csswg-drafts/css-regions/Overview.src.html at 23534b7e8272bd82dbc8f2b56edf02d394222c6f · xfq/csswg-drafts · GitHub

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<!DOCTYPE html public '-//W3C//DTD HTML 4.01//EN'
  'http://www.w3.org/TR/html4/strict.dtd'>
<html lang="en">
<head profile="http://www.w3.org/2006/03/hcard">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>CSS Regions Module Level 1</title>
  <link rel="stylesheet" type="text/css" href="../shared/style/default.css">
  <script defer=defer
          src="http://test.csswg.org/harness/annotate.js#css-regions-1_dev"
          type="text/javascript"></script>

       <style type="text/css">
         #region-style-example p, #region-style-example pre {
         	clear: both;
         }

         #region_styling_illustration {
         	margin: 0px auto;
         	width: 70ex;
         }
       </style>

        <link rel="stylesheet" type="text/css"
         href="issues.css">
       <link rel="stylesheet" type="text/css"
        href="http://www.w3.org/StyleSheets/TR/W3C-ED.css">


       <link id="st" href="../shared/style/alternate-spec-style.css" rel="stylesheet"
              type="text/css" title="alternate spec style">
</head>


<body>
    <div class="head" id="div-head">
        <!--begin-logo-->
        <p><a href="http://www.w3.org/"><img id="mobile-logo" alt="W3C" height="53" src=
        "http://www.w3.org/2008/site/images/logo-w3c-mobile-lg" width="90"></a>


        <p><a href="http://www.w3.org/"><img id="logo" alt="W3C" height="48" src=
        "http://www.w3.org/Icons/w3c_home" width="72"></a> <!--end-logo--></p>

        <h1 id="css-regions-module">CSS Regions Module Level 1</h1>

        <h2 class="no-num no-toc" id="longstatus-date">[LONGSTATUS]
        [DATE]</h2>

        <dl>
            <dt>This version:</dt>

            <dd><a href=
            "[VERSION]">[VERSION]</a></dd>

            <dt>Latest version:</dt>

            <dd><a href=
            "http://www.w3.org/TR/css3-regions/">http://www.w3.org/TR/css3-regions/</a></dd>

            <dt>Previous version:</dt>

            <dd><a href="http://www.w3.org/TR/2012/WD-css3-regions-20120823/">http://www.w3.org/TR/2012/WD-css3-regions-20120823/</a></dd>


            <dt>Editors:</dt>

            <dd class="vcard"><span class="fn">Vincent Hardy</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">vhardy@adobe.com</span></dd>
			<dd class="vcard"><span class="fn">Rossen Atanassov</span>,
            <span class="org">Microsoft Corporation</span>, <span class=
            "email">ratan@microsoft.com</span></dd>
            <dd class="vcard"><span class="fn">Alan Stearns</span>,
            <span class="org">Adobe Systems, Inc.</span>, <span class=
            "email">stearns@adobe.com</span></dd>

		    <dt>Authors and former editors:</dt>
			<dd class="vcard"><span class="fn">Alex Mogilevsky</span>,
            <span class="org">Microsoft Corporation</span>, <span class=
            "email">alexmog@microsoft.com</span></dd>

            <dt>Issues List:</dt>
                <dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&amp;component=Regions&amp;resolution=---">Bugzilla Bugs for CSS regions</a></dd>
            <dt>Feedback:
            <dd><a href="mailto:www-style@w3.org?subject=%5Bcss-regions%5D%20feedback">www-style@w3.org</a>
                with subject line &ldquo;<kbd>[css-regions] <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
                (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)</dd>

            <dt>Test Suite:</dt>

                <dd><a href="http://test.csswg.org/suites/css3-regions/nightly-unstable/">http://test.csswg.org/suites/css3-regions/nightly-unstable/</a></dd>
        </dl><!--begin-copyright--><!--end-copyright-->
        <hr title="Separator for header">
    </div>

    <h2 class="no-num no-toc" id="abstract">Abstract</h2>

    <p>The CSS regions module allows content to flow across multiple areas
    called regions. The regions 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 [[CSS3COL]] or the Grid Layout Module
    [[CSS3-GRID-LAYOUT]] to position the regions where content flows.</p>

