    <li><a href="#multi-column-regions"><span class=secno>4.
     </span>Multi-column regions</a>
    <li><a href="#pseudo_elements"><span class=secno>5.
     </span>Pseudo-elements</a>
     <ul class=toc>
      <li><a href="#processing-model"><span class=secno>5.1. </span>Processing
       model</a>
     </ul>
    <li><a href="#cssom_view_and_css_regions"><span class=secno>6.
     </span>CSSOM</a>
     <ul class=toc>
      <li><a href="#the-namedflow-interface"><span class=secno>6.1. </span>The
       NamedFlow interface</a>
      <li><a href="#the-region-interface"><span class=secno>6.2. </span>The
       Region interface</a>
      <li><a href="#region-style-rule-interface"><span class=secno>6.3.
       </span>The CSSRegionStyleRule interface</a>
      <li><a href="#region-flow-layout-events"><span class=secno>6.4.
       </span>Region flow layout events</a>
      <li><a href="#cssomview-and-regions"><span class=secno>6.5.
       </span>Clarifications on pre-existing APIs</a>
       <ul class=toc>
        <li><a
         href="#cssomview-getclientrects-and-getboundingclientrect"><span
         class=secno>6.5.1. </span><code class=idl>getClientRects()</code> and
         <code>getBoundingClientRects()</code></a>
        <li><a href="#cssomview-offset-attributes"><span class=secno>6.5.2.
         </span><code class=idl>offsetTop</code>, <code
         class=idl>offsetLeft</code>, <code class=idl>offsetWidth</code> and
         <code class=idl>offsetWidth</code></a>
       </ul>
     </ul>
    <li><a href="#regions-visual-formatting-details"><span class=secno>7.
     </span>Regions visual formatting details</a>
     <ul class=toc>
      <li><a href="#regions-flow-content-box"><span class=secno>7.1.
       </span>The Region Flow Content Box (RFBC)</a>
       <ul class=toc>
        <li><a href="#rfbc-width-resolution"><span class=secno>7.1.1.
         </span>RFCB ‘<code class=property>width</code>’ resolution</a>
       </ul>
      <li><a href="#regions-visual-formatting-steps"><span class=secno>7.2.
       </span>Regions visual formatting steps</a>
       <ul class=toc>
        <li><a href="#rfcb-flow-fragment-height-resolution"><span
         class=secno>7.2.1. </span>Step 1: RFCB flow fragment height
         resolution</a>
         <ul class=toc>
          <li><a href="#rfcb-flow-fragment-height-resolution-phase-1"><span
           class=secno>7.2.1.1. </span>RFCB flow fragment height resolution,
           Phase 1</a>
          <li><a href="#rfcb-flow-fragment-height-resolution-phase-2"><span
           class=secno>7.2.1.2. </span>RFCB flow fragment height resolution,
           Phase 2</a>
         </ul>
        <li><a href="#regions-boxes-layout"><span class=secno>7.2.2.
         </span>Step 2: region boxes layout</a>
        <li><a href="#named-flows-layout"><span class=secno>7.2.3. </span>Step
 : named flows layout</a>
       </ul>
      <li><a href="#regions-visual-formatting-implementation-note"><span
       class=secno>7.3. </span>Regions visual formatting: implementation
       note</a>
      <li><a href="#regions-visual-formatting-examples"><span class=secno>7.4.
       </span>Regions visual formatting example</a>
       <ul class=toc>
        <li><a href="#step-1--phase-1-laying-out-rfcbs-with-us"><span
         class=secno>7.4.1. </span>Step 1 - Phase 1: Laying out RFCBs with
         used height of zero</a>
        <li><a href="#step-1--phase-2-layout-flow-to-compute-t"><span
         class=secno>7.4.2. </span>Step 1 - Phase 2: Layout flow to compute
         the RFCBs' flow fragments heights</a>
        <li><a href="#step-2-layout-document-and-regions-witho"><span
         class=secno>7.4.3. </span>Step 2: Layout document and regions without
         named flows</a>
        <li><a href="#step-3-named-flows-layout"><span class=secno>7.4.4.
         </span>Step 3: named flows layout</a>
       </ul>
     </ul>
    <li><a href="#relation-to-document-events"><span class=secno>8.
     </span>Relation to document events</a>
    <li><a href="#relation-to-other-specifications"><span class=secno>9.
     </span>Relation to other specifications</a>
    <li><a href="#usecases"><span class=secno>10. </span>Use Cases</a>
    <li><a href="#conformance"><span class=secno>11. </span>Conformance</a>
    <li><a href="#changes"><span class=secno>12. </span>Changes</a>
     <ul class=toc>
      <li><a href="#changes_from_Aug_28_2012"><span class=secno>12.1.
       </span>Changes from August 28<sup>th</sup> 2012 version</a>
      <li><a href="#changes_from_May_03_2012"><span class=secno>12.2.
       </span>Changes from May 3<sup>rd</sup> 2012 version</a>
      <li><a href="#changes_from_November_29_2011"><span class=secno>12.3.
       </span>Changes from November 29<sup>th</sup> 2011 version</a>
      <li><a href="#changes_from_June_09_2011"><span class=secno>12.4.
       </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="#intro-example-code">Appendix A. Example Code
     for Introduction</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>Displaying the complex layouts of a typical magazine, newspaper, or
    textbook on the web requires capabilities beyond those available in
    existing CSS modules. Dynamic magazine layout in particular requires
    flexibility in placement of boxes for content flows. This is the purpose
    of the CSS regions module.
   <div class=issue-marker data-bug_id=15733 data-bug_status=NEW> <a
    href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15733">Issue-15733</a>
    <div class=issue-details>
     <p class=short-desc>Should the region specification define a mechanism to
      create blocks that can be regions with CSS syntax?
    </div>
   </div>
   <p>For more complex layouts, content needs to flow from one area of the
    page to the next without limitation on the areas' sizes and positions.
    These arbitrary areas are the target of specific content flows which this
    document calls <a href="#named-flow0">named flows</a>. Each <a
    href="#named-flow0">named flow</a> can be associated with a set of CSS
    Regions called a region chain. CSS Regions are based on the rectangular
    geometry of the CSS box model. Elements in a <a href="#named-flow0">named
    flow</a> are taken out of the normal visual formatting and rendered in a
    chain of CSS Regions.
   <h3 id=named-flows-and-regions><span class=secno>1.1. </span>Named flows
    and region chains</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/intro-example-layout.svg" width=400>
    <p class=caption>Layout requiring sophisticated content flow
   </div>
   <p>The designer's intent is to position an image in box ‘<code
    class=property>A</code>’ and to flow an article's content from box
    ‘<code class=css>1</code>’ through boxes ‘<code
    class=css>2</code>’, ‘<code class=css>3</code>’ and ‘<code
    class=css>4</code>’. Note that the second box should have two columns,
    and the image is not contained in the article. Box ‘<code
    class=css>4</code>’ should auto-size to render the remainder of the
    article content that did not fit in the earlier boxes.
   <p>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
   </div>
   <p>There is no existing mechanism in CSS to associate the content with
    these boxes arranged in this manner so that content flows as intended. The
    CSS regions module properties provide that mechanism.
   <p>The following example illustrates how the content of an
    <code>&lt;article&gt;</code> element becomes a <a
    href="#named-flow0">named flow</a> and how boxes marked with ‘<code
    class=css>region1</code>’, ‘<code class=css>region2</code>’,
    ‘<code class=css>region3</code>’ and ‘<code
    class=css>region4</code>’ IDs become CSS Regions that consume the
    ‘<code class=css>article_flow</code>’ content.
   <div class=example>
    <pre>
 &lt;style&gt;
   article {
     <strong>flow-into: article_flow;</strong>
   }
   #region1, #region2, #region3, #region4 {
     <strong>flow-from: article_flow;</strong>
   }
 &lt;/style&gt;
       </pre>
   </div>
   <p>The ‘<code class=css>article_flow</code>’ value on the ‘<a
    href="#flow-into"><code class=property>flow-into</code></a>’ property
    directs the <code>article</code> element to the ‘<code
    class=css>article_flow</code>’ <a href="#named-flow0">named flow</a>.
    Setting the ‘<a href="#flow-from"><code
    class=property>flow-from</code></a>’ property on block containers to
    ‘<code class=css>article_flow</code>’ makes them CSS Regions and
    associates the resulting region chain with the <a
    href="#named-flow0">named flow</a>: the flow is ‘<code
    class=css>poured</code>’ into the region chain.
   <p>See <a href="#intro-example-code">Appendix A</a> for sample code using
    grid layout that implements this example
   <div class=issue-marker data-bug_id=16858 data-bug_status=ASSIGNED> <a
    href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16858">Issue-16858</a>
    <div class=issue-details>
     <p class=short-desc>Should creation of regions from elements be
      disallowed?
    </div>
   </div>
   <div class=note><span class=note-prefix>Note </span>
    <p><strong>CSS Regions are independent from layout</strong>
    <p>Any of the CSS layout facilities can be used to create, position and
     size boxes that can become CSS Regions. Using a grid layout <a
     href="#CSS3-GRID-LAYOUT"
     rel=biblioentry>[CSS3-GRID-LAYOUT]<!--{{CSS3-GRID-LAYOUT}}--></a> is just
     an example. The example could use a flexible box layout <a
     href="#CSS3-FLEXBOX"
     rel=biblioentry>[CSS3-FLEXBOX]<!--{{CSS3-FLEXBOX}}--></a> instead.
    <p>The CSS regions specification does not define a layout mechanism and is
     meant to integrate with existing and future CSS layout facilities.
    <p><strong>CSS Regions do not have to be elements</strong>
    <p>The CSS regions module is independent of the layout of boxes and the
     mechanism used to create them. For simplicity, our example code in <a
     href="#intro-example-code">Appendix A</a> uses elements to define the
     boxes.
    <p>While the example uses elements for CSS Regions (since <a
     href="#CSS3-GRID-LAYOUT"
     rel=biblioentry>[CSS3-GRID-LAYOUT]<!--{{CSS3-GRID-LAYOUT}}--></a>
     requires elements to create grid items) it is important to note that this
     is not required. CSS Regions can be pseudo-elements, such as ‘<code
     class=css>::before</code>’ and ‘<code class=css>::after</code>’.
     The only requirement for an element or pseudo-element to become a CSS
     Region is that it needs to be subject to CSS styling to receive the ‘<a
     href="#flow-from"><code class=property>flow-from</code></a>’ property.
     The CSS page template module (see <a href="#CSS3-PAGE-TEMPLATE"
     rel=biblioentry>[CSS3-PAGE-TEMPLATE]<!--{{CSS3-PAGE-TEMPLATE}}--></a>)
     proposes another mechanism to create stylable boxes that can become CSS
     Regions with the <code class=css>@slot</code> syntax.
   </div>
   <h3 id=region-styling><span class=secno>1.2. </span>CSS Region names and
    styling</h3>
   <div class=issue-marker data-bug_id=16859 data-bug_status=NEW> <a
    href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16859">Issue-16859</a>
    <div class=issue-details>
     <p class=short-desc>Reconsider using @rule for region styling
    </div>
   </div>
   <p>Content can be styled depending on the CSS Region it flows into. It is
    an extension of pseudo-element styling such as <a
    href="http://www.w3.org/TR/css3-selectors/#first-line"> <code
    class=css>::first-line</code> </a> which applies a particular style to a
    fragment of content. With CSS Region styling, additional selectors may
    apply depending on the CSS Region into which content flows.
   <p>In our example, the designer wants to make text flowing into #region1
    dark blue and bold.
   <p>This design can be expressed as shown below.
   <div class=example>
    <pre>
 &lt;style&gt;
   @region #region1 {
       p {
           color: #0C3D5F;
           font-weight: bold;
       }
 &lt;/style&gt;</pre>
   </div>
   <p>The <code class=css>@region #region1</code> rule limits its selectors to
    content flowing into <code class=css>#region1</code>. The following figure
    shows how the rendering changes if we apply styling specific to
    <code>#region1</code>. Note how less text fits into this box now that the
    ‘<code class=property>font-weight</code>’ is bold instead of normal.
   <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
   </div>
   <h2 id=css-regions-concepts><span class=secno>2. </span>CSS regions
    concepts</h2>
   <h3 id=regions><span class=secno>2.1. </span>Regions</h3>
   <div class="issue-marker wrapper">
    <div class=issue-marker data-bug_id=15186 data-bug_status=NEW> <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15186">Issue-15186</a>
     <div class=issue-details>
      <p class=short-desc>Is a mechanism to auto-generate regions necessary in
       order to support reusable style sheets?
     </div>
    </div>
   </div>
   <p id=region> A <dfn id=css-region title=region>CSS Region</dfn> is a block
    container that has an associated <a href="#named-flow0"><em><span>named
    flow</span></em></a> (see the ‘<a href="#flow-from"><code
    class=property>flow-from</code></a>’ property).
   <h3 id=region-chain><span class=secno>2.2. </span>Region chain</h3>
   <p>A <dfn id=region-chain0>region chain</dfn> is the sequence of regions
    that are associated with a <a href="#named-flow0">named flow</a>. <a
    href="#css-region" title=region>CSS Regions</a> in a region chain receive
    content from the named flow following their order in the chain. <a
    href="#css-region" title=region>CSS Regions</a> are organized into a <span
    title=region-chain>region chain</span> according to the document order.
   <h3 id=named-flow><span class=secno>2.3. </span>Named flows</h3>
   <p>A <dfn id=named-flow0>named flow</dfn> is the ordered sequence of
    elements associated with a flow with a given identifier. Elements in a <a
    href="#named-flow0">named flow</a> are ordered according to the document
    order.
   <p>Elements are placed into a <a href="#named-flow0">named flow</a> with
    the ‘<a href="#flow-into"><code class=property>flow-into</code></a>’
    property. The elements in a <a href="#named-flow0">named flow</a> are laid
    out in the <span title=region-chain>region chain</span> that is associated
    with this <a href="#named-flow0">named flow</a>.
   <p>Content from a <a href="#named-flow0">named flow</a> is broken up
    between regions according to the regions flow breaking rules.
   <p>A <a href="#named-flow0">named flow</a> is created when at least one
    element is moved into the flow with the given identifier or when at least
    one CSS Region requests content from that flow.
   <h3 id=regions-flow-breaking-rules><span class=secno>2.4. </span>Regions
    flow breaking rules</h3>
   <p>Breaking a <a href="#named-flow0">named flow</a> across a region chain
    is similar to breaking a document's content across pages (see <a
    href="#CSS3PAGE" rel=biblioentry>[CSS3PAGE]<!--{{CSS3PAGE}}--></a>) or a
    multi-column element's content across column boxes (see <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>). One difference is that
    page boxes are generated based on the available content whereas a region
    chain is a set of recipient boxes for the <a href="#named-flow0">named
    flow</a> content whose dynamic generation is not in the scope of this
    specification.
   <p>Each CSS Region in turn consumes content from its associated <a
    href="#named-flow0">named flow</a>. The <a href="#named-flow0">named
    flow</a> content is positioned in the <dfn id=current-region
    title=current-region>current region</dfn> until a natural or forced region
    break occurs, at which point the next region in the <span
    title=region-chain>region chain</span> becomes the <a
    href="#current-region" title=current-region>current region</a>. If there
    are no more <a href="#css-region">region</a>s 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-fragment"><code
    class=property>region-fragment</code></a>’ property on the last <a
    href="#css-region">region</a> in the chain.
   <p>The CSS regions module follows the fragmentation rules defined in the
    CSS Fragmentation Module Level 3 (see <a href="#CSS3-BREAK"
    rel=biblioentry>[CSS3-BREAK]<!--{{!CSS3-BREAK}}--></a>).
   <h2 id=properties-and-rules><span class=secno>3. </span>Properties and
    rules</h2>
   <h3 id=the-flow-into-property><span class=secno>3.1. </span>The ‘<a
    href="#flow-into"><code class=property>flow-into</code></a>’ property</h3>
   <div class="issue-marker wrapper">
    <div class=issue-marker data-bug_id=16527 data-bug_status=ASSIGNED> <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16527">Issue-16527</a>
     <div class=issue-details>
      <p class=short-desc>[Shadow]: getFlowByName and shadow DOM
     </div>
    </div>
   </div>
   <p>The ‘flow-into’ property can place an element into a <a
    href="#named-flow0">named flow</a>. Elements that belong to the same flow
    are laid out in the region chain associated with that flow.
   <table class=propdef summary="flow property definition">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn id=flow-into>flow-into</dfn>
     <tr>
      <th>Value:
      <td>&lt;ident&gt; | none | inherit
     <tr>
      <th>Initial:
      <td>none
     <tr>
      <th>Applies to:
      <td>any element.