    <h2 class="no-num no-toc" id="status-of-this-document">Status of this
    document</h2>

    <!--status-->

    <p>This draft is related to the drafts about Multi-column Layout
    [[CSS3COL]], Grid Layout [[CSS3GRID]], Flexible Box Layout
    [[CSS3-FLEXBOX]], and Template Layout [[CSS3LAYOUT]].</p>
    <h2 class="no-num no-toc" id="table-of-contents">Table of contents</h2>
    <!--begin-toc--><!--end-toc-->

    <h2 id="introduction">Introduction</h2>

    <p><em>This section is non-normative.</em></p>

    <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.</p>

    <div class="issue-marker">
        <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?</p>
        </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 <span>named flows</span>.
      Each <span>named flow</span> 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.
      Content in a <span>named flow</span>
      is taken out of the normal visual formatting
      and rendered in a chain of CSS Regions.</p>

    <h3 id="named-flows-and-regions">Named flows and region chains</h3>

    <p>Consider the layout illustrated in figure 1.</p>


    <div class="figure">
        <img src="images/intro-example-layout.svg" width="400" alt=
        "multiple chained regions which should receive content from a flow">

        <p class="caption">Layout requiring sophisticated content flow</p>
    </div>

    <p>The designer's intent is
      to position an image in box 'A'
      and to flow an article's content
      from box '1' through boxes '2', '3' and '4'.
      Note that the second box should have two columns,
      and the image is not contained in the article.
      Box '4' should auto-size to render
      the remainder of the article content
      that did not fit in the earlier boxes.</p>

    <p>Figure 2 shows an example
      of the intended visual rendering
      of the content.</p>

    <div class="figure">
        <img src="images/regions-intro-rendering.png" width="450" alt=
        "Sample rendering showing a single thread of text flowing through a chain of regions">

        <p class="caption">Sample rendering for desired layout</p>
    </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>

    <p>The following example illustrates
      how an <code>&lt;article&gt;</code> element
      is placed in a <span>named flow</span>
      and how boxes marked with 'region1', 'region2', 'region3' and 'region4' IDs
      become CSS Regions that consume the 'article-flow' content.</p>

    <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 'article-flow' value on the 'flow-into' property
      directs the <code>article</code> element
      to the 'article-flow' <span>named flow</span>.
      Setting the 'flow-from' property on block containers
      to 'article-flow' makes them CSS Regions
      and associates the resulting region chain
      with the <span>named flow</span>:
      the flow is ''poured''
      into the region chain.</p>

    <p>See <a href="#intro-example-code">Appendix A</a>
      for sample code using grid layout
      that implements this example</p>

    <div class="issue-marker">
    <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?</p>
    </div>
    </div>

    <div class="note"><span class="note-prefix">Note </span>
      <p><strong>CSS Regions are independent from layout</strong></p>

      <p>Any of the CSS layout facilities can be used
        to create, position and size boxes that can become CSS Regions.
        For instance,
        the code in <a href="#intro-example-code">Appendix A</a>
        uses a grid layout [[CSS3-GRID-LAYOUT]] as an example.
        This example could use
        a flexible box layout [[CSS3-FLEXBOX]] instead.</p>
      <p>The CSS regions specification does not
        define a layout mechanism
        and is meant to integrate
        with existing and future CSS layout facilities.</p>
    </div>

    <div class="note"><span class="note-prefix">Note </span>
      <p><strong>CSS Regions do not have to be elements</strong></p>

      <p>The CSS regions module is independent
        of the layout of boxes and
        the mechanisms used to create them.

      <p>For simplicity,
        our example code in <a href="#intro-example-code">Appendix A</a>
        uses elements to define the boxes.
        Any other mechanism available
        in markup or style
        to create stylable boxes could be used instead,
        such as pseudo-elements
        or the @slot rule proposed
        by the CSS Page Template Module [[CSS3-PAGE-TEMPLATE]].</p>


      <p>The only requirement
        for box to become a CSS Region
        is that the 'flow-from' property applies to the box. </p>
    </div>

    <h3 id="region-styling">CSS Region names and styling</h3>

    <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>

    <p>In our example,
      the designer wants to make
      text flowing into #region1
      dark blue and bold.</p>

    <p>This design can be expressed as shown below.</p>

    <div class="example">
        <pre>
&lt;style&gt;
  #region1::region(p) {
    color: #0C3D5F;
    font-weight: bold;
  }
&lt;/style&gt;</pre>
    </div>

    <p>The <code class="css">::region()</code> functional pseudo-element
      takes a <code class="css">p</code> selector as an argument in this example.
      The color and font-weight declarations will apply
      to any fragments of paragraphs that are
      displayed in <code class="css">#region1</code>.
      The following figure shows how
      the rendering changes
      if we apply this styling specific to <code>#region1</code>.
      Note how less text fits into this box
      now that the 'font-weight' is
      bold instead of normal.</p>

    <div class="figure">
        <img src="images/region-styling.png" width="450" alt=
        "Illustrate how changing region styling affects the flow of content.">

        <p class="caption">Different rendering with a different region
        styling</p>
    </div>

    <h2 id="css-regions-concepts">CSS regions concepts</h2>

    <h3 id="regions">Regions</h3>

    <p id="region">
      A <dfn title="region">CSS Region</dfn>
      is a block container
      that has an associated
      <em><span>named flow</span></em>
      (see the 'flow-from' property).</p>

    <h3 id="region-chain">Region chain</h3>

    <p>A <dfn>region chain</dfn> is the sequence of regions that are associated with
        a <span>named flow</span>. <span title="region">CSS Regions</span> in a
        region chain receive content from the
        named flow following their order in the chain. <span title="region">CSS Regions</span> are organized
          into a <span title="region-chain">region chain</span>
          according to the document order.</p>

    <h3 id="named-flow">Named flows</h3>

    <p>A <dfn>named flow</dfn> is the ordered sequence of content
    associated with a flow with a given identifier.
    Contents in a <span>named flow</span> are ordered
    according to the document order.</p>

    <p>Content is placed into a <span>named flow</span>
      with the 'flow-into' property.
      The content in a <span>named flow</span> is laid out
      in the <span title="region-chain">region chain</span>
      that is associated with this <span>named flow</span>
      using the 'flow-from' property.
      </p>

    <p>Content from a <span>named flow</span>
      is broken up between regions
      according to the regions flow breaking rules.</p>

    <p>A <span>named flow</span> is created when
      some content is moved
      into the flow with the given identifier
      or when at least one CSS Region
      requests content from that flow.</p>

    <h3 id="regions-flow-breaking-rules">Regions flow breaking rules</h3>
    <p>Breaking a <span>named flow</span> across a region chain
      is similar to breaking a document's content across pages
      (see [[CSS3PAGE]])
      or a multi-column element's content across column boxes
      (see [[CSS3COL]]).
      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 <span>named flow</span> content
      whose dynamic generation is not in the scope
      of this specification.</p>

    <p>Each CSS Region in turn
      consumes content from its associated <span>named flow</span>.
      The <span>named flow</span> content is positioned
      in the current region
      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 <span title="current-region">current region</span>.
      If there are no more <span>region</span>s
      in the region chain
      and there is still content in the flow,
      the positioning of the remaining content
      is controlled by the 'region-fragment' property
      on the last <span>region</span> in the chain.</p>

    <p>The CSS regions module follows
      the fragmentation rules defined
      in the CSS Fragmentation Module Level 3
      (see [[!CSS3-BREAK]]).</p>


    <h2 id="properties-and-region-styling">Properties and Region Styling</h2>

    <h3 id="the-flow-into-property">The 'flow-into' property</h3>

    <div class="issue-marker wrapper">
    <div class="issue-marker">
    <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</p>
    </div>
    </div>
    </div>

    <p>The ‘flow-into’ property can place an element
      or its contents
      into a <span>named flow</span>.
      Content that belongs to the same flow
      is laid out in the region chain
      associated with that flow.</p>

    <table class="propdef" summary="flow property definition">
        <tr>
            <th>Name:</th>