       <p>The ‘<a href="#flow-into"><code
        class=property>flow-into</code></a>’ property does not apply to any
        <a
        href="http://www.w3.org/TR/selectors/#pseudo-elements">pseudo-element</a>
        such as <code class=css>::first-line</code>, <code
        class=css>::first-letter</code>, <code class=css>::before</code> or
        <code class=css>::after</code>.
     <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>none
    <dd>The element is not moved to a <a href="#named-flow0">named flow</a>
     and normal CSS processing takes place.
    <dt><a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
    <dd>The element is taken out of its parent's flow and placed into the flow
     with the name ‘<code class=css><a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a></code>’.
     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>’, ‘<code class=property>auto</code>’
     and ‘<code class=property>initial</code>’ are invalid flow names.
   </dl>
   <p>A <a href="#named-flow0">named flow</a> needs to be associated with a
    region chain (one or more CSS regions) for its elements to be visually
    formatted. If no region chain is associated with a given <a
    href="#named-flow0">named flow</a>, the elements in the <a
    href="#named-flow0">named flow</a> 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 the same specified flow as one of its ancestors, it
    becomes a sibling of its ancestor for the purpose of layout in the region
    chain laying out content from that flow.
   <p>The ‘<a href="#flow-into"><code class=property>flow-into</code></a>’
    property does not affect the CSS cascade and inheritance for the elements
    on which it is specified. The ‘<a href="#flow-into"><code
    class=property>flow-into</code></a>’ property affects the visual
    formatting of elements placed into a <a href="#named-flow0">named flow</a>
    and of the region chain laying out content from a <a
    href="#named-flow0">named flow</a>.
   <p>The edges of the first CSS region in a region chain associated with a <a
    href="#named-flow0">named flow</a> establish the rectangle that is the <a
    href="http://www.w3.org/TR/CSS2/visudet.html#containing-block-details">containing
    block</a> used for absolutely positioned elements in the <a
    href="#named-flow0">named flow</a> which do not have an ancestor with a
    ‘<code class=property>position</code>’ of ‘<code
    class=property>absolute</code>’, ‘<code
    class=property>relative</code>’ or ‘<code
    class=property>fixed</code>’ (see <a href="#CSS21"
    rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>). That first CSS region
    rectangle is used as the containing block instead of the initial
    containing block. This does not affect fixed position elements in the <a
    href="#named-flow0">named flow</a> - they are still positioned relative to
    the viewport or page area even if they have been redirected to a named
    flow.
   <p>The first region defines the <a
    href="http://www.w3.org/TR/css3-writing-modes/#writing-mode">writing
    mode</a> for the entire flow. The writing mode on subsequent regions is
    ignored.
   <p>Elements in a <a href="#named-flow0">named flow</a> are sequenced in
    document order. The structure of a named flow is equivalent to the result
    of moving the elements to a common parent. The visual formatting model
    uses the relationships between elements in the named flow structure as
    input, rather than the elements’ original positions.
   <div class=note><span class=note-prefix>Note </span>
    <p>The ‘<a href="#flow-into"><code
     class=property>flow-into</code></a>’ property moves an element into the
     flow and the interplay with selectors should be considered carefully.
    <p>For example,
    <pre>table {flow-into: table-content}</pre>
    <p>will move all tables in the ‘<code
     class=property>table-content</code>’ <a href="#named-flow0">named
     flow</a>. However, the
    <pre>table &gt; * {flow-into: table-content} ...</pre>
    <p>selector will move all immediate children of all table elements into
     the ‘table-content’ <a href="#named-flow0">named flow</a> (which may
     be useful as it will usually result, if the immediate children are rows,
     in merging rows of multiple tables), but the
    <pre>table * {flow-into: table-content}</pre>
    <p>selector will move all descendants of table elements into the
     ‘table-content’ <a href="#named-flow0">named flow</a>, transforming
     the element tree into a flat list in order of opening tags (which is
     probably not intentional). This will make all the descendants of table
     elements siblings in the <a href="#named-flow0">named flow</a>. Having
     the descendants become siblings in the <a href="#named-flow0">named
     flow</a> results in a different processing (see <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/tables.html#anonymous-boxes">CSS
 .1‘<code class=css>s anonymous table objects</code></a>). This note
     illustrates how authors must exercise caution when choosing a particular
     selector for setting the ’flow-into' property to avoid unintended
     results.
   </div>
   <div class=note><span class=note-prefix>Note </span>
    <p>Another consequence of moving elements into named flows is that
     surrounding whitespace is not moved into the named flow. If you have code
     like this:
    <pre>span {flow-into: span-content}
 &lt;span&gt;one&lt;/span&gt;
 &lt;span&gt;two&lt;/span&gt;
 </pre>
    Then the ‘<code class=property>span-content</code>’ named flow
    contents will contain this:
    <pre>&lt;span&gt;one&lt;/span&gt;&lt;span&gt;two&lt;/span&gt;
 </pre>
    Which will change the display from "one two" to "onetwo". If whitespace is
    significant, then moving the parent that contains the whitespace to the
    named flow is required.</div>
   <div class=note><span class=note-prefix>Note </span>
    <p>A future CSS Regions module level will define a mechanism for creating
     a named flow from an external resource.
   </div>
   <h3 id=the-flow-from-property><span class=secno>3.2. </span>The ‘<a
    href="#flow-from"><code class=property>flow-from</code></a>’ property</h3>
   <p>The ‘<a href="#flow-from"><code class=property>flow-from</code></a>’
    property makes a block container a region and associates it with a <a
    href="#named-flow0">named flow</a>.
   <table class=propdef summary="flow-from property definition">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn id=flow-from>flow-from</dfn>
     <tr>
      <th>Value:
      <td>&lt;ident&gt; | none | inherit
     <tr>
      <th>Initial:
      <td>none
     <tr>
      <th>Applies to:
      <td>Non-replaced <a
       href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">block
       containers</a>. <br>
       This might be expanded in future versions of the specification to allow
       other types of containers to receive flow content.
     <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><strong>none</strong>
    <dd>The block container is not a <a href="#css-region"
     title=region>region</a>.
    <dt><strong><a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a></strong>
    <dd> If the ‘<code class=property>content</code>’ property computes to
     something else than ‘<code class=property>normal</code>’ (or ‘<code
     class=property>none</code>’ for a pseudo-element), the block container
     does not become a <span>CSS Region</span>. If the ‘<code
     class=property>content</code>’ property computes to ‘<code
     class=property>normal</code>’ (or ‘<code
     class=property>none</code>’ for a pseudo-element), then the block
     container becomes a <span>CSS Region</span> and is ordered in a <a
     href="#region-chain0">region chain</a> according to its document order.
     The content from the flow with the <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
     name will be <a href="#region-flow-break">broken into fragments</a> and
     visually formatted in the <a
     href="http://www.w3.org/TR/CSS21/visuren.html#principal-box">principal
     boxes</a> of the <a href="#css-region">regions</a> in the <a
     href="#region-chain0">region chain</a>. <br>
     If there is no flow with name <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>,
     then the block container does not format any content visually.
   </dl>
   <p>A <span>CSS Region</span>‘<code class=css>s document children are not
    visually formatted unless they are directed to a <a
    href="#named-flow0">named flow</a> with an associated <a
    href="#region-chain0">region chain</a>.</code>
   <div class=note><span class=note-prefix>Note </span>
    <p>An block container becomes a <span>CSS Region</span> when its
     ’flow-from' property is set to a valid <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
     value, even if there is no content contributing to the referenced flow.
     For example:
    <pre>
 &lt;style&gt;
   .article{
     flow-into: thread;
   }
   .region{
     flow-from: thread;
   }
 &lt;/style&gt;
 &lt;html&gt;
   &lt;body&gt;
     &lt;div class=region&gt;div content&lt;/div&gt;
   &lt;/body&gt;
 &lt;/html&gt;
         </pre>
    There is no element matching the <code>.article</code> selector and
    therefore no content in the <code>thread</code> flow. However, the block
    container matching the <a href="#css-region"><code>.region</code></a>
    selector is still associated with that empty <a href="#named-flow0">named
    flow</a> and, consequently, its children are not formatted visually.</div>
   <div class=note><span class=note-prefix>Note </span>
    <p>At the time of this note-writing, the <code>display</code> values that
     always result in a non-replaced block container include
     <code>block</code>, <code>inline-block</code>, <code>table-cell</code>,
     <code>table-caption</code>, and <code>list-item</code>. All of these
     display values work as regions with non-replaced elements.
   </div>
   <div class="issue-marker wrapper">
    <div class=issue-marker data-bug_id=15824 data-bug_status=NEW> <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15824">Issue-15824</a>
     <div class=issue-details>
      <p class=short-desc>Should regions not create a new stacking context?
     </div>
    </div>
    <div class=issue-marker data-bug_id=15827 data-bug_status=NEW> <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15824">Issue-15827</a>
     <div class=issue-details>
      <p class=short-desc>Specify behavior of stacking contexts that are split
       between regions
     </div>
    </div>
   </div>
   <p><a href="#css-region" title=region>CSS Regions</a> create a new <a
    href="http://www.w3.org/TR/CSS2/visuren.html#z-index">stacking
    context</a>. <a href="#css-region" title=region>CSS Regions</a> establish
    a new <a
    href="http://www.w3.org/TR/CSS2/visuren.html#block-formatting">block
    formatting Context</a>.
   <div class=note><span class=note-prefix>Note </span>
    <p>With <a href="#region-chain0">region chains</a>, an element may be
     split across multiple boxes and these boxes may overlap (for example if
     they are absolutely positioned). So fragments of the same element can
     overlap each other. Since each element has a single z-index, it would be
     required to find another mechanism to decide in which order
     the fragments are rendered. Since each <span>CSS Region</span> creates a
     new stacking context, it is clear that each fragment is rendered
     separately and their rendering order follows the regular CSS rendering
     model.
   </div>
   <p>Floats or other exclusions (see <a href="#CSS3-EXCLUSIONS"
    rel=biblioentry>[CSS3-EXCLUSIONS]<!--{{CSS3-EXCLUSIONS}}--></a>)
    potentially impact the content laid out in region chains, just as for
    non-regions.
   <p>See the <a href="#regions-visual-formatting-details">regions visual
    formatting details</a> section for a description of how ‘<code
    class=property>width</code>’ and ‘<code
    class=property>height</code>’ values are resolved for <span>CSS
    Region</span> boxes.
   <h4 id=circular-dependencies><span class=secno>3.2.1. </span>Cycle
    Detection</h4>
   <p><a href="#named-flow0">Named flows</a> containing elements where the
    value of ‘<a href="#flow-from"><code
    class=property>flow-from</code></a>’ computes to an <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
    can produce nonsensical circular relationships, such as a <a
    href="#named-flow0">named flow</a> containing <span>CSS Regions</span> in
    its own <a href="#region-chain0">region chain</a>. These relationships can
    be easily and reliably detected and resolved, however, by keeping track of
    a dependency graph and using common cycle-detection algorithms.
   <p>The dependency graph consists of edges such that:
   <ul>
    <li>Every <a href="#named-flow0">named flow</a> depends on its elements
     where the value of ‘<a href="#flow-from"><code
     class=property>flow-from</code></a>’ computes to an <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
     .
    <li>Every element in a <a href="#named-flow0">named flow</a> where the
     value of ‘<a href="#flow-from"><code
     class=property>flow-from</code></a>’ computes to an <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
     depends on the <a href="#named-flow0">named flow</a> with the <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
     name.
   </ul>
   <p>If the graph contains a cycle, any elements where the value of ‘<a
    href="#flow-from"><code class=property>flow-from</code></a>’ computes to
    an <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
    participating in the cycle do not become <span>CSS Regions</span>.
   <h4 id=fragmenting-regions><span class=secno>3.2.2. </span>Nested
    fragmentation contexts</h4>
   <p>A <span>CSS Region</span> that contains a fragment of a <a
    href="#named-flow0">named flow</a> can itself be fragmented if it is
    nested within a fragmentation context <a href="#CSS3-BREAK"
    rel=biblioentry>[CSS3-BREAK]<!--{{!CSS3-BREAK}}--></a>, such as when a
    layout using a region chain is printed. In these cases break opportunities
    in the named flow fragment contained by the <span>CSS Region</span> are
    determined using the standard <a
    href="http://www.w3.org/TR/css3-break/">fragmentation rules</a>. In other
    words, each region box and its associated fragment should break as if it
    were a simple div containing the fragment contents. This can be controlled
    by using an avoid break value on the <span>CSS Region</span>, if that is
    desired.
   <h3 id=region-flow-break><span class=secno>3.3. </span>Region flow break
    properties: ‘<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>’</h3>
   <p>User agents laying out content in multiple regions must determine where
    content breaks occur. The problem of breaking content into fragments
    fitting in regions is similar to breaking content into pages or columns.
   <p>Each break ends layout in the current region and causes remaining pieces
    of content from the <a href="#named-flow0">named flow</a> to be visually
    formatted in the following region in the region chain, if there is one.
   <p>The following extends the ‘<a href="#break-before"><code
    class=property>break-before</code></a>’, ‘<a href="#break-after"><code
    class=property>break-after</code></a>’ and ‘<a
    href="#break-inside"><code class=property>break-inside</code></a>’
    properties from the <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a> specification to account
    for regions. The additional values are described below.
   <table class=propdef summary="break-before property definition">
    <tbody>
     <tr>
      <td><a href="#dom-named-flow-name"><em>Name:</em></a>
      <td><dfn id=break-before>break-before</dfn>
     <tr>
      <td><em>Value:</em>
      <td>auto | always | avoid | left | right | page | column | region |
       avoid-page | avoid-column | avoid-region
     <tr>
      <td><em>Initial:</em>
      <td>auto
     <tr>
      <td><em>Applies to:</em>
      <td>block-level elements
     <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="break-after property definition">
    <tbody>
     <tr>
      <td><a href="#dom-named-flow-name"><em>Name:</em></a>
      <td><dfn id=break-after>break-after</dfn>
     <tr>
      <td><em>Value:</em>
      <td>auto | always | avoid | left | right | page | column | region |
       avoid-page | avoid-column | avoid-region
     <tr>
      <td><em>Initial:</em>
      <td>auto
     <tr>
      <td><em>Applies to:</em>
      <td>block-level elements
     <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="break-inside property definition">
    <tbody>
     <tr>
      <td><a href="#dom-named-flow-name"><em>Name:</em></a>
      <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
     <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 and region break behavior
    before/after/inside the generated box. These values are normatively
    defined in <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a>:
   <p>This specification adds the following new values:
   <dl>
    <dt>region
    <dd>Always force a region break before (after) the generated box.
    <dt>avoid-region
    <dd>Avoid a region break before (after, inside) the generated box.
   </dl>
   <p>The behavior of region breaks with regards to regions is identical to
    the behavior of page breaks with regards to pages, as defined in the <a
    href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
   <h3 id=the-region-fragment-property><span class=secno>3.4. </span>The
    region-fragment property</h3>
   <table class=propdef summary="break-after property definition">
    <tbody>
     <tr>
      <td><a href="#dom-named-flow-name"><em>Name:</em></a>
      <td><dfn id=region-fragment>region-fragment</dfn>
     <tr>
      <td><em>Value:</em>
      <td>auto | break
     <tr>
      <td><em>Initial:</em>
      <td>auto
     <tr>
      <td><em>Applies to:</em>
      <td><em title="region element">CSS Regions</em>
     <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>The ‘<a href="#region-fragment"><code
    class=property>region-fragment</code></a>’ property controls the
    behavior of the <em id=last-region>last region</em> associated with a <a
    href="#named-flow0">named flow</a>.
   <dl>
    <dt>auto
    <dd>Content flows as it would in a regular content box. If the content
     exceeds the container box, it is subject to the <a
     href="http://www.w3.org/TR/CSS21/visufx.html#propdef-overflow">overflow</a>
     property's computed value on the CSS Region. Region breaks must be
     ignored on the last region.
    <dt>break
    <dd>
     <p>If the content fits within the <em>CSS Region</em>, then this property
      has no effect. If the content does not fit within the <em>CSS
      Region</em>, the content breaks as if flow was going to continue in a
      subsequent region. See the <a
      href="#regions-flow-breaking-rules">breaking rules</a> section. A forced
      region break takes precedence over a natural break point.
     <p>Flow content that follows the last break in the last region is not
      rendered.
   </dl>
   <p>The ‘<a href="#region-fragment"><code
    class=property>region-fragment</code></a>’ property does not influence
    the size of the region it applies to.
   <p>The following code sample illustrates the usage of the ‘<a
    href="#region-fragment"><code class=property>region-fragment</code></a>’
    property.
   <div class=example>
    <pre>
 &lt;style&gt;
 article {
   flow-into: "article";
 }
 #region_1, #region_2 {
   flow-from: article;
   <strong>region-fragment: break;</strong> /* or auto */
   <strong>overflow: visible;</strong> /* or hidden */
 }
 &lt;/style&gt;
 &lt;article&gt;...&lt;/article&gt;
 &lt;div id="region_1"&gt;&lt;/div&gt;
 &lt;div id="region_2"&gt;&lt;/div&gt;
       </pre>
   </div>
   <div class=figure>
    <table style="border: 1px solid gray;width: 100%;"
     summary="Different values for the region-fragment property - Illustration.">
     <tbody>
      <tr>
       <td>‘<code class=css>flow-into: "article"</code>’
       <td><code class=html>region_1</code> and <code
        class=html>region_2</code>
       <td>‘<code class=css>region-fragment: auto</code>’<br>
        ‘<code class=css>overflow:visible</code>’
      <tr>
       <td rowspan=3 style="vertical-align: top;"><img
        alt="regions receiving the flow content"
        src="images/region-overflow-flow.png">
       <td><img alt="result if region-overflow is set to 'break'"
        src="images/region-overflow-regions.png">
       <td><img alt="regions receiving the flow content"
        src="images/region-overflow-auto-overflow-visible.png">
      <tr>
       <td>‘<code class=css>region-fragment: break</code>’
       <td>‘<code class=css>region-fragment: auto</code>’<br>
        ‘<code class=css>overflow:hidden</code>’
      <tr>
       <td><img alt="result if region-overflow is set to 'break'"
        src="images/region-overflow-break.png">
       <td><img alt="regions receiving the flow content"
        src="images/region-overflow-auto-overflow-hidden.png">
    </table>
    <p class=caption>Different values for the region-fragment property
   </div>
   <div class=note><span class=note-prefix>Note </span>
    <p>The ‘<code class=property>overflow</code>’ property is honored on a
     region: if region content overflows, such as the borders of elements on
     the last line, the ‘<code class=property>overflow</code>’ property
     controls the visibility of the overflowing content. See the ‘<code
     class=property>overflow</code>’ property definition (<a href="#CSS21"
     rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>).
   </div>
   <h3 id=the-at-region-style-rule><span class=secno>3.5. </span>The @region
    rule</h3>
   <p>An ‘<code class=css>@region</code>’ rule contains style declarations
    specific to particular regions.
   <pre>
 @region &lt;selector&gt; {
     ... CSS styling rules ...
 }
     </pre>
   <p>The ‘<code class=css>@region</code>’ rule consists of the keyword
    ‘<code class=css>@region</code>’ followed by a <a
    href="http://www.w3.org/TR/css3-selectors/#selector-syntax">selector</a>
    and a block of style rules. The ‘<code class=css>@region</code>’ rule
    and the selector constitute the region's ‘<code class=css>flow
    fragment</code>’ selector. The region's flow fragment selector specifies
    which range of elements in the flow are subject to the style rules in the
    following block: it applies to the range (see <a href="#DOM"
    rel=biblioentry>[DOM]<!--{{!DOM}}--></a>) from the region's flow that
    flows in the selected region(s).
   <div class=issue-marker data-bug_id=15713 data-bug_status=NEW> <a
    href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15713">Issue-15713</a>
    <div class=issue-details>
     <p class=short-desc>Model for styling element fragments
    </div>
   </div>
   <p>Elements that are fully or partially in the ‘<code class=css>flow
    fragment</code>’ may match any of the selectors found in the style rule.
    However, the style rules only apply to the portion of the element that
    falls into the corresponding region.
   <p>Only a limited list of properties can be set in a region style rule:
   <ol>
    <li><a href="http://www.w3.org/TR/CSS2/fonts.html">font properties</a>
    <li><a href="http://www.w3.org/TR/CSS2/colors.html">color property</a>
    <li><a href="http://www.w3.org/TR/css3-color/#transparency">opacity
     property</a>
    <li><a href="http://www.w3.org/TR/css3-background/#backgrounds">background
     property</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/text.html#propdef-word-spacing">‘<code
     class=property>word-spacing</code>’</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/text.html#propdef-letter-spacing">‘<code
     class=property>letter-spacing</code>’</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/text.html#propdef-text-decoration">‘<code
     class=property>text-decoration</code>’</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/text.html#propdef-text-transform">‘<code
     class=property>text-transform</code>’</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height">‘<code
     class=property>line-height</code>’</a>
    <li><a href="http://www.w3.org/TR/css3-text/#justification">alignment and
     justification properties</a>
    <li><a href="http://www.w3.org/TR/css3-background/#borders">border
     properties</a>
    <li><a href="http://www.w3.org/TR/css3-background/#corners">rounded corner
     properties</a>
    <li><a href="http://www.w3.org/TR/css3-background/#border-images">border
     images properties</a>
    <li><a href="http://www.w3.org/TR/CSS2/box.html#margin-properties">margin
     properties</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/box.html#padding-properties">padding
     properties</a>
    <li><a href="http://www.w3.org/TR/css3-text/#text-shadow">‘<code
     class=property>text-shadow</code>’ property</a>
    <li><a
     href="http://www.w3.org/TR/css3-background/#the-box-shadow">‘<code
     class=property>box-shadow</code>’ property</a>
    <li><a
     href="http://www.w3.org/TR/css3-background/#the-box-decoration-break">‘<code
     class=property>box-decoration-break</code>’ property</a>
    <li><a
     href="http://www.w3.org/TR/CSS2/visudet.html#the-width-property">‘<code
     class=property>width</code>’ property</a>
   </ol>
   <div class=example id=region-style-example>
    <p>In the following example, the <a href="#named-flow0">named flow</a>
     ‘<code class=css>article_flow</code>’ flows through ‘<code
     class=css>region_1</code>’ and ‘<code class=css>region_2</code>’.
    <pre>
 &lt;style&gt;
     #div_1 {
         <strong>flow-into: article_flow;</strong>
     }
     #region_1, #region_2 {
         <strong>flow-from: article_flow;</strong>
     }
     /* region style "RSA" */
     <strong>@region #region_1, #region_2</strong> {
         div {...}
         p {...}
     }
     /* region style "RSB" */
     <strong>@region #region_1</strong> {
         p {...}
     }
 &lt;/style&gt;
 &lt;div id="div_1"&gt;
     &lt;p id="p_1"&gt;...&lt;/p&gt;
     &lt;p id="p_2"&gt;...&lt;/p&gt;
 &lt;/div&gt;
 &lt;div id="region_1"&gt;&lt;/div&gt;
 &lt;div id="region_2"&gt;&lt;/div&gt;
 </pre>
    <div id="region_styling_illustration"> <img
     alt="Illustration showing how a named flow content fits into regions to illustrate the @region styling."
     id="region_styling_img_2" name="region_styling_img_2"
     src="images/region-styling-2.png">
     <ul class=swatch-list>
      <li><span class=swatch style="background:#1C75BC"> </span>div div_1
      <li><span class=swatch style="background:crimson"> </span>paragraph p_1
      <li><span class=swatch style="background:white"> </span>paragraph p_2
      <li><span class=swatch style="background:#E6E7E8"> </span>range of flow
       that fits into region_1
      <li><span class=swatch style="background:#BCBEC0"> </span>range of flow
       that fits into region_2
     </ul>
    </div>
    <p>The region style ‘<code class=css>RSA</code>’ applies to flow
     content that is laid out in either ‘<code class=css>region_1</code>’
     or ‘<code class=css>region_2</code>’.
    <p>The first rule set ‘<code class=css>div {...}</code>’ applies to
     all <code class=html>&lt;div&gt;</code> elements that fit partially or
     fully into ‘<code class=css>region_1</code>’ or ‘<code
     class=css>region_2</code>’. <code class=html>div_1</code> is split
     between ‘<code class=css>region_1</code>’ and ‘<code
     class=css>region_2</code>’ and gets the style from this style rule.
    <p>The second rule set ‘<code class=css>p {...}</code>’ applies to all
     <code class=html>&lt;p&gt;</code> elements that fit into ‘<code
     class=css>region_1</code>’ or ‘<code class=css>region_2</code>’. In
     our example, both <code class=html>p_1</code> and <code
     class=html>p_2</code> are selected.
    <p>The region style ‘<code class=css>RSB</code>’ applies to flow
     content that fits in ‘<code class=css>region_1</code>’.
    <p>The first rule set ‘<code class=css>p {...}</code>’ matches <code
     class=html>p_1</code> and <code class=html>p_2</code> because these
     paragraphs flow into ‘<code class=css>region_1</code>’. Only the
     fragment of <code class=html>p_2</code> that flows into <code
     class=html>region_1</code> is styled with this rule.
   </div>
   <div class=issue-marker data-bug_id=15734 data-bug_status=NEW> <a
    href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15734">Issue-15734</a>
    <div class=issue-details>
     <p class=short-desc>@region and specificity
    </div>
   </div>
   <p>The specificity of the selectors in a ‘<code
    class=css>@region</code>’ rule is calculated as <a
    href="http://www.w3.org/TR/css3-selectors/#specificity">defined</a> in the
    CSS Selectors module (see <a href="#SELECT"
    rel=biblioentry>[SELECT]<!--{{SELECT}}--></a>). In other words, the
    ‘<code class=css>@region</code>’ rule adds an extra condition to the
    selector's matching, but does not change the selector's specificity. This
    is the same behavior as selectors appearing in ‘<code
    class=css>@media</code>’ rules declaration blocks (see <a href="#MEDIAQ"
    rel=biblioentry>[MEDIAQ]<!--{{MEDIAQ}}--></a>), where the rule does not
    influence the selectors' specificity.
   <p>Consequently, selectors that match a given element (as describe above),
    participate in the <a
    href="http://www.w3.org/TR/2011/REC-CSS2-20110607/cascade.html#cascading-order">CSS
    Cascading order</a> as defined in <a href="#CSS21"
    rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
   <div> Region styling does not apply to nested regions. For example, if a
    region ‘<code class=property>A</code>’ receives content from a flow
    that contains region ‘<code class=property>B</code>’, the content that
    flows into ‘<code class=property>B</code>’ does not receive the region
    styling specified for region ‘<code class=property>A</code>’.</div>
   <h2 id=multi-column-regions><span class=secno>4. </span>Multi-column
    regions</h2>
   <p>A <a
    href="http://dev.w3.org/csswg/css3-multicol/#multi-column-element">multi-column</a>
    <a href="#CSS3COL" rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a> element
    can have an assigned <a href="#named-flow0"><em><span>named
    flow</span></em></a>. The element becomes part of the region chain for
    that named flow, and flows its content fragments through columns according
    to the multi-column specification <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a>. In particular, when
    computing the <a href="#rfcb-flow-fragment-height-resolution">flow
    fragment height</a> of a multi-column element that is associated with a <a
    href="#named-flow0"><em><span>named flow</span></em></a>, the <a
    href="http://dev.w3.org/csswg/css3-multicol/#column-fill">‘<code
    class=property>column-fill</code>’</a> <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a> property is honored to
    balance the fragments of content that would flow through its columns.
   <div class=example>
    <p>The following example:
    <pre>
 &lt;style&gt;
    #multi-col {
        column-count: 2;
        flow-from: article;
        height: 6em;
        column-gap: 1em;
    }
    #remainder {
        flow-from: article;
        height: auto;
    }
 &lt;/style&gt;
 &lt;article&gt;...&lt;/article&gt;
 &lt;div id="multicol"&gt;&lt;/div&gt;
 &lt;div id="remainder"&gt;&lt;/div&gt;</pre>
    <p>is equivalent in rendering to, for example:
    <pre>
 &lt;style&gt;
    #flex {
        display: flex;
        flex-pack: justify;
        height: 6em;
    }
    #flex > div {
        flow-from: article;
        width: calc(50% - 0.5em);
    }
    #remainder {
        flow-from: article;
        height: auto;
    }
 &lt;/style&gt;
 &lt;article&gt;...&lt;/article&gt;
 &lt;div id="flex"&gt;
    &lt;div /&gt;
    &lt;div /&gt;
 &lt;/div&gt;
 &lt;div id="remainder"&gt;&lt;/div&gt;</pre>
   </div>
   <p>Overflow of multicol regions is mostly handled according to the same
    rules as other CSS Regions. If the remainder of the named flow does not
    fit in the multicol region, then the rest of the content flows into the
    remaining region chain. However, if a multicol region is the last region
    in a <a href="#region-chain0">region chain</a>, then the multicol region
    must follow the <a
    href="http://dev.w3.org/csswg/css3-multicol/#overflow-columns">overflow
    column rules</a> <a href="#CSS3COL"
    rel=biblioentry>[CSS3COL]<!--{{!CSS3COL}}--></a>.
   <h2 id="pseudo_elements"><span class=secno>5. </span>Pseudo-elements</h2>
   <div class=issue-marker data-bug_id=15188 data-bug_status=NEW> <a
    href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15188">Issue-15188</a>
    <div class=issue-details>
     <p class=short-desc>add functionality for having textual description like
      "continued here" or "continued on page x" to regions.
    </div>
   </div>
   <p>It can be useful to visually mark the content to highlight that a
    content thread is flowing through the region chain. For example, a marker
    such as <em>‘<code class=css>continued below</code>’</em> clearly
    indicates, at the end of a CSS Region, that there is more content in the
    flow which can be found by scrolling past whatever content interrupts the
    region chain.
   <p>The ‘<code class=css>::before</code>’ and ‘<code
    class=css>::after</code>’ pseudo-elements (see <a href="#SELECT"
    rel=biblioentry>[SELECT]<!--{{!SELECT}}--></a>) let content authors mark
    the beginning and end of a region with such markers.
   <h3 id=processing-model><span class=secno>5.1. </span>Processing model</h3>
   <p>The ‘<code class=css>::before</code>’ content is laid out in the
    region prior to any other content coming from the flow. Note that it is
    possible to set the ‘<code class=css>::before</code>’ pseudo-element's
    ‘<code class=property>display</code>’ property to ‘<code
    class=property>run-in</code>’ to align it with the content's inline
    boxes.
   <p>The ‘<code class=css>::after</code>’ content is laid out in the
    region after laying out the flow fragment content. Then, flow content is
    removed from the fragment to accommodate the ‘<code
    class=css>::after</code>’ content. Accommodating means that the ‘<code
    class=css>::after</code>’ content is laid out without overflowing the
    region. The ‘<code class=property>display</code>’ property of the
    ‘<code class=css>::after</code>’ content can be set to ‘<code
    class=property>run-in</code>’ to align with the region's content's
    inline boxes. In that case, the ‘<code class=css>::after</code>’
    content becomes the last inline box of the previous element in the flow
    that has some visual rendering in the region and can accommodate for the
    ‘<code class=css>::after</code>’ box.
   <p>If there is not enough room to accommodate the ::before content, the
    ‘<code class=css>::after</code>’ content after removing all flow
    fragment content, or a combination of the two, then the ::before and/or
    ::after content overflows.
   <h2 id="cssom_view_and_css_regions"><span class=secno>6. </span>CSSOM</h2>
   <div class=issue-marker-wrapper>
    <div class=issue-marker data-bug_id=15679 data-bug_status=NEW> <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15679">Issue-15679</a>
     <div class=issue-details>
      <p class=short-desc>elementFromPoint and CSS regions
     </div>
    </div>
   </div>
   <p>Since content may flow into multiple regions, authors need a way to
    determine if there are enough regions to flow all the content from a named
    flow. This is especially important considering that the size of regions or
    the size of the content may change depending on the display context. For
    example, flowing the same content on a mobile phone with a small screen
    may require more regions than on a large desktop display. Another example
    is the user changing the font size of text flowing through regions.
    Depending on the change, new regions may be needed to accommodate for the
    additional space required to fit the larger text or some regions may need
    to be removed for smaller text.
   <h3 id=the-namedflow-interface><span class=secno>6.1. </span>The NamedFlow
    interface</h3>
   <p>A <a href="#named-flow0">named flow</a> is created when it becomes
    referenced by the ‘<a href="#flow-into"><code
    class=property>flow-into</code></a>’ and/or ‘<a
    href="#flow-from"><code class=property>flow-from</code></a>’ computed
    values. The following APIs allow scripts to reference a <a
    href="#dom-named-flow"><code class=idl>NamedFlow</code></a> object
    representation of a <a href="#named-flow0">named flow</a>.
   <p>An additional method on the <a
    href="http://www.w3.org/TR/dom/#interface-document"><code
    class=idl>Document</code></a> interface provide access to <a
    href="#named-flow0">named flows</a>.