            <td><dfn id="flow-into">flow-into</dfn></td>
        </tr>

        <tr>
            <th>Value:</th>

            <td>none | &lt;ident&gt; [element|content]?</td>
        </tr>

        <tr>
            <th>Initial:</th>

            <td>none</td>
        </tr>

        <tr>
            <th>Applies to:</th>

            <td>All elements, but not
              <a href="http://www.w3.org/TR/selectors/#pseudo-elements">pseudo-elements</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>.</td>
        </tr>

        <tr>
            <th>Inherited:</th>

            <td>no</td>
        </tr>

        <tr>
            <th>Percentages:</th>

            <td>N/A</td>
        </tr>

        <tr>
            <th>Media:</th>

            <td>visual</td>
        </tr>

        <tr>
            <th>Computed&nbsp;value:</th>

            <td>as specified</td>
        </tr>
    </table>

    <dl>
        <dt>none</dt>

        <dd>The element is not moved
          to a <span>named flow</span>
          and normal CSS processing takes place.</dd>

        <dt><a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a></dt>

        <dd>If the keyword 'element' is present
          or neither keyword is present,
          then the element is taken out
          of its parent's flow
          and placed into the flow
          with the name '<a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>'.
          If the keyword 'content' is present,
          then only the element's contents
          are placed into the named flow.
          The element or content is said to have
          a <dfn id="specified-flow">specified flow</dfn>.
          The values 'none', 'inherit', 'default', 'auto' and 'initial'
          are invalid flow names.</dd>
    </dl>

    <p>The 'flow-into' property does not affect
      the CSS cascade and inheritance
      for the elements on which it is specified.
      The 'flow-into' property does not affect the
      <a href="http://www.w3.org/TR/2012/WD-dom-20120405/#introduction-to-the-dom">DOM</a>
      [[!DOM]] position of an element or its contents.
      The 'flow-into' property affects
      the visual formatting of elements or contents
      placed into a <span>named flow</span>
      and of the region chain laying out content
      from a <span>named flow</span>.</p>

    <p>A <span>named flow</span> needs to be associated
      with a region chain
      (one or more CSS regions)
      for its content to be visually formatted.
      If no region chain is associated
      with a given <span>named flow</span>,
      the content in the <span>named flow</span>
      is not rendered:
      it does not generate boxes
      and is not displayed.</p>

    <p>The children of an element or content
      with a specified flow
      may themselves have a specified flow.
      If an element or content has
      the same specified flow
      as one of its ancestors,
      it becomes the next sibling
      of its closest ancestor
      with the same specified flow.</p>

    <p>Content in a <span>named flow</span>
      is sequenced in document order.
      The visual formatting model
      uses the relationships between content
      in the named flow as input,
      rather than the contents&rsquo; position
      in the DOM.
    </p>

    <p>The edges of the first CSS region in a region chain
      associated with a <span>named flow</span>
      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 <span>named flow</span>
      which do not have an ancestor
      with a 'position' of 'absolute', 'relative' or 'fixed'
      (see [[!CSS21]]).
      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 <span>named flow</span> -
      they are still positioned relative
      to the viewport or page area even if
      they have been redirected
      to a named flow.</p>

    <p>The first region defines the principal
      <a href="http://www.w3.org/TR/css3-writing-modes/#writing-mode1">writing mode</a>
      for the entire flow.
      The writing mode
      on subsequent regions is ignored.</p>

    <div class="note"><span class="note-prefix">Note </span>
        <p>The 'flow-into' property moves an element into the flow
          and the interplay with selectors should be considered carefully.</p>
        <p>For example, </p>
        <pre>table {flow-into: table-content}</pre>
        <p>will move all tables in the 'table-content'
          <span>named flow</span>.
          However, the</p>
        <pre>table &gt; * {flow-into: table-content} ...</pre>
        <p>selector will move all immediate children
          of all table elements
          into the ‘table-content’ <span>named flow</span>
          (which may be useful as it will usually result,
          if the immediate children are rows,
          in merging rows of multiple tables),
          but the</p>
        <pre>table * {flow-into: table-content}</pre>
        <p>selector will move all descendants
          of table elements into the ‘table-content’ <span>named flow</span>,
          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 <span>named flow</span>.
          Having the descendants become siblings
          in the <span>named flow</span>
          results in a different processing
          (see <a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/tables.html#anonymous-boxes">CSS 2.1's anonymous table objects</a>).
          This note illustrates how authors must exercise caution
          when choosing a particular selector
          for setting the 'flow-into' property
          to avoid unintended results.</p>
    </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 'span-content' 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>