   <pre class=idl>
 partial interface <a
    href="http://www.w3.org/TR/dom/#interface-document">Document</a> {
   <a
    href="#dom-named-flow-collection">NamedFlowCollection</a> <a
    href="#document-getnamedflows">getNamedFlows()</a>;
 };
 </pre>
   <p>The <dfn id=document-getnamedflows><code
    class=idl>getNamedFlows()</code></dfn> method on the <a
    href="http://www.w3.org/TR/dom/#interface-document"><code
    class=idl>Document</code></a> interface returns a static snapshot of all
    the current <a href="#named-flow0">named flows</a> in the document.
   <p>The <a href="#dom-named-flow"><code class=idl>namedFlows</code></a>
    collection must include all <a href="#named-flow0">named flows</a> that
    are currently in the <code>CREATED</code> state. The list must not include
    <a href="#named-flow0">named flows</a> that are in the <code>NULL</code>
    state.
   <ul>
    <li><code class=idl>NULL</code>: the user agent must return
     <code>null</code>.
    <li><code class=idl>CREATED</code>: the user agent must return a valid <a
     href="#dom-named-flow"><code class=idl>NamedFlow</code></a> instance.
   </ul>
   <p>The <dfn id=dom-named-flow-collection><code
    class=idl>NamedFlowCollection</code></dfn> interface provides a list of
    current <a href="#dom-named-flow"><code class=idl>NamedFlow</code></a>
    instances in the document. The collection is a snapshot of the data.
   <pre class=idl>
 interface <a
    href="#dom-named-flow-collection">NamedFlowCollection</a> {
   readonly attribute unsigned long <a
    href="#dom-named-flow-collection-length">length</a>;
   [IndexGetter] <a
    href="#dom-named-flow">NamedFlow?</a> <a
    href="#dom-named-flow-collection-item">item</a> (unsigned long index);
   [NameGetter] <a
    href="#dom-named-flow">NamedFlow?</a> <a
    href="#dom-named-flow-collection-named-item">namedItem</a> (DOMString name);
 };</pre>
   <p>The <dfn id=dom-named-flow-collection-length><code
    class=idl>length</code></dfn> attribute returns the number of items in the
    collection.
   <p>The <dfn id=dom-named-flow-collection-item><code
    class=idl>item(index)</code></dfn> method returns the <a
    href="#dom-named-flow"><code>NamedFlow</code></a> instance at index
    <em>index</em> in the collection or <code class=idl>undefined</code> if
    <em>index</em> is out of range. An object <code>collection</code>
    implementing <a
    href="#dom-named-flow-collection"><code>NamedFlowCollection</code></a>
    supports indices in the range <code>0 ≤ index <
    collection.length</code>.
   <p>The <dfn id=dom-named-flow-collection-named-item><code
    class=idl>namedItem(name)</code></dfn> method returns the <a
    href="#dom-named-flow"><code>NamedFlow</code></a> instance in the
    collection whose name attribute matches the supplied <a
    href="#dom-named-flow-name"><em>name</em></a>, or <code
    class=idl>undefined</code> if there is no match.
   <p>The <dfn id=dom-named-flow><code class=idl>NamedFlow</code></dfn>
    interface offers a representation of a <a href="#named-flow0">named
    flow</a> instance.
   <p>The <a href="#dom-named-flow"><code class=idl>NamedFlow</code></a>
    interface can be used for different purposes. For example, the <a
    href="#dom-named-flow-get-regions-by-content"><code>getRegionsByContent()</code></a>
    method can help navigate by bookmark: a script can find the <a
    href="#css-region">region</a>s that display a particular anchor and bring
    them to view.
   <p>Likewise, the interface allows authors to check if all content has been
    fitted into existing regions. If it has, the <a
    href="#dom-named-flow-overset"><code>overset</code></a> attribute would be
    false.
   <pre class=idl>
 interface <a href="#dom-named-flow">NamedFlow</a> : <a
    href="http://www.w3.org/TR/dom/#interface-eventtarget">EventTarget</a> {
   readonly attribute DOMString <a
    href="#dom-named-flow-name">name</a>;
   readonly attribute boolean <a
    href="#dom-named-flow-overset">overset</a>;
   sequence&lt;<a
    href="#region-interface">Region</a>&gt; <a
    href="#dom-named-flow-get-regions">getRegions()</a>;
   readonly attribute integer <a
    href="#dom-named-flow-first-empty-region-index">firstEmptyRegionIndex</a>;
   <a
    href="http://www.w3.org/TR/dom/#interface-nodelist">NodeList</a> <a
    href="#dom-named-flow-get-content">getContent()</a>;
   sequence&lt;<a
    href="#region-interface">Region</a>&gt; <a
    href="#dom-named-flow-get-regions-by-content">getRegionsByContent</a>(<a
    href="http://www.w3.org/TR/dom/#interface-node">Node</a> node);
 };</pre>
   <p>The <dfn id=dom-named-flow-name><code class=idl>name</code></dfn>
    attribute returns the name of the <a href="#dom-named-flow"><code
    class=idl>NamedFlow</code></a> instance.
   <p>The <dfn id=dom-named-flow-overset><code class=idl>overset</code></dfn>
    attribute returns true if there are <a href="#named-flow0">named flow</a>
    fragments that do not fit in the associated region chain. The attribute
    also returns true if there is no associated region chain. Otherwise, it
    returns false.
   <p>The <dfn id=dom-named-flow-get-regions><code
    class=idl>getRegions()</code></dfn> method returns the sequence of regions
    in the <a href="#region-chain0">region chain</a> associated with the <a
    href="#named-flow0">named flow</a>. Note that the returned values is a
    static sequence in document order.
   <p>The <dfn
    id=dom-named-flow-first-empty-region-index>firstEmptyRegionIndex</dfn> is
    the index of the first region in the region chain with the <a
    href="#dom-region-regionoverset"><code>regionOverset</code></a> attribute
    set to <code>empty</code>. If all regions have the <a
    href="#dom-region-regionoverset"><code>regionOverset</code></a> attribute
    set to <code>fit</code> or <a
    href="#dom-named-flow-overset"><code>overset</code></a>, the value for <a
    href="#dom-named-flow-first-empty-region-index"><code>firstEmptyRegionIndex</code></a>
    is <code>-1</code>. If there are no regions in the <a
    href="#region-chain0">region chain</a>, the value is <code>-1</code> as
    well.
   <p>The <dfn id=dom-named-flow-get-content>getContent()</dfn> method returns
    an ordered collection of nodes that constitute the <a
    href="#named-flow0">named flow</a>. The returned list is a static snapshot
    of the <a href="#named-flow0">named flow</a> content at the time the
    method is invoked. This list contains the elements that were moved to the
    <a href="#named-flow0">named flow</a> by their flow-into property but not
    their descendants (unless the descendants are themselves moved to the <a
    href="#named-flow0">named flow</a>).
   <p>The <dfn
    id=dom-named-flow-get-regions-by-content>getRegionsByContent()</dfn>
    method returns the sequence of regions that contain at least part of the
    target content node if it belongs to the <a href="#named-flow0">named
    flow</a> directly (i.e., its ‘<a href="#flow-into"><code
    class=property>flow-into</code></a>’ computed value is the <a
    href="#named-flow0">named flow</a>) or one of its ancestors belongs to the
    <a href="#named-flow0">named flow</a>. Otherwise, the method returns an
    empty sequence. The returned value is a static sequence in document order.
   <p>The <a href="#named-flow0">named flow</a> states are :
   <ul>
    <li><code class=idl>NULL</code>: the <a href="#named-flow0">named flow</a>
     is not referenced by any ‘<a href="#flow-into"><code
     class=property>flow-into</code></a>’ or ‘<a href="#flow-from"><code
     class=property>flow-from</code></a>’ computed value.
    <li><code class=idl>CREATED</code>: the <a href="#named-flow0">named
     flow</a> is referenced by at least one ‘<a href="#flow-into"><code
     class=property>flow-into</code></a>’ or ‘<a href="#flow-from"><code
     class=property>flow-from</code></a>’ computed value.
   </ul>
   <p>A <a href="#dom-named-flow"><code class=idl>NamedFlow</code></a> object
    is live: it always represents the <a href="#named-flow0">named flow</a>
    with the corresponding name even if that <a href="#named-flow0">named
    flow</a> has transitioned to the <code>NULL</code> state.
   <p>If a script holds a reference to a <a href="#dom-named-flow"><code
    class=idl>NamedFlow</code></a> instance that has entered the
    <code>NULL</code> state, its attributes and methods should behave as
    follows:
   <ul>
    <li>the <a href="#dom-named-flow-name"><code class=idl>name</code></a>
     attribute returns its value as normal.
    <li>the <a href="#dom-named-flow-overset"><code
     class=idl>overset</code></a> attribute returns <code
     class=idl>false</code>.
    <li>the <code class=idl>getRegions</code> method returns an empty
     sequence.
    <li>the <a href="#dom-named-flow-first-empty-region-index"><code
     class=idl>firstEmptyRegionIndex</code></a> attribute returns <code
     class=idl>-1</code>.
    <li>the <a href="#dom-named-flow-get-content"><code
     class=idl>getContent()</code></a> method returns an empty <code
     class=idl>NodeList</code> instance.
    <li>the <a href="#dom-named-flow-get-regions-by-content"><code
     class=idl>getRegionsByContent()</code></a> method retuns an empty
     sequence.
   </ul>
   <h3 id=the-region-interface><span class=secno>6.2. </span>The Region
    interface</h3>
   <p>The <dfn id=region-interface> <code class=idl>Region</code>
    interface</dfn> is a <a
    href="http://www.w3.org/TR/WebIDL/#idl-implements-statements">supplemental
    interface</a> which must be implemented by all objects (<a
    href="http://www.w3.org/TR/dom/#interface-element"> <code
    class=idl>Elements</code></a>, pseudo-elements or other CSS constructs
    such as <a
    href="http://dev.w3.org/csswg/css3-page-template/#templates-and-slots">slots</a>)
    in an implementation which can be CSS Regions.
   <pre class=idl>
 [NoInterfaceObject]
 interface <a
    href="#region-interface">Region</a> {
   readonly attribute DOMString <a
    href="#dom-region-regionoverset">regionOverset</a>;
   sequence&lt;<a
    href="#">Range</a>&gt;? <a
    href="#dom-region-getregionflowranges">getRegionFlowRanges()</a>;
   <a
    href="http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface">CSSStyleDeclaration</a> <a
    href="#dom-region-getComputedRegionStyle">getComputedRegionStyle</a>(Element elt);
   <a
    href="http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface">CSSStyleDeclaration</a> <a
    href="#dom-region-getComputedRegionStyle">getComputedRegionStyle</a>(Element elt, DOMString pseudoElt);
 };
 Element implements Region;</pre>
   <p>The <dfn id=dom-region-regionoverset><code
    class=idl>regionOverset</code></dfn> attribute returns one of the
    following values:
   <dl>
    <dt>‘<a href="#dom-named-flow-overset"><code
     class=property>overset</code></a>’
    <dd>The region is the last one in the <span title=region-chain>region
     chain</span> and not able to fit the remaining content from the <a
     href="#named-flow0">named flow</a>. Note that the region's <a
     href="http://www.w3.org/TR/CSS21/visufx.html#overflow"><code
     class=idl>overflow</code></a> property value can be used to control the
     visibility of the overflowing content and the ‘<a
     href="#region-fragment"><code
     class=property>region-fragment</code></a>’ property controls whether or
     not fragmentation happens on the content that overflows the last region.
    <dt>‘<code class=property>fit</code>’
    <dd>The region's flow fragment content fits into the region's <a
     href="http://www.w3.org/TR/CSS21/box.html#box-dimensions">content
     box</a>. If the region is the last one in the <span
     title=region-chain>region chain</span>, it means that the content fits
     without overflowing. If the region is not the last one in the region
     chain, that means the <a href="#named-flow0">named flow</a> content is
     further fitted in subsequent regions. In this last case, note that the <a
     href="#named-flow0">named flow</a> fragment may be empty (for example if
     the region is too small to accommodate any content). This value is
     returned if the <a href="#css-region">region</a> object is not (or no
     longer) a region.
    <dt>‘<code class=property>empty</code>’
    <dd>All content from the <a href="#named-flow0">named flow</a> was fitted
     in prior regions.
   </dl>
   <p>Note that if there is no content in the <a href="#named-flow0">named
    flow</a>, all regions associated with that <a href="#named-flow0">named
    flow</a> should have their <a href="#dom-region-regionoverset"><code
    class=idl>regionOverset</code></a> attribute return ‘<code
    class=property>empty</code>’. If there is content in the flow but that
    content does not generate any box for visual formatting, the ‘<a
    href="#dom-named-flow-overset"><code class=property>overset</code></a>’
    attribute on the first region in the region chain associated with the flow
    will return ‘<code class=property>fit</code>’.
   <p>The <dfn id=dom-region-getregionflowranges>getRegionFlowRanges</dfn>
    method returns an array of <a
    href="http://www.w3.org/TR/dom/#interface-range">Range</a> instances
    corresponding to fragment from the <a href="#named-flow0">named flow</a>
    that is laid out in the region. If the region has not received a fragment
    because it is too small to accommodate any, the method returns a single
    <code class=idl>Range</code> where the <code>startContainer</code> and
    <code>startOffset</code> have the same values as the
    <code>endContainer</code> and <code>endOffset</code> and therefore the
    collapsed attribute on the <code class=idl>Range</code> is true. In that
    situation, if the region is the first in the <a
    href="#region-chain0">region chain</a>, the <code>startContainer</code> is
    the first <code>Node</code> in the <a href="#named-flow0">named flow</a>
    and the <code>startOffset</code> is zero. If the region is the last region
    in the region chain (but not the first and only one), the
    <code>startContainer</code> and <code>startOffset</code> are the same
    values as the <code>endContainer</code> and <code>endOffset</code> on the
    previous region in the <a href="#region-chain0">region chain</a>. The
    method returns null if the <a href="#css-region">region</a> object is not
    (or no longer) a region.
   <p>The <dfn
    id=dom-region-getComputedRegionStyle>getComputedRegionStyle</dfn> methods
    on the Region interface work the same as the <a
    href="http://dev.w3.org/csswg/cssom/#extensions-to-the-window-interface"><code
    class=idl>getComputedStyle</code></a> <a href="#CSSOM"
    rel=biblioentry>[CSSOM]<!--{{!CSSOM}}--></a> methods on the <a
    href="http://www.whatwg.org/specs/web-apps/current-work/multipage/browsers.html#window"><code
    class=idl>Window</code></a> interface <a href="#HTML5"
    rel=biblioentry>[HTML5]<!--{{!HTML5}}--></a> with the following
    exceptions. For the <a href="#css-region"><code
    class=idl>Region</code></a> interface the <a
    href="http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface">CSSStyleDeclaration</a>
    returned must include the result of <a
    href="#the-at-region-style-rule">region styling</a>. If the element is
    fragmented across region boundaries, the <a
    href="http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface">CSSStyleDeclaration</a>
    returned must apply only to the fragment that flows through the <span>CSS
    Region</span>. The method returns null if the <a
    href="#css-region">region</a> object is not (or no longer) a region.
   <div class=note>
    <p>If the element is not contained in the <span>CSS Region</span> at all,
     the method returns the <a href="#the-at-region-style-rule">region
     styling</a> that would apply to the element if it were contained in the
     <span>CSS Region</span> (following how <a
     href="http://dev.w3.org/csswg/cssom/#extensions-to-the-window-interface"><code
     class=idl>getComputedStyle</code></a> works with elements not contained
     in the window's document).
   </div>
   <p>A <a href="#css-region"><code class=idl>Region</code></a> instance may
    represent an object that is no longer a <a href="#css-region">region</a>.
    This may happen for example if the ‘<a href="#flow-from"><code
    class=property>flow-from</code></a>’ property on the corresponding
    pseudo-element, element or other construct becomes ‘<code
    class=property>none</code>’ but a script is still holding a reference to
    the <a href="#css-region"><code class=idl>Region</code></a> object.
   <h3 id=region-style-rule-interface><span class=secno>6.3. </span>The
    CSSRegionStyleRule interface</h3>
   <p>The <dfn id=dom-region-style-rule-interface> <code
    class=idl>CSSRegionStyleRule</code> interface</dfn> represents an ‘<code
    class=css>@region</code>’ rule in a CSS style sheet. This rule type is
    added to the <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrule-interface"><code>CSSRule</code></a>
    <a href="#CSSOM" rel=biblioentry>[CSSOM]<!--{{!CSSOM}}--></a> interface.
   <pre class=idl>
 partial interface <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrule-interface">CSSRule</a> {
   const unsigned short <a
    href="#region-style-rule-const">REGION_STYLE_RULE</a> = 16;
 };
 </pre>
   <dl>
    <dt><dfn id=region-style-rule-const>REGION_STYLE_RULE</dfn>
    <dd>The rule is a CSSRegionStyleRule
   </dl>
   <pre class=idl>
 interface <a
    href="#dom-region-style-rule-interface">CSSRegionStyleRule</a> : <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrule-interface">CSSRule</a> {
   attribute DOMString <a
    href="#region-style-rule-selectorText">selectorText</a>;
   readonly attribute <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrulelist-sequence">CSSRuleList</a> <a
    href="#region-style-rule-cssRules">cssRules</a>;
   unsigned long <a
    href="#region-style-rule-insertRule">insertRule</a>(DOMString rule,  unsigned long index)
     raises(<a
    href="http://www.w3.org/TR/dom/#exception-domexception">DOMException</a>);
   void <a
    href="#region-style-rule-deleteRule">deleteRule</a>(unsigned long index)
     raises(<a
    href="http://www.w3.org/TR/dom/#exception-domexception">DOMException</a>);
 };
 </pre>
   <p>The <dfn id=region-style-rule-selectorText> <code
    class=idl>selectorText</code></dfn> attribute gets and sets the associated
    selector as defined in section <a
    href="http://dev.w3.org/csswg/cssom/#widl-CSSStyleRule-selectorText">6.4.3</a>
    of <a href="#CSSOM" rel=biblioentry>[CSSOM]<!--{{!CSSOM}}--></a>.
   <p>The <dfn id=region-style-rule-cssRules> <code
    class=idl>cssRules</code></dfn> attribute must return a <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrulelist-sequence"><code>CSSRuleList</code></a>
    <a href="#CSSOM" rel=biblioentry>[CSSOM]<!--{{!CSSOM}}--></a> object for
    the list of <a
    href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleRule"><code
    class=idl>CSSStyleRules</code></a> specified in the region style block.
   <p>The <dfn id=region-style-rule-insertRule> <code
    class=idl>insertRule(rule, index)</code></dfn> method inserts a <a
    href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleRule"><code
    class=idl>CSSStyleRule</code></a> into the region style block before the
    specified index. If the index is equal to the length of the <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrulelist-sequence"><code>CSSRuleList</code></a>
    the <a
    href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleRule"><code
    class=idl>CSSStyleRule</code></a> is inserted at the end of the list.
   <p>Possible exceptions:
   <ul>
    <li><a
     href="http://www.w3.org/TR/dom/#dom-domexception-hierarchy_request_err"><code
     class=idl>HIERARCHY_REQUEST_ERR</code></a>: Raised if the rule to insert
     is not a <a
     href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleRule"><code
     class=idl>CSSStyleRule</code></a>.
    <li><a
     href="http://www.w3.org/TR/dom/#dom-domexception-index_size_err"><code
     class=idl>INDEX_SIZE_ERR</code></a>: Raised if the specified index is not
     a valid insertion point.
    <li><a href="http://www.w3.org/TR/dom/#dom-domexception-syntax_err"><code
     class=idl>SYNTAX_ERR</code></a>: Raised if the specified rule has a
     syntax error and is unparsable.
   </ul>
   <p>The <dfn id=region-style-rule-deleteRule> <code
    class=idl>deleteRule(index)</code></dfn> method deletes the <a
    href="http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-CSSStyleRule"><code
    class=idl>CSSStyleRule</code></a> in the <a
    href="http://dev.w3.org/csswg/cssom/#the-cssrulelist-sequence"><code>CSSRuleList</code></a>
    at the specified index.
   <p>Possible exception:
   <ul>
    <li><a
     href="http://www.w3.org/TR/dom/#dom-domexception-index_size_err"><code
     class=idl>INDEX_SIZE_ERR</code></a>: Raised if the specified index does
     not correspond to a rule in the region style block.
   </ul>
   <h3 id=region-flow-layout-events><span class=secno>6.4. </span>Region flow
    layout events</h3>
   <p><a href="#dom-named-flow"><code class=idl>NamedFlow</code></a> objects
    are <a
    href="http://www.w3.org/TR/dom/#interface-eventtarget">EventTargets</a>
    which dispatch <code class=idl>regionLayoutUpdate</code> events when there
    is a possible layout change in their region chain. The event is
    asynchronous, and fires in the same step of the <a
    href="http://www.w3.org/TR/html5/webappapis.html#processing-model-2">event
    loop</a> <a href="#HTML5" rel=biblioentry>[HTML5]<!--{{!HTML5}}--></a> as
    when <a
    href="http://www.w3.org/TR/2012/WD-dom-20120405/#mutation-observers">MutationObservers</a>
    <a href="#DOM" rel=biblioentry>[DOM]<!--{{!DOM}}--></a> are invoked.
   <p>If region chain nesting occurs (the contents of a <a
    href="#dom-named-flow"><code class=idl>NamedFlow</code></a> contains
    regions for a different <a href="#dom-named-flow"><code
    class=idl>NamedFlow</code></a>) then the <code
    class=idl>regionLayoutUpdate</code> event for the nested flow(s) must be
    dispatched before the <code class=idl>regionLayoutUpdate</code> event for
    the containing flow is dispatched.
   <table border=0 cellpadding=2 cellspacing=0 class=event-desc
    summary="This table contains information about the semantics of the given event type">
    <tbody>
     <tr class="assert must">
      <th>Type
      <td class=eventname><strong><code>regionLayoutUpdate</code></strong>
     <tr class="assert must">
      <th>Interface
      <td><code><a
       href="http://www.w3.org/TR/DOM-Level-3-Events/#events-UIEvent">UIEvent</a></code>
       (see <a href="#DOM-LEVEL-3-EVENTS"
       rel=biblioentry>[DOM-LEVEL-3-EVENTS]<!--{{!DOM-LEVEL-3-EVENTS}}--></a>)
     <tr class="assert must">
      <th>Sync / Async
      <td>Async
     <tr class="assert must">
      <th>Bubbles
      <td>No
     <tr class="assert must">
      <th>Target
      <td><a href="#dom-named-flow"><code class=idl>NamedFlow</code></a>
     <tr class="assert must">
      <th>Cancelable
      <td>Yes
     <tr class="assert must">
      <th>Default action
      <td>none
     <tr class="assert must">
      <th>Context info
      <td>
       <ul>
        <li><code class=attribute-name>Event.target</code>: <a
         href="#dom-named-flow"><code class=idl>NamedFlow</code></a> whose
         layout may have changed
       </ul>
   </table>
   <h3 id=cssomview-and-regions><span class=secno>6.5. </span>Clarifications
    on pre-existing APIs</h3>
   <h4 id=cssomview-getclientrects-and-getboundingclientrect><span
    class=secno>6.5.1. </span><code class=idl>getClientRects()</code> and
    <code>getBoundingClientRects()</code></h4>
   <p>The <a href="http://www.w3.org/TR/cssom-view/">CSSOM View Module</a>
    defines how user agents compute the bounding client rectangle for an
    element (<code class=idl>getBoundingClientRect()</code>) and its generated
    boxes (<code class=idl>getClientRects()</code>).
   <p>This definition applies to the (possibly) multiple boxes generated for
    an element in a named flow flowing through a region chain. The <code
    class=idl>getClientRects()</code> method returns the list of boxes
    generated for each of the element fragments laid out in different regions.
    The <code>getBoundingClientRect()</code> method operates as specified in
    the <a href="http://www.w3.org/TR/cssom-view/">CSSOM View Module</a> as
    well and is computed from the set of rectangles returned by <code
    class=idl>getClientRects()</code>.
   <h4 id=cssomview-offset-attributes><span class=secno>6.5.2. </span><code
    class=idl>offsetTop</code>, <code class=idl>offsetLeft</code>, <code
    class=idl>offsetWidth</code> and <code class=idl>offsetWidth</code></h4>
   <p>The computation of the <a
    href="http://dev.w3.org/csswg/cssom-view/#extensions-to-the-htmlelement-interface">offset
    attributes</a> for elements laid out in a <a href="#named-flow0">named
    flow</a> follow the <a
    href="http://dev.w3.org/csswg/cssom-view/#extensions-to-the-htmlelement-interface">specification</a>
    <a href="#CSSOM" rel=biblioentry>[CSSOM]<!--{{!CSSOM}}--></a>. For the
    purpose of the algorithm, the <em>first CSS layout box</em> associated
    with an element laid out in a <a href="#named-flow0">named flow</a> is the
    box generated for the first region the element is laid out into.
   <h2 id=regions-visual-formatting-details><span class=secno>7.
    </span>Regions visual formatting details</h2>
   <p>Regions are laid out by CSS and take part in the normal box model and
    other layout models offered by CSS modules such as flexible boxes (<a
    href="#CSS3-FLEXBOX"
    rel=biblioentry>[CSS3-FLEXBOX]<!--{{CSS3-FLEXBOX}}--></a>). However, <a
    href="#css-region">regions</a> lay out a fragment of their <a
    href="#named-flow0">named flow</a> instead of laying out descendant
    content as happens with other boxes.
   <p>This section describes the model for laying out <a
    href="#css-region">regions</a> and for laying out <a
    href="#named-flow0">named flow</a> content into regions. The descriptions
    in this section are biased towards a horizontal writing mode, using
    <span>width</span> for logical width (or measure) and <span>height</span>
    for logical height (or extent) as <a
    href="http://www.w3.org/TR/css3-writing-modes/#abstract-dimensions">defined</a>
    in the CSS Writing Modes Module <a href="#CSS3-WRITING-MODES"
    rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>).
    To use this model in a vertical writing mode apply the principles <a
    href="http://www.w3.org/TR/css3-writing-modes/#vertical-layout">described</a>
    in that specification.
   <h3 id=regions-flow-content-box><span class=secno>7.1. </span>The Region
    Flow Content Box (RFBC)</h3>
   <p>A region box lays out the following boxes:
   <ul>
    <li>The boxes generated by its <code class=css>::before</code> and <code
     class=css>::after</code> pseudo-elements, if any.
    <li>The anonymous <dfn id=region-flow-content-box>region flow content
     box</dfn> (called <dfn id=rfcb>RFCB</dfn> in this document) which
     contains the fragment of the <a href="#named-flow0">named flow</a> that
     the region receives.
   </ul>
   <div class=figure> <img
    alt="The ::before, RFCB and ::after boxes contained in the Region Box"
    src="images/RFCB.svg" width=600px>
    <p class=caption>The Region Flow Content Box (RFCB)
   </div>
   <p>Laying out a <a href="#css-region">region</a> box follows the same
    processing rules as for any other <a
    href="http://www.w3.org/TR/CSS2/visuren.html#block-boxes">block container
    box</a>.
   <p>The <a href="#rfcb">RFCB</a> is a <a
    href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">block container
    box</a> with a <a href="">computed</a> ‘<code
    class=property>width</code>’ of ‘<code class=property>auto</code>’
    and a whose <a href="">used</a> ‘<code class=property>height</code>’
    is resolved as detailed below.
   <h4 id=rfbc-width-resolution><span class=secno>7.1.1. </span>RFCB ‘<code
    class=property>width</code>’ resolution</h4>
   <p>At various points in the visual formatting of documents containing
    regions, the used ‘<code class=property>width</code>’ of RFCBs and
    regions need to be resolved. In all cases, the resolution is done
    following the rules for <a
    href="http://www.w3.org/TR/CSS2/visudet.html#Computing_widths_and_margins">calculating
    widths and margins</a> (see <a href="#CSS21"
    rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>). Sometimes, resolving the
    used ‘<code class=property>width</code>’ value requires measuring the
    content's <code class=css>min-content</code> and <code
    class=css>max-content</code> values (as <a
    href="http://www.w3.org/TR/css3-writing-modes/#intrinsic-sizing">defined</a>
    in the CSS Writing Modes Module <a href="#CSS3-WRITING-MODES"
    rel=biblioentry>[CSS3-WRITING-MODES]<!--{{!CSS3-WRITING-MODES}}--></a>).
    For an RFCB, <strong>these measures are made on the <em>entire</em>
    associated <a href="#named-flow0">named flow</a> content</strong>.
   <p>As a consequence, all <a href="#rfcb">RFCBs</a> of <a
    href="#css-region">regions</a> associated with a given <a
    href="#named-flow0">named flow</a> share the same <code
    class=css>min-content</code> and <code class=css>max-content</code>
    measures.
   <p>This approach is consistent with the <a
    href="http://www.w3.org/TR/css3-break/#varying-size-boxes">box model for
    breaking</a> (<a href="#CSS3-BREAK"
    rel=biblioentry>[CSS3-BREAK]<!--{{!CSS3-BREAK}}--></a>).
   <h3 id=regions-visual-formatting-steps><span class=secno>7.2.
    </span>Regions visual formatting steps</h3>
   <p>Formatting documents that contain <a href="#named-flow0">named flows</a>
    laid out in regions is a three-step process:
   <ul>
    <li><em>Step 1: RFCB <dfn id=flow-fragment-height>flow fragment
     height</dfn> resolution</em>. In this step, the heights of fragments
     fitting in the regions' RFCBs are resolved.
    <li><em>Step 2: document and regions layout</em>. In this step, the
     document content and regions are laid out. However, <a
     href="#named-flow0">named flow</a> content is not laid out in regions
     yet.
    <li><em>Step 3: <a href="#named-flow0">named flow</a> layout</em>. In this
     step, the content of <a href="#named-flow0">named flows</a> is laid out
     in their respective <a href="#region-chain0">region chains</a>.
   </ul>
   <div class=figure> <img
    alt="visual representation of the three-step process"
    src="images/regions-layout-three-steps.svg" width=600px>
    <p class=caption>Regions visual formatting steps
   </div>
   <h4 id=rfcb-flow-fragment-height-resolution><span class=secno>7.2.1.
    </span>Step 1: RFCB flow fragment height resolution</h4>
   <p>Conceptually, resolving the flow fragment height is a two phase process.
   <h5 id=rfcb-flow-fragment-height-resolution-phase-1><span
    class=secno>7.2.1.1. </span>RFCB flow fragment height resolution, Phase 1</h5>
   <p>The document is laid out with a <a
    href="http://www.w3.org/TR/CSS2/cascade.html#used-value">used</a> height
    of zero for all <a href="#rfcb">RFCB</a>s. In this phase, the content of
    <a href="#named-flow0">named flows</a> is not laid out in <a
    href="#css-region">regions</a>. This phase yields resolved position and
    sizes for all regions and their RFCBs in the document.
   <h5 id=rfcb-flow-fragment-height-resolution-phase-2><span
    class=secno>7.2.1.2. </span>RFCB flow fragment height resolution, Phase 2</h5>
   <p><a href="#named-flow0">Named flows</a> are laid out in <a
    href="#css-region">regions</a>. The user agent resolves the <a
    href="#flow-fragment-height"><em><span>flow fragment
    height</span></em></a> for the <a href="#rfcb">RFCB</a>s using the
    remainder of the flow and accounting for <a
    href="http://www.w3.org/TR/css3-break/">fragmentation rules</a>. This
    process accounts for constraints such as the ‘<code
    class=property>height</code>’ or ‘<code
    class=property>max-height</code>’ values, as described in the CSS 2.1
    section on <a
    href="http://www.w3.org/TR/CSS2/visudet.html#Computing_heights_and_margins">calculating
    heights and margins</a> (see the <a
    href="http://www.w3.org/TR/CSS2/visudet.html#normal-block">Block-level
    non-replaced elements in normal flow when ‘<code
    class=property>overflow</code>’ computes to ‘<code
    class=property>visible</code>’</a> section and the <a
    href="http://www.w3.org/TR/CSS2/visudet.html#block-root-margin">complicated
    cases</a> section). During this phase, generated content is laid out
    according to the <a href="#processing-model">rules</a> described earlier
    in this document.
   <h4 id=regions-boxes-layout><span class=secno>7.2.2. </span>Step 2: region
    boxes layout</h4>
   <p>In this step, the document is laid out according to the normal CSS
    layout rules.
   <p>If a measure of the content is required to resolve the used ‘<code
    class=property>width</code>’ of the region box, the value is resolved as
    described in the <a href="#rfbc-width-resolution">RFCB width
    resolution</a> section.
   <p>If a measure of the content is required to resolve the used height of
    the RFCB (for example if the region box is absolutely positioned), the <a
    href="#flow-fragment-height">flow fragment height</a> resolved in Step 1
    is used for the vertical content measure of the RFCB.