    <h3 id="the-flow-from-property">The 'flow-from' property</h3>

      a block container a region
      and associates it with
      a <span>named flow</span>.</p>

    <table class="propdef" summary="flow-from property definition">
        <tr>
            <th>Name:</th>

            <td><dfn id="flow-from">flow-from</dfn></td>
        </tr>

        <tr>
            <th>Value:</th>

            <td>&lt;ident&gt; | none | inherit</td>
        </tr>

        <tr>
            <th>Initial:</th>

            <td>none</td>
        </tr>

        <tr>
            <th>Applies to:</th>

            <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.</td>
        </tr>

        <tr>
            <th>Inherited:</th>

            <td>no</td>
        </tr>

        <tr>
            <th>Percentages:</th>

            <td>N/A</td>
        </tr>

        <tr>
            <th>Media:</th>

            <td>visual</td>
        </tr>

        <tr>
            <th>Computed&nbsp;value:</th>

            <td>as specified</td>
        </tr>
    </table>

    <dl>
        <dt><strong>none</strong></dt>

        <dd>The block container is not a <span title="region">region</span>.</dd>

        <dt><strong><a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a></strong></dt>

        <dd>
            If the 'content' property computes
            to something else than 'normal'
            (or 'none' for a pseudo-element),
            the block container does not become a <span>CSS Region</span>.
            If the 'content' property computes to 'normal'
            (or 'none' for a pseudo-element),
            then the block container becomes a <span>CSS Region</span>
            and is ordered in a <span>region chain</span>
            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 <span>regions</span>
            in the <span>region chain</span>.
            <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.
        </dd>
    </dl>

    <p>A <span>CSS Region</span>&rsquo;s document children
      are not visually formatted
      unless they are directed
      to a <span>named flow</span>
      with an associated <span>region chain</span>.</p>

    <p>Block container pseudo-elements where
      the value of 'flow-from' computes to an
      <a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
      and the value of 'content' computes to 'none'
      are generated as CSS Regions,
      which is an update to the behavior
      described in [[!CSS21]].</p>

    <div class="note"><span class="note-prefix">Note </span>
      <p>A 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:</p>
        <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 <code>.region</code> selector
        is still associated with
        that empty <span>named flow</span>
        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.

     <p>The <code>flex</code> and <code>grid</code> display values do not
      result in block containers (they are defined as flex containers and grid
      elements, respectively). So ‘<a href="#flow-from"><code
      class=property>flow-from</code></a>’ combined with those display values
      does not result in a CSS Region.
    </div>

    <p><span title="region">CSS Regions</span>
      create a new
      <a href="http://www.w3.org/TR/CSS2/visuren.html#z-index">stacking context</a>.
      <span title="region">CSS Regions</span>
      establish a new
      <a href="http://www.w3.org/TR/CSS2/visuren.html#block-formatting">block formatting Context</a>.
      Exclusions (see [[CSS3-EXCLUSIONS]])
      potentially impact the content
      laid out in region chains,
      just as for non-regions.</p>

    <div class="note"><span class="note-prefix">Note </span>
        <p>With <span>region chains</span>,
          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.</p>
        <p>Fragments rendering separately
          is also relevant to elements that might normally
          be rendered as a unit
          (for example,
          an element with its own stacking context,
          or with transparency).
          Each fragment of these elements
          is separately contained in the stacking context
          created by the <span>CSS Region</span>,
          so each fragment of these elements
          is rendered separately.</p>
    </div>

    <p>See the
      <a href="#regions-visual-formatting-details">regions visual formatting details</a>
      section for a description of how
      'width' and 'height' values are resolved
      for <span>CSS Region</span> boxes.</p>