   <p>At the end of this step, regions are laid out and ready to receive
    content from their associated <a href="#named-flow0">named flows</a>.
   <h4 id=named-flows-layout><span class=secno>7.2.3. </span>Step 3: named
    flows layout</h4>
   <p>In this final step, the content of <a href="#named-flow0">named
    flows</a> is laid out in the <a href="#css-region">regions</a>‘<code
    class=css> RFCBs along with the generated content boxes.</code>
   <p>The used ’width' for RFCBs is resolved <a
    href="#rfbc-width-resolution">as described before</a>.
   <p>The used ‘<code class=property>height</code>’ of RFCBs is resolved
    such that none of the boxes in the region box's normal flow overflow the
    region's box. In other words, the RFCB boxes are stretched vertically to
    accommodate as much of the flow as possible without overflowing the region
    box and accounting for <a
    href="http://www.w3.org/TR/css3-break/">fragmentation rules</a> and
    generated content boxes.
   <p>During this phase, generated content is laid out according to the <a
    href="#processing-model">rules</a> described earlier in this document.
   <div class=note>
    <p>The model for resolving auto sized regions will cause, under certain
     circumstances, the flow content to be overset or underset. In other
     words, it will not fit tightly. The model prevents having circular
     dependencies in the layout model. Implementations may decide to apply
     further layout steps to ensure that the whole flow content is displayed
     to the user, even in edge cases.
   </div>
   <h3 id=regions-visual-formatting-implementation-note><span class=secno>7.3.
    </span>Regions visual formatting: implementation note</h3>
   <p>The process for resolving an RFCB's ‘<code
    class=property>height</code>’ and the three-step process used to lay out
    documents containing regions and <a href="#named-flow0">named flows</a>
    are <em>conceptual</em> descriptions of what the layout should yield and
    implementations should optimize to reduce the number of steps and phases
    necessary wherever possible.
   <h3 id=regions-visual-formatting-examples><span class=secno>7.4.
    </span>Regions visual formatting example</h3>
   <p>This section is non-normative.
   <p>This example considers a document where content flows between three
    regions, and region boxes are intermixed with the normal document content.
   <div class=example>
    <pre>
 &lt;style&gt;
 article {
     flow-into: article;
 }
 #rA, #rB, #rC {
     flow-from: article;
     height: auto;
     margin: 1em 1em 0em 1em;
     padding: 1em;
     border: 3px solid #46A4E9;
 }
 #rA {
     width: auto;
     height: auto;
 }
 #rB {
     float: left;
     width: 15em;
     height: auto;
     max-height: 150px;
 }
 #rC {
     float: right;
     width: 12em;
     height: auto;
 }
 #main-flow {
     padding: 0em 1em 0em 1em;
 }
 &lt;/style&gt;
 &lt;body&gt;
     &lt;article&gt;
         &lt;p style="break-after:region;"&gt;I am not a ... &lt;/p&gt;
         &lt;p&gt;...&lt;/p&gt;
     &lt;/article&gt;
     &lt;div id="rA"&gt;&lt;/div&gt;
     &lt;div id="rB"&gt;&lt;/div&gt;
     &lt;div id="rC"&gt;&lt;/div&gt;
     &lt;div id="main-flow"&gt;
         &lt;p&gt;Lorem ipsum dolor ...&lt;/p&gt;
     &lt;/div&gt;
 &lt;/body&gt;
         </pre>
   </div>
   <p>The following sections and figures illustrate the intermediate results
    for the visual formatting process. In the following, we call RFCB-A,
    RFCB-B and RFCB-C the <a href="#rfcb">RFCBs</a> for regions rA, rB and rC
    respectively.
   <h4 id=step-1--phase-1-laying-out-rfcbs-with-us><span class=secno>7.4.1.
    </span>Step 1 - Phase 1: Laying out RFCBs with used height of zero</h4>
   <p>Applying the rules for Step 1, Phase 1, the computed ‘<code
    class=property>auto</code>’ ‘<code class=property>width</code>’
    values for the RFCBs are resolved to used values according to the normal
    <a
    href="http://www.w3.org/TR/CSS2/visudet.html#Computing_widths_and_margins">CSS
    layout rules</a> meaning they stretch to the width of their containing
    block's content box.
   <ol>
    <li>RFCB-A: stretches to fit the rA content box.
     <p>Since rA also has an ‘<code class=property>auto</code>’ ‘<code
      class=property>width</code>’, its own used ‘<code
      class=property>width</code>’ is stretched to fit the
      <code>&lt;body&gt;</code> content box.
    <li>RFCB-B: stretches to fit the <code>rB</code> content box.
    <li>RFCB-C: stretches to fit the <code>rC</code> content box.
   </ol>
   <p>Also applying the rules for Step 1, Phase 1, the used values for the
    RFCBs ‘<code class=property>height</code>’ properties are all zero.
   <p>Conceptually, this produces the layout illustrated below.
   <div class=figure> <img
    alt="Step 1 - Phase 1: Layout RFBCs with used heights of 0"
    src="images/flow-fragment-height-phase-1.png" width=500px>
    <p class=caption>Step 1 - Phase 1: Layout RFBCs with used heights of 0
   </div>
   <h4 id=step-1--phase-2-layout-flow-to-compute-t><span class=secno>7.4.2.
    </span>Step 1 - Phase 2: Layout flow to compute the RFCBs' flow fragments
    heights</h4>
   <p>In this second phase of Step 1, the named flow is laid out in <a
    href="#css-region">regions</a> and the height of each fragment falling in
    each RFCB is computed.
   <p>The user agent lays out as much of the flow into an area with RFCB-A's
    used ‘<code class=property>width</code>’. rA's ‘<code
    class=property>height</code>’ computes to ‘<code
    class=property>auto</code>’ and there is no vertical maximum height for
    RFCA's ‘<code class=property>height</code>’. However, because there is
    a break after the first paragraph in the ‘<code
    class=property>article</code>’ <a href="#named-flow0"><code>named
    flow</code></a>, only this first paragraph is laid out in RFCB-A and FH-A
    (the flow fragment height for RFCB-A) is resolved by laying out this first
    paragraph in the used ‘<code class=property>width</code>’.
   <p>At this point, the user agent lays out as much of the remaining flow as
    possible in RFCB-B. Because rB's ‘<code
    class=property>max-height</code>’ computed value is ‘<code
    class=css>150px</code>’, the user agent only lays out the ‘<code
    class=property>article</code>’ named flow using RFCB-B's used ‘<code
    class=property>width</code>’ until the point where laying out additional
    content would cause RFCB-B to overflow rB's box. The fragment height for
    RFCB-B is resolved: FH-B (<code class=css>150px</code>).
   <p>Finally, the user agent lays out the remainder of the flow in RFCB-C.
    Because rC has no other constraints and no region breaks, the remaining
    content is laid out in RFCB-C's used ‘<code
    class=property>width</code>’. This results in a resolved flow fragment
    height: FH-C.
   <div class=figure> <img
    alt="Step 1 - Phase 2: Measure flow fragments heights"
    src="images/flow-fragment-height-phase-2.png" width=370px>
    <p class=caption>Step 1 - Phase 2: Measure flow fragments heights
   </div>
   <h4 id=step-2-layout-document-and-regions-witho><span class=secno>7.4.3.
    </span>Step 2: Layout document and regions without named flows</h4>
   <p>The used ‘<code class=property>width</code>’ of RFCB-A, RFCB-B and
    RFCB-C are resolved as in the previous step. However, the ‘<code
    class=property>height</code>’ is resolved differently.
   <p>Resolving the ‘<code class=property>height</code>’ of rA <a
    href="http://www.w3.org/TR/CSS2/visudet.html#normal-block">requires a
    content measure</a> which is FH-A (the flow fragment height for RFCB-A).
   <p>The ‘<code class=property>height</code>’ of rB results from first
    computing its <a
    href="http://www.w3.org/TR/CSS2/visudet.html#block-root-margin">content
    measure</a> and then applying the <a
    href="http://www.w3.org/TR/CSS2/visudet.html#min-max-heights">rules for
    ‘<code class=property>max-height</code>’</a>. Here, the vertical
    content measure evaluates to FH-B. After applying the rules for ‘<code
    class=property>max-height</code>’ and accounting for margins, padding
    and borders, the used ‘<code class=property>height</code>’ of rB is
    resolved to LH-B (<code class=css>150px</code>).
   <p>The ‘<code class=property>height</code>’ of rC's box results from <a
    href="http://www.w3.org/TR/CSS2/visudet.html#block-root-margin">calculating
    its content measure</a>: FH-C becomes rC's used ‘<code
    class=property>height</code>’.
   <div class=figure> <img
    alt="Step 2: Layout document and regions without named flows"
    src="images/regions-visual-formatting-step-2.png" width=370px>
    <p class=caption>Step 2: Layout document and regions without <a
     href="#named-flow0">named flows</a>
   </div>
   <h4 id=step-3-named-flows-layout><span class=secno>7.4.4. </span>Step 3:
    named flows layout</h4>
   <p>In this final step, the <code class=css>article</code> <a
    href="#named-flow0">named flow</a> is laid out in its <a
    href="#region-chain0">region chain</a>. The used ‘<code
    class=property>width</code>’ for each of the RFCB is resolved as in step
 above.
   <p>The used ‘<code class=property>height</code>’ for the RFCB is a
    result of laying out the as much of the content in the <a
    href="#css-region">region</a> without overflowing its content box and
    following the <a href="http://www.w3.org/TR/css3-break/">fragmentation
    rules</a>.
   <p>Because the computed ‘<code class=property>width</code>’ of the RFCB
    has not changed and the fragmentation rules applied are the same as in
    Phase 1, Step 2, the used ‘<code class=property>height</code>’ for
    RFCB-A, RFCB-B and RFCB-C are LH-A, LH-B and LH-C, respectively.
   <p>There may be situations where the used ‘<code
    class=property>height</code>’ of RFCBs resolved in Step 3 are different
    from the <a href="#flow-fragment-height">flow fragment height</a> computed
    in Step 1 Phase 2.
   <div class=figure> <img
    alt="Step 3: Final result after laying out named flows in regions"
    src="images/regions-visual-formatting-step-3.png" width=370px>
    <p class=caption>Step 3: Final result after laying out named flows in
     regions
   </div>
   <h2 id=relation-to-document-events><span class=secno>8. </span>Relation to
    document events</h2>
   <p>The CSS regions module does not alter the normal processing of events in
    the document tree. In particular, if an event occurs on an element that is
    part of a <a href="#named-flow0">named flow</a>, the <a
    href="http://www.w3.org/TR/dom/#events">event's bubble and capture
    phases</a> happen following the document tree order.
   <h2 id=relation-to-other-specifications><span class=secno>9.
    </span>Relation to other specifications</h2>
   <p>This specification is related to other specifications as described in
    the <a href="#references">references</a> section. In addition, it is
    related to the following specifications:
   <ol>
    <li>CSS Fragmentation Module Level 3 <a href="#CSS3-BREAK"
     rel=biblioentry>[CSS3-BREAK]<!--{{CSS3-BREAK}}--></a>. This module
     defines the rules for fragmenting content over multiple containers and
     applies to CSS regions in addition to applying to multi-column and paged
     media.
    <li>CSS Pagination Templates Module Level 3 <a href="#CSS3-PAGE-TEMPLATE"
     rel=biblioentry>[CSS3-PAGE-TEMPLATE]<!--{{CSS3-PAGE-TEMPLATE}}--></a>.
     This module defines a syntax to define layout templates which can be used
     when paginating content. The page templates use regions.
    <li>CSS Exclusions Module <a href="#CSS3-EXCLUSIONS"
     rel=biblioentry>[CSS3-EXCLUSIONS]<!--{{CSS3-EXCLUSIONS}}--></a>. This
     module defines a generic way to define arbitrarily shaped exclusions into
     which content can flow or around which content can flow. This can be seen
     as an extension to the way CSS floats provide rectangular areas into
     which content flows and around which content flows. In advanced layout
     designs, it is expected that the CSS Exclusions module will be commonly
     combined with the CSS regions module.
    <li>CSS Line Grid Module <a href="#CSS3-LINE-GRID"
     rel=biblioentry>[CSS3-LINE-GRID]<!--{{CSS3-LINE-GRID}}--></a>. This
     module defines a concept of line grid to align the position of lines in
     different elements. The line grid functionality is related and needed for
     aligning content flowing in separate regions.
   </ol>
   <h2 id=usecases><span class=secno>10. </span>Use Cases</h2>
   <p>Use cases are described on <a
    href="http://wiki.csswg.org/spec/css3-regions/regions-use-cases">these</a>
    <a
    href="http://wiki.csswg.org/spec/css3-regions/regions-print-use-cases">pages</a>.
   <h2 id=conformance><span class=secno>11. </span>Conformance</h2>
   <h2 id=changes><span class=secno>12. </span>Changes</h2>
   <h3 id="changes_from_Aug_28_2012"><span class=secno>12.1. </span>Changes
    from <a href="http://www.w3.org/TR/2012/WD-css3-regions-20120823/">August
 <sup>th</sup> 2012</a> version</h3>
   <ul>
    <li>Added section on fragmenting the fragmenters
    <li>Added section on handling circular flow-from and flow-into situations
    <li>Added alignment and justification to region styling properties
    <li>Added timing for regionLayoutUpdate event
    <li>Clarified interaction between content and flow-from for
     pseudo-elements
    <li>Changed NamedFlowCollection getters to return undefined when there is
     no NamedFlow.
    <li>Changed region-overflow property to region-fragment.
   </ul>
   <h3 id="changes_from_May_03_2012"><span class=secno>12.2. </span>Changes
    from <a href="http://www.w3.org/TR/2012/WD-css3-regions-20120503/">May
 <sup>rd</sup> 2012</a> version</h3>
   <ul>
    <li>Removed exceptions from the Region interface.
    <li>Changed NamedFlowCollection from live to a static snapshot.
    <li>Changed NamedFlow to inherit from EventTarget.
    <li>Removed flowFrom from Region interface and changed method name to
     getComputedRegionStyle().
    <li>Region interface is now a supplemental interface with the
     [NoInterfaceObject] extended attribute.
    <li>Added note for regionLayoutUpdate dispatching in nested flows.
    <li>Removed Document.getFlowByName() in favor of
     NamedFlowCollection.namedItem().
    <li>Changed to overset:false for NULL NamedFlow.
    <li>Changed regionLayoutUpdate to not bubble.
   </ul>
   <h3 id="changes_from_November_29_2011"><span class=secno>12.3.
    </span>Changes from <a
    href="http://www.w3.org/TR/2011/WD-css3-regions-20111129/">November
 <sup>th</sup> 2011</a> version</h3>
   <ul>
    <li>Modified region styling examples to use element selector instead of of
     pseudo-code selectors (this had been omitted in the previous pass at
     removing pseudo-code from the examples).
    <li>Removed divs with class set to "issue moved", "issue stale" and "issue
     resolved" since these divs where not displayed.
    <li>Minor updates to the alternate stylesheet.
    <li>Removed "This section is normative" paragraphs since in CSS
     specifications, sections are normative unless otherwise specified.
    <li>Removed "This section is informative" paragraphs since in CSS
     specifications notes are always informative.
    <li>Reworded the text about flow-into: &lt;ident&gt; and removed obsolete
     text about the interaction with the ‘<code
     class=property>content</code>’ property.
    <li>Removed "this section is non-normative" from the "Regions Concepts"
     section.
    <li>In the section on region breaks, removed descriptions of break values
     normatively defined in external specifications. Removed the section about
     split boxes and replaced with paragraph referencing the page breaking
     behavior. Removed the discussion about how the ‘<code
     class=property>overflow</code>’ property applies to content flown in
     regions from the break section, since this is covered in the section on
     ‘<code class=property>region-overflow</code>’ already. See <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Dec/0477.html">mailing
     list feedback.</a>
    <li>Clarified that @region style rules only apply to the ‘<code
     class=property>portion</code>’ of an element that falls into the
     corresponding region and added an issue that the model for ‘<code
     class=property>partial</code>’ styling needs to be defined. See <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Dec/0480.html">mailing
     list feedback</a>.
    <li>Clarified that the <code>NodeList</code> returned by
     <code>getRegionsByContentNode</code> is live.
    <li>Added a name property to the <a
     href="#dom-named-flow"><code>NamedFlow</code></a> interface. Added a <a
     href="#dom-named-flow-collection"><code>NamedFlowCollection</code></a>
     interface and added a <code>getNamedFlows</code> method on the
     <code>Document</code> interface, as per <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15828">Bug
 </a>.