    <h4 id="circular-dependencies">Cycle Detection</h4>

    <p><span>Named flows</span> containing elements
      where the value of 'flow-from' 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 <span>named flow</span> containing <span>CSS Regions</span>
      in its own <span>region chain</span>.
      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>

    <p>The dependency graph consists of edges such that:</p>

    <ul>
      <li>Every <span>named flow</span> depends on its elements
        where the value of 'flow-from' computes to an
        <a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
        .</li>
      <li>Every element in a <span>named flow</span>
        where the value of 'flow-from' 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 <span>named flow</span> with the
        <a href="http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-identifier">&lt;ident&gt;</a>
        name.</li>
    </ul>

    <p>If the graph contains a cycle,
      any elements where the value of 'flow-from'
      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>.</p>

    <div class="note"><span class="note-prefix">Note </span>
      <p>For example, styling like this:</p>
        <pre>
#id {
    flow-into: foolish;
    flow-from: foolish;
}
        </pre>
      <p>would move the <code>#id</code> element to a 'foolish' <span>named flow</span>, and try to make the <code>#id</code> element  a <span>CSS Region</span> for the 'foolish' <span>named flow</span>. The 'foolish' <span>named flow</span> would then contain its own region, creating a cycle. So the <code>#id</code> element does not become a <span>CSS Region</span>.
    </div>

    <h4 id="fragmenting-regions">Nested fragmentation contexts</h4>

    <p>A <span>CSS Region</span> that contains
      a fragment of a <span>named flow</span>
      can itself be fragmented if it is nested
      within a fragmentation context [[!CSS3-BREAK]],
      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.</p>
    <p>A <span>CSS Region</span> can be part
      of the contents of a separate named flow,
      as long as there are no cycles broken
      by the <a href="#circular-dependencies">Cycle Detection</a>
      described above.
      This case is a <dfn>nested region context</dfn>,
      which has an effect
      on the <a href="#regions-visual-formatting-steps">Visual Formatting Steps</a>
      described below.</p>

    <h3 id="region-flow-break">Region flow break properties: 'break-before', 'break-after', 'break-inside'</h3>

    <div class="note"><span class="note-prefix">Note </span>
      <p>This section is also defined in [[!CSS3-BREAK]].
        If that specification moves to last call
        with the region values,
        the section here can be replaced by a reference.</p>
    </div>

    <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>

    <p>Each break ends layout in the current region and causes remaining pieces
    of content from the <span>named flow</span> to be visually formatted in the following
    region in the region chain, if there is one.</p>

    <p>The following extends the 'break-before', 'break-after' and
    'break-inside' properties from the [[!CSS3COL]] specification to account
    for regions. The additional values are described below.</p>

    <table class="propdef" summary="break-before property definition">
        <tr>
            <td><em>Name:</em></td>

            <td><dfn>break-before</dfn></td>
        </tr>

        <tr>
            <td><em>Value:</em></td>

            <td>auto | always | avoid | left | right | page | column | region |
            avoid-page | avoid-column | avoid-region</td>
        </tr>

        <tr>
            <td><em>Initial:</em></td>

            <td>auto</td>
        </tr>

        <tr>
            <td><em>Applies to:</em></td>

            <td>block-level elements</td>
        </tr>

        <tr>
            <td><em>Inherited:</em></td>

            <td>no</td>
        </tr>

        <tr>
            <td><em>Percentages:</em></td>

            <td>N/A</td>
        </tr>

        <tr>
            <td><em>Media:</em></td>

        </tr>

        <tr>
            <td><em>Computed&nbsp;value:</em></td>

            <td>specified value</td>
        </tr>
    </table>

    <table class="propdef" summary="break-after property definition">
        <tr>
            <td><em>Name:</em></td>

            <td><dfn>break-after</dfn></td>
        </tr>

        <tr>
            <td><em>Value:</em></td>

            <td>auto | always | avoid | left | right | page | column | region |
            avoid-page | avoid-column | avoid-region</td>
        </tr>

        <tr>
            <td><em>Initial:</em></td>

            <td>auto</td>
        </tr>

        <tr>
            <td><em>Applies to:</em></td>