    <li>Modified wording about containing block resolution for absolutely
     positioned elements in a named flow.
    <li>Modified initial examples as per <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15131">Bug 15131</a>
    <li>Multiple editorial changes following <a
     href="http://lists.w3.org/Archives/Public/www-style/2012Feb/0001.html">mailing
     list review comments</a>
    <li>Fixed DOM references to now point to the DOM TR
    <li>Fixed Web IDL issues as reported in <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15931">Issue
 </a>
    <li>Added text to explain support for multi-column elements as required by
     <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15191">Issue
 </a> and <a
     href="http://www.w3.org/Style/CSS/Tracker/actions/375">Action 375</a>.
    <li>Renamed ‘<code class=property>regionOverflow</code>’ to ‘<a
     href="#dom-region-regionoverset"><code
     class=property>regionOverset</code></a>’ to avoid confusion between
     fitting a flow in regions and the concept of visual overflow that the
     word ‘<code class=property>overflow</code>’ (and the property) carry.
    <li>Reworked the partial document interface following the <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14948">Issue
 </a> on <code>getFlowByName</code>.
    <li>Updated the object model section as proposed on the <a
     href="http://wiki.csswg.org/spec/css3-regions/css-om">wiki</a> and in
     particular:
     <ul>
      <li>introduced a <a href="#css-region"><code>Region</code></a> interface
       to replace the supplemental Element interface
      <li>Added a ‘<code class=property>flowFrom</code>’ attribute on the
       Region interface
      <li>NamedFlow.getRegions() was added
      <li>Renamed getContentNode to getContent and getRegionsByContentNode to
       getRegionsByContent as per <a
       href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15879">Isseu
 </a>
      <li>NamedFlow now returns static lists (see <a
       href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16286">Issue
 </a>)
     </ul>
    <li>Modified region layout event to be dispatched on <a
     href="#dom-named-flow"><code>NamedFlow</code></a> instead of region as
     before. Was requested by <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15938">Issue
 </a> and required in the general effort to have the DOM APIs work
     with non-element regions.
    <li>Changed paragraph on pseudo-elements to disallow ‘<a
     href="#flow-into"><code class=property>flow-into</code></a>’ on all
     pseudo-elements because moving a ‘<code class=css>::before</code>’
     element (for a example) to a named flow does not seem useful and causes
     specification and implementation complexity.
    <li>Added section about getClientRects(), getBoundingClientRect(),
     offsetWidth, offsetHeight, offsetTop and offsetLeft.
    <li>Added ‘<code class=css>Regions visual formatting details</code>’
     section to better describe the model for resolving auto sizing on
     regions.
    <li>Reworked the initial specification example.
    <li>Changed break and region-overflow properties to apply to visual media
     instead of paged.
    <li>Added opacity to region styling.
    <li>Added possibility of ::before content contributing to overflow.
    <li>Added CSSRegionStyleRule
   </ul>
   <h3 id="changes_from_June_09_2011"><span class=secno>12.4. </span>Changes
    from <a href="http://www.w3.org/TR/2011/WD-css3-regions-20110609/">June
 <sup>th</sup> 2011</a> version</h3>
   <ul>
    <li>Editorial changes (typos, rephrasings).
    <li>Made ‘<code class=property>content-order</code>’ a &lt;integer&gt;
     and not a &lt;float&gt; following a working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0329.html">resolution</a>
    <li>Added Alex Mogilevsky as an editor
    <li>Flow names became &lt;ident&gt; instead of &lt;string&gt; following a
     working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0413.html">resolution</a>
    <li>Removed issue about possibly altering the DOM Events model for region
     events following a working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0413.html">resolution</a>
    <li>Made the "relation to document events" section informative following a
     working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0413.html">resolution</a>
    <li>Removed issue in section "The NamedFlow interface" following the
     working group's <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0413.html">resolution</a>
     to have both NamedFlow and the Element interface extension
    <li>Following a working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0413.html">resolution</a>:
     <ul>
      <li>Turned the first issue in the "Extensions to the Element interface"
       into a note explaining that the NamedFlow interface can be used when
       regions are pseudo-elements.
      <li>Added NamedFlowUpdate
     </ul>
    <li>Excluded ‘<code class=property>none</code>’, ‘<code
     class=property>inherit</code>’ and ‘<code
     class=property>initial</code>’ from the possible identifier names on
     the flow property following <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jun/0583.html">discussion</a>
     on the mailing list.
    <li>Simplified integration discussion on multi-column layout and just
     state that since column boxes are not addressable by selectors, they
     cannot be regions.
    <li>Added specification of how the ‘<a href="#flow-into"><code
     class=property>flow-into</code></a>’ property interacts with object,
     embed and iframe elements.
    <li>Excluded ‘<code class=property>default</code>’ from the possible
     identifier names on the flow property because it <a
     href="http://www.w3.org/TR/2006/WD-css3-values-20060919/#keywords">may
     get reserved</a>.
    <li>Added to the definition of ‘<code class=property>auto</code>’ on
     ‘<code class=property>region-overflow</code>’ specifying that region
     breaks must be ignored.
    <li>Renamed ‘<code class=css>Document.flowWithName</code>’ to ‘<code
     class=css>Document.getFlowByName</code>’ since it is not a property.
    <li>Added a note that a ‘<a href="#dom-named-flow"><code
     class=property>NamedFlow</code></a>’ instance is live.
    <li>Added an ‘<code class=property>undefined</code>’ string value to
     the regionOverflow property on the Element interface extension.
    <li>Renamed NamedFlowEvent to regionLayoutUpdate to avoid having ‘<code
     class=property>Event</code>’ in the event name.
    <li>Added description for special behavior of iframe/object/embed as flow
     source
    <li>Removed issue on copying content to a named flow instead of moving
     elements to named flow following working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0069.html">resolution</a>.
    <li>Removed issue on content:flow-from v.s., flow-from property following
     working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0069.html">resolution</a>.
    <li>Renamed ‘<code class=property>flow</code>’ to ‘<a
     href="#flow-into"><code class=property>flow-into</code></a>’ following
     working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0069.html">resolution</a>.
    <li>Updated the css3-grid-align example following working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0069.html">resolution</a>
     that it should use &lt;div&gt; instead of grid-cell pseudo elements that
     were removed from the CSS Grid Layout specification.
    <li>Renamed ‘<code class=property>from-flow</code>’ to ‘<a
     href="#flow-from"><code class=property>flow-from</code></a>’ following
     a working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0069.html">resolution</a>.
    <li>Limited the applicability of ‘<code class=css>content:
     flow-from()</code>’ to block container box and added a note that this
     might be expanded in the future, following a working group <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0069.html">resolution</a>.
    <li>Removed issue about API to find which region displays an element in a
     named flow since <a
     href="http://www.w3.org/Style/CSS/Tracker/actions/350">ACTION-350</a> was
     created to add this API.
    <li>In the ‘<code class=property>flow</code>’ property description,
     removed the required wrapper anonymous block as agreed on <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0161.html">mailing
     list discussion</a>.
    <li>Reworded the paragraph on how regions create a new stacking context,
     as per the <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0025.html">mailing
     list discussion</a>.
    <li>Reworked the "CSS regions Model" section to now be "CSS regions
     Layout". Moved the definition of a region as the first sub-section.
    <li>Removed the "Visual Formatting Model and Flows" section to match the
     new Regions Model (now CSS regions Layout) section.
    <li>Moved the sections on allowed region breaks, forced region breaks and
     "best" regions breaks to follow the property definitions to follow the
     formatting of the paged media section in CSS 2.1.
    <li>Added note about why regions create a new stacking context following
     the discussion on the <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jul/0158.html">mailing
     list</a>.
    <li>Removed sentence about content:none making elements disconnected
     following the discussion on the <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jul/0158.html">mailing
     list</a>.
    <li>Removed sentence about content:none making elements disconnected
     following the discussion on the <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jul/0158.html">mailing
     list</a>.
    <li>Added the ::region-before and a ::region-after pseudo-elements.
    <li>Added note of caution when using selectors and the ‘<a
     href="#flow-into"><code class=property>flow-into</code></a>’ property
     following a <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Jul/0191.html">mailing
     list discussion</a>.
    <li>Removed sections about allowed, forced and best region breaks to align
     with the multi-column specification approach and until the group agrees
     on where and how the general issue of breaks (regions/pages/columns)
     should be addressed.
    <li>Removed the section on Integration with other specifications since
     specifications that was superfluous especially since there is no specific
     integration with multi column, grid or template layout.
    <li>Added a note that regions establish a new block formatting context.
    <li>Renamed content-order to region-order.
    <li>Added a note about overflowing content in regions (e.g., for content
     with borders).
    <li>Added a note that a region cannot layout content it is part of (to
     avoid creating a circular dependency) in the flow-from description,
     specifying that if flow-from references the flow an element is part of,
     then the element does not format anything visually.
    <li>Replaced ‘<code class=css>content:flow-from(&lt;ident&gt;)</code>’
     with ‘<code class=css>flow-from: &lt;ident&gt;</code>’ following a <a
     href="http://krijnhoetmer.nl/irc-logs/css/20110824">working group
     resolution</a>.
    <li>Added more specific wording about auto width and auto height,
     following <a
     href="http://www.w3.org/Style/CSS/Tracker/actions/351">ACTION 351</a>.
    <li>Reworked section on region markers to now use ‘<code
     class=css>::before</code>’ and ‘<code class=css>::after</code>’ and
     explain how ‘<code class=css>display:run-in</code>’ works with
     regions.
    <li>Modified the @region style rule to remove the ::region-lines
     pseudo-selector.
    <li>Removed the ‘<code class=property>region-order</code>’ property
     following implementation feedback.
    <li>Specified that region-overflow does not influence a region's size.
    <li>Specified that the flow's writing mode is defined by the first
     region's writing mode following <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Aug/0306.html">mailing
     list discussion</a>.
    <li>Made iframe, object and embed support of flow-into optional following
     <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Sep/0073.html">mailing
     list discussion</a>.
    <li>Clarified that flow content following the last break in the last
     region is not rendered, following <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Oct/0167.html">mailing
     list discussions.</a>
    <li>Modified the rule for computing the width and height of a region when
     they are set to auto, following <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Oct/0216.html">a
     mailing list discussion.</a>
    <li>Added ‘<code class=property>auto</code>’ to the list of invalid
     flow identifiers.
    <li>Made ‘<code class=property>none</code>’ the initial value for
     ‘<a href="#flow-into"><code class=property>flow-into</code></a>’ and
     aligned with ‘<a href="#flow-from"><code
     class=property>flow-from</code></a>’, as explained in this <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Oct/0576.html">email</a>.
     Also allowed the ‘<code class=property>inherit</code>’ value on ‘<a
     href="#flow-from"><code class=property>flow-from</code></a>’ and ‘<a
     href="#flow-into"><code class=property>flow-into</code></a>’ (same
     email).
    <li>Added note about nested region styling following <a
     href="http://lists.w3.org/Archives/Public/www-style/2011Oct/0650.html">a
     mailing list discussion</a>.
    <li>Added additional DOM interface following <a
     href="http://www.w3.org/Style/CSS/Tracker/actions/350">Action 350</a>.
    <li>Clarified that a region becomes a region only if its ‘<code
     class=property>content</code>’ property computes to normal, following
     the resolution of <a
     href="http://wiki.csswg.org/spec/css3-regions#issue-22content-vs-flow-from-precedence">Issue
 </a>.
    <li>Removed text about special iframe behavior as a result of <a
     href="http://www.w3.org/Style/CSS/Tracker/actions/376">ACTION 376</a>.
    <li>Made the selectors explicit in the initial section code examples,
     following discussion in <a
     href="http://krijnhoetmer.nl/irc-logs/css/20111030">San Jose, October
 </a> face to face meeting.
    <li>Added section on use cases following <a
     href="http://www.w3.org/Style/CSS/Tracker/actions/377">ACTION-377</a>.
   </ul>
   <h2 class=no-num id=acknowledgments>Acknowledgments</h2>
   <p>The editors are grateful to the CSS working group for their feedback and
    help with the genesis of this specification.
   <p>In addition, the editors would like to thank the following individuals
    for their contributions, either during the conception of CSS regions or
    during its development and specification review process:
   <p>Rossen Atanassov, Tab Atkins, Mihai Balan, Andrei Bucur, Razvan Caliman,
    Alexandru Chiculita, Phil Cupp, Arron Eicholz, John Jansen, Dimitri
    Glazkov, Daniel Glazman, Arno Gourdol, David Hyatt, Brian Heuston, Ian
    Hickson, Jonathan Hoersch, Michael Jolson, Brad Kemper, Håkon Wium Lie,
    Markus Mielke, Robert O'Callahan, Edward O'Connor, Mihnea Ovidenie, Virgil
    Palanciuc, Olga Popiv, Christoph Päper, Anton Prowse, Peter Sorotokin,
    Christian Stockwell, Eugene Veselov, Boris Zbarsky, Stephen Zilles and the
    CSS Working Group members.
   <h2 class=no-num id=intro-example-code>Appendix A. Example Code for
    Introduction</h2>
   <p>The following is one possible way to code the example from the <a
    href="#introduction">introduction</a>. This code uses grid cells to
    define, size and position the region areas but table layout, absolute
    positioning, or any other CSS layout facility could be used.
   <div class=example>
    <pre>
 &lt;style&gt;
   #grid {
     width: 80vw;
     height: 60vw;
     grid-template: "aaa.d"
                    "....d"
                    "bbb.d"
                    "....d"
                    "ccc.d";
     grid-rows: 52% 4% 20% 4% 20%;
     grid-columns: 30% 5% 30% 5% 30%;
   }
   #region1 { grid-cell: a; }
   #region2 { grid-cell: b; }
   #boxA    { grid-cell: c; }
   #region3 { grid-cell: d; }
   #region4 {
     width: 80vw;
   }
   #region2 {
     column-count: 2;
   }
   <span
     class=highlight>/*
    * Creates the named flow
    */</span>
   article {
     <strong>flow-into: article_flow;</strong>
   }
   <span
     class=highlight>/*
    * Associate it with the intended CSS Regions.
    * This creates a region chain for the named flow.
    */</span>
   #region1, #region2, #region3, #region4 {
     <strong>flow-from: article_flow;</strong>
   }
 &lt;/style&gt;
 <span
     class=highlight>&lt;!--
      The following code element is the content to flow
      through the region chain.
 --&gt;</span>
 &lt;article&gt;
   &lt;h1&gt;Introduction&lt;/h1&gt;
   &lt;p&gt;This is an example ...&lt;/p&gt;
   &lt;h2&gt;More Details&lt;/h2&gt;
   &lt;p&gt;This illustrates ...&lt;/p&gt;
   &lt;p&gt;Then, the example ...&lt;/p&gt;
   &lt;p&gt;Finally, this ...&lt;/p&gt;
 &lt;/article&gt;
 <span
     class=highlight>&lt;!--
      For this example, we layout the regions with a grid.
      Regions could be pseudo-elements and could be laid out
      with another layout module (e.g., flexible boxes)
 --&gt;</span>
 &lt;div id="grid"&gt;
   &lt;div id="region1"&gt;&lt;/div&gt;
   &lt;div id="region2"&gt;&lt;/div&gt;
   &lt;div id="boxA"&gt;&lt;/div&gt;
   &lt;div id="region3"&gt;&lt;/div&gt;
 &lt;/div&gt;
 &lt;div id="region4"&gt;&lt;/div&gt;
     </pre>
   </div>
   <div class=note><span class=note-prefix>Note </span>
    <p> Note that a multi-column element is used for #region2, which is a bit
     gratuitous here (because grid cells could be used). The reason to use a
     multi-column element is to illustrate that regions can be multi-column.
   </div>
   <h2 class=no-num id=references>References</h2>
   <div class="issue-marker wrapper">
    <div class=issue-marker data-bug_id=18767 data-bug_status=NEW> <a
     href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=18767">Issue-18767</a>
     <div class=issue-details>
      <p class=short-desc>Something appears to be wrong with the
       checkSpecificationIssues() call
     </div>
    </div>
   </div>
   <h3 class=no-num id=normative-references>Normative references</h3>
   <!--begin-normative-->
   <!-- Sorted by label -->
   <dl class=bibliography>
    <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
     <!---->
    <dt id=CSS21>[CSS21]
    <dd>Bert Bos; et al. <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607"><cite>Cascading Style
     Sheets Level 2 Revision 1 (CSS 2.1) Specification.</cite></a> 7 June
 . W3C Recommendation. URL: <a
     href="http://www.w3.org/TR/2011/REC-CSS2-20110607">http://www.w3.org/TR/2011/REC-CSS2-20110607</a>
    </dd>
    <!---->
    <dt id=CSS3-BREAK>[CSS3-BREAK]
    <dd>Rossen Atanassov; Elika J. Etemad. <a
     href="http://www.w3.org/TR/2012/WD-css3-break-20120823/"><cite>CSS
     Fragmentation Module Level 3.</cite></a> 23 August 2012. W3C Working
     Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-css3-break-20120823/">http://www.w3.org/TR/2012/WD-css3-break-20120823/</a>
    </dd>
    <!---->
    <dt id=CSS3-WRITING-MODES>[CSS3-WRITING-MODES]
    <dd>Elika J. Etemad; Koji Ishii. <a
     href="http://www.w3.org/TR/2012/WD-css3-writing-modes-20120501/"><cite>CSS
     Writing Modes Module Level 3.</cite></a> 1 May 2012. W3C Working Draft.
     (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-css3-writing-modes-20120501/">http://www.w3.org/TR/2012/WD-css3-writing-modes-20120501/</a>
    </dd>
    <!---->
    <dt id=CSS3COL>[CSS3COL]
    <dd>Håkon Wium Lie. <a
     href="http://www.w3.org/TR/2011/CR-css3-multicol-20110412"><cite>CSS
     Multi-column Layout Module.</cite></a> 12 April 2011. W3C Candidate
     Recommendation. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2011/CR-css3-multicol-20110412">http://www.w3.org/TR/2011/CR-css3-multicol-20110412</a>
    </dd>
    <!---->
    <dt id=CSSOM>[CSSOM]
    <dd>Anne van Kesteren. <a
     href="http://www.w3.org/TR/2011/WD-cssom-20110712/"><cite>CSSOM.</cite></a>
 July 2011. W3C Working Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2011/WD-cssom-20110712/">http://www.w3.org/TR/2011/WD-cssom-20110712/</a>
    </dd>
    <!---->
    <dt id=DOM>[DOM]
    <dd>Anne van Kesteren; Aryeh Gregor; Ms2ger. <a
     href="http://www.w3.org/TR/2012/WD-dom-20120405/"><cite>DOM4.</cite></a>
 April 2012. W3C Working Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-dom-20120405/">http://www.w3.org/TR/2012/WD-dom-20120405/</a>
    </dd>
    <!---->
    <dt id=DOM-LEVEL-3-EVENTS>[DOM-LEVEL-3-EVENTS]
    <dd>Travis Leithead; et al. <a
     href="http://www.w3.org/TR/2012/WD-DOM-Level-3-Events-20120906/"><cite>Document
     Object Model (DOM) Level 3 Events Specification.</cite></a> 6 September
 . W3C Working Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-DOM-Level-3-Events-20120906/">http://www.w3.org/TR/2012/WD-DOM-Level-3-Events-20120906/</a>
    </dd>
    <!---->
    <dt id=HTML5>[HTML5]
    <dd>Ian Hickson. <a
     href="http://www.w3.org/TR/2011/WD-html5-20110525/"><cite>HTML5.</cite></a>
 May 2011. W3C Working Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2011/WD-html5-20110525/">http://www.w3.org/TR/2011/WD-html5-20110525/</a>
    </dd>
    <!---->
    <dt id=SELECT>[SELECT]
    <dd>Tantek Çelik; et al. <a
     href="http://www.w3.org/TR/2011/REC-css3-selectors-20110929/"><cite>Selectors
     Level 3.</cite></a> 29 September 2011. W3C Recommendation. URL: <a
     href="http://www.w3.org/TR/2011/REC-css3-selectors-20110929/">http://www.w3.org/TR/2011/REC-css3-selectors-20110929/</a>
    </dd>
    <!---->
   </dl>
   <!--end-normative-->
   <h3 class=no-num id=other-references>Other references</h3>
   <!--begin-informative-->
   <!-- Sorted by label -->
   <dl class=bibliography>
    <dt style="display: none"><!-- keeps the doc valid if the DL is empty -->
     <!---->
    <dt id=CSS3-EXCLUSIONS>[CSS3-EXCLUSIONS]
    <dd>Vincent Hardy; Rossen Atanassov; Alan Stearns. <a
     href="http://www.w3.org/TR/2012/WD-css3-exclusions-20120503/"><cite>CSS
     Exclusions and Shapes Module Level 3.</cite></a> 3 May 2012. W3C Working
     Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-css3-exclusions-20120503/">http://www.w3.org/TR/2012/WD-css3-exclusions-20120503/</a>
    </dd>
    <!---->
    <dt id=CSS3-FLEXBOX>[CSS3-FLEXBOX]
    <dd>Tab Atkins Jr.; Elika J. Etemad; Alex Mogilevsky. <a
     href="http://www.w3.org/TR/2012/WD-css3-flexbox-20120612/"><cite>CSS
     Flexible Box Layout Module.</cite></a> 12 June 2012. W3C Working Draft.
     (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-css3-flexbox-20120612/">http://www.w3.org/TR/2012/WD-css3-flexbox-20120612/</a>
    </dd>
    <!---->
    <dt id=CSS3-GRID-LAYOUT>[CSS3-GRID-LAYOUT]
    <dd>Alex Mogilevsky; et al. <a
     href="http://www.w3.org/TR/2012/WD-css3-grid-layout-20120322/"><cite>CSS
     Grid Layout.</cite></a> 22 March 2012. W3C Working Draft. (Work in
     progress.) URL: <a
     href="http://www.w3.org/TR/2012/WD-css3-grid-layout-20120322/">http://www.w3.org/TR/2012/WD-css3-grid-layout-20120322/</a>
    </dd>
    <!---->
    <dt id=CSS3-LINE-GRID>[CSS3-LINE-GRID]
    <dd>Koji Ishii. <a href="http://dev.w3.org/csswg/css-line-grid/"><cite>CSS
     Line Grid Module.</cite></a> Proposal for a CSS module. (Retrieved 26
     October 2011) URL: <a
     href="http://dev.w3.org/csswg/css-line-grid/">http://dev.w3.org/csswg/css-line-grid/</a>
    </dd>
    <!---->
    <dt id=CSS3-PAGE-TEMPLATE>[CSS3-PAGE-TEMPLATE]
    <dd>Alan Stearns. <a
     href="http://dev.w3.org/csswg/css3-page-template/"><cite>CSS Pagination
     Templates Module Level 3.</cite></a> Proposal for a CSS module.
     (Retrieved 4 April 2012) URL: <a
     href="http://dev.w3.org/csswg/css3-page-template/">http://dev.w3.org/csswg/css3-page-template/</a>
    </dd>
    <!---->
    <dt id=CSS3GRID>[CSS3GRID]
    <dd>Alex Mogilevsky; Markus Mielke. <a
     href="http://www.w3.org/TR/2007/WD-css3-grid-20070905"><cite>CSS Grid
     Positioning Module Level 3.</cite></a> 5 September 2007. W3C Working
     Draft. (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2007/WD-css3-grid-20070905">http://www.w3.org/TR/2007/WD-css3-grid-20070905</a>
    </dd>
    <!---->
    <dt id=CSS3LAYOUT>[CSS3LAYOUT]
    <dd>Bert Bos; César Acebal. <a
     href="http://www.w3.org/TR/2010/WD-css3-layout-20100429"><cite>CSS
     Template Layout Module.</cite></a> 29 April 2010. W3C Working Draft.
     (Work in progress.) URL: <a
     href="http://www.w3.org/TR/2010/WD-css3-layout-20100429">http://www.w3.org/TR/2010/WD-css3-layout-20100429</a>
    </dd>
    <!---->
    <dt id=CSS3PAGE>[CSS3PAGE]
    <dd>Håkon Wium Lie; Melinda Grant. <a
     href="http://www.w3.org/TR/2006/WD-css3-page-20061010"><cite>CSS3 Module:
     Paged Media.</cite></a> 10 October 2006. W3C Working Draft. (Work in
     progress.) URL: <a
     href="http://www.w3.org/TR/2006/WD-css3-page-20061010">http://www.w3.org/TR/2006/WD-css3-page-20061010</a>
    </dd>
    <!---->
    <dt id=MEDIAQ>[MEDIAQ]
    <dd>Florian Rivoal. <a
     href="http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/"><cite>Media
     Queries.</cite></a> 19 June 2012. W3C Recommendation. URL: <a
     href="http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/">http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/</a>
    </dd>
    <!---->
   </dl>
   <!--end-informative-->
   <h2 class=no-num id=index>Index</h2>
   <!--begin-index-->
   <ul class=indexlist>
    <li>break-after, <a href="#break-after"
     title=break-after><strong>3.3.</strong></a>
    <li>break-before, <a href="#break-before"
     title=break-before><strong>3.3.</strong></a>
    <li>break-inside, <a href="#break-inside"
     title=break-inside><strong>3.3.</strong></a>
    <li><code class=idl>CSSRegionStyleRule</code> interface, <a
     href="#dom-region-style-rule-interface"
     title="CSSRegionStyleRule interface"><strong>6.3.</strong></a>
    <li><a href="#region-style-rule-cssRules"><code
     class=idl>cssRules</code></a>, <a href="#region-style-rule-cssRules"
     title=cssRules><strong>6.3.</strong></a>
    <li>current-region, <a href="#current-region"
     title=current-region><strong>2.4.</strong></a>
    <li><a href="#region-style-rule-deleteRule"><code
     class=idl>deleteRule(index)</code></a>, <a
     href="#region-style-rule-deleteRule"
     title="deleteRule(index)"><strong>6.3.</strong></a>
    <li>firstEmptyRegionIndex, <a
     href="#dom-named-flow-first-empty-region-index"
     title=firstEmptyRegionIndex><strong>6.1.</strong></a>
    <li>flow fragment height, <a href="#flow-fragment-height"
     title="flow fragment height"><strong>7.2.</strong></a>
    <li>flow-from, <a href="#flow-from"
     title=flow-from><strong>3.2.</strong></a>
    <li>flow-into, <a href="#flow-into"
     title=flow-into><strong>3.1.</strong></a>
    <li>getComputedRegionStyle, <a href="#dom-region-getComputedRegionStyle"
     title=getComputedRegionStyle><strong>6.2.</strong></a>
    <li>getContent(), <a href="#dom-named-flow-get-content"
     title="getContent()"><strong>6.1.</strong></a>
    <li><a href="#document-getnamedflows"><code
     class=idl>getNamedFlows()</code></a>, <a href="#document-getnamedflows"
     title="getNamedFlows()"><strong>6.1.</strong></a>
    <li>getRegionFlowRanges, <a href="#dom-region-getregionflowranges"
     title=getRegionFlowRanges><strong>6.2.</strong></a>
    <li><a href="#dom-named-flow-get-regions"><code
     class=idl>getRegions()</code></a>, <a href="#dom-named-flow-get-regions"
     title="getRegions()"><strong>6.1.</strong></a>
    <li>getRegionsByContent(), <a
     href="#dom-named-flow-get-regions-by-content"
     title="getRegionsByContent()"><strong>6.1.</strong></a>
    <li><a href="#region-style-rule-insertRule"><code
     class=idl>insertRule(rule, index)</code></a>, <a
     href="#region-style-rule-insertRule"
     title="insertRule(rule, index)"><strong>6.3.</strong></a>
    <li><a href="#dom-named-flow-collection-item"><code
     class=idl>item(index)</code></a>, <a
     href="#dom-named-flow-collection-item"
     title="item(index)"><strong>6.1.</strong></a>
    <li><a href="#dom-named-flow-collection-length"><code
     class=idl>length</code></a>, <a href="#dom-named-flow-collection-length"
     title=length><strong>6.1.</strong></a>
    <li><a href="#dom-named-flow-name"><code class=idl>name</code></a>, <a
     href="#dom-named-flow-name" title=name><strong>6.1.</strong></a>
    <li><a href="#dom-named-flow"><code class=idl>NamedFlow</code></a>, <a
     href="#dom-named-flow" title=NamedFlow><strong>6.1.</strong></a>
    <li>named flow, <a href="#named-flow0"
     title="named flow"><strong>2.3.</strong></a>
    <li><a href="#dom-named-flow-collection"><code
     class=idl>NamedFlowCollection</code></a>, <a
     href="#dom-named-flow-collection"
     title=NamedFlowCollection><strong>6.1.</strong></a>
    <li><a href="#dom-named-flow-collection-named-item"><code
     class=idl>namedItem(name)</code></a>, <a
     href="#dom-named-flow-collection-named-item"
     title="namedItem(name)"><strong>6.1.</strong></a>
    <li><a href="#dom-named-flow-overset"><code class=idl>overset</code></a>,
     <a href="#dom-named-flow-overset" title=overset><strong>6.1.</strong></a>
    <li>region, <a href="#css-region" title=region><strong>2.1.</strong></a>
    <li>region chain, <a href="#region-chain0"
     title="region chain"><strong>2.2.</strong></a>
    <li>region flow content box, <a href="#region-flow-content-box"
     title="region flow content box"><strong>7.1.</strong></a>
    <li>region-fragment, <a href="#region-fragment"
     title=region-fragment><strong>3.4.</strong></a>
    <li><a href="#css-region"><code class=idl>Region</code></a> interface, <a
     href="#region-interface"
     title="Region interface"><strong>6.2.</strong></a>
    <li><a href="#dom-region-regionoverset"><code
     class=idl>regionOverset</code></a>, <a href="#dom-region-regionoverset"
     title=regionOverset><strong>6.2.</strong></a>
    <li>REGION_STYLE_RULE, <a href="#region-style-rule-const"
     title="REGION_STYLE_RULE"><strong>6.3.</strong></a>
    <li>RFCB, <a href="#rfcb" title=RFCB><strong>7.1.</strong></a>
    <li><a href="#region-style-rule-selectorText"><code
     class=idl>selectorText</code></a>, <a
     href="#region-style-rule-selectorText"
     title=selectorText><strong>6.3.</strong></a>
    <li>specified flow, <a href="#specified-flow"
     title="specified flow"><strong>3.1.</strong></a>
   </ul>
   <!--end-index-->
   <h2 class=no-num id=property-index>Property index</h2>
   <!--begin-properties-->
   <table class=proptable>
    <thead>
     <tr>
      <th>Property
      <th>Values
      <th>Initial
      <th>Applies to
      <th>Inh.
      <th>Percentages
      <th>Media
    <tbody>
     <tr>
      <th><a class=property href="#break-after">break-after</a>
      <td>auto | always | avoid | left | right | page | column | region |
       avoid-page | avoid-column | avoid-region
      <td>auto
      <td>block-level elements
      <td>no
      <td>N/A
      <td>visual
     <tr>
      <th><a class=property href="#break-before">break-before</a>
      <td>auto | always | avoid | left | right | page | column | region |
       avoid-page | avoid-column | avoid-region
      <td>auto
      <td>block-level elements
      <td>no
      <td>N/A
      <td>visual
     <tr>
      <th><a class=property href="#break-inside">break-inside</a>
      <td>auto | avoid | avoid-page | avoid-column | avoid-region
      <td>auto
      <td>block-level elements
      <td>no
      <td>N/A
      <td>visual
     <tr>
      <th><a class=property href="#flow-from">flow-from</a>
      <td>&lt;ident&gt; | none | inherit
      <td>none
      <td>Non-replaced block containers. This might be expanded in future
       versions of the specification to allow other types of containers to
       receive flow content.
      <td>no
      <td>N/A
      <td>visual
     <tr>
      <th><a class=property href="#flow-into">flow-into</a>
      <td>&lt;ident&gt; | none | inherit
      <td>none
      <td>any element. The ‘flow-into’ property does not apply to any
       pseudo-element such as ::first-line, ::first-letter, ::before or
       ::after.
      <td>no
      <td>N/A
      <td>visual
     <tr>
      <th><a class=property href="#region-fragment">region-fragment</a>
      <td>auto | break
      <td>auto
      <td>CSS Regions
      <td>no
      <td>N/A
      <td>visual
   </table>
   <!--end-properties-->
   <!-- template markup for issues pulled from Bugzilla -->
   <!-- template markup for issues pulled from Bugzilla -->
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     </script>
   <div class=issue-marker data-bug_id="{{bug_id}}"
    data-bug_status="{{bug_status}}"> <a
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    <div class=issue-details>
     <p class=short-desc>{{short_desc}}
    </div>
   </div>
   <script src="../shared/scripts/BugzillaTrackerUtil.js"
    type="text/javascript"></script>
   <script src="../shared/scripts/BugzillaTracker.js" type="text/javascript"></script>
   <script type="text/javascript">
         checkSpecificationIssues(‘</script>
   <p><code class=property>CSS</code>’, ‘<a href="#css-region"><code
    class=property>Regions</code></a>’);
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