csswg-drafts/css3-regions/Overview.html at a51f1eba94c4a7df684521022166c3a8c8495b3d · w3c/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 dir=ltr lang=en>
 <head profile="http://www.w3.org/2006/03/hcard"><meta content="IE=edge"
  http-equiv=X-UA-Compatible>
  <meta content="text/html; charset=utf-8" http-equiv=Content-Type>

  <title>CSS Regions Module</title>
  <link href="http://dev.w3.org/csswg/default.css" rel=stylesheet
  type="text/css">
  <link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=Stylesheet
  type="text/css">
  <link href=alternate-spec-style.css id=st rel=stylesheet title="additional
  spec styles" type="text/css">
  <script src=style-toggle.js type="text/javascript">
</script>

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

   <h1>CSS Regions Module</h1>

   <h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 10 May 2011</h2>

   <dl>
    <dt>This version:

    <dd><a
     href="http://www.w3.org/TR/2011/ED-css3-regions-20110510/">http://www.w3.org/csswg/css3-regions</a>

    <dt>Latest version:

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

    <dt>Previous version:

    <dd>none

    <dt>Editors:

    <dd class=vcard><span class=fn>Vincent Hardy</span>, <span
     class=org>Adobe Systems, Inc.</span>, <span
     class=email>vhardy@adobe.com</span>

    <dt>Authors and Former Editors:

    <dd class=vcard><span class=fn>Stephen Zilles</span>, <span
     class=org>Adobe Systems, Inc.</span>, <span
     class=email>szilles@adobe.com</span>

    <dd class=vcard><span class=fn>Alexandru Chiculita</span>, <span
     class=org>Adobe Systems, Inc.</span>, <span
     class=email>achicu@adobe.com</span>

    <dd class=vcard><span class=fn>Andrei Bucur</span>, <span class=org>Adobe
     Systems, Inc.</span>, <span class=email>abucur@adobe.com</span>

    <dd class=vcard><span class=fn>Mihnea Ovidenie</span>, <span
     class=org>Adobe Systems, Inc.</span>, <span
     class=email>mihnea@adobe.com</span>

    <dd class=vcard><span class=fn>Peter Sorotokin</span>, <span
     class=org>Adobe Systems, Inc.</span>, <span
     class=email>psorotok@adobe.com</span>

    <dd class=vcard><span class=fn>Virgil Palanciuc</span>, <span
     class=org>Adobe Systems, Inc.</span>, <span
     class=email>virgilp@adobe.com</span>

    <dd class=vcard><span class=fn>Arno Gourdol</span>, <span class=org>Adobe
     Systems, Inc.</span>, <span class=email>agourdol@adobe.com</span>
   </dl>
   <!--begin-copyright-->
   <p class=copyright><a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
    rel=license>Copyright</a> &copy; 2011 <a
    href="http://www.w3.org/"><acronym title="World Wide Web
    Consortium">W3C</acronym></a><sup>&reg;</sup> (<a
    href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
    of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
    title="European Research Consortium for Informatics and
    Mathematics">ERCIM</acronym></a>, <a
    href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
    <a
    href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
    and <a
    href="http://www.w3.org/Consortium/Legal/copyright-documents">document
    use</a> rules apply.</p>
   <!--end-copyright-->
   <hr title="Separator for header">
  </div>

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

  <p>The CSS Regions module allows content to flow across multiple areas
   called regions. The regions do not necessarily follow the document order.
   The CSS Regions module provides an advanced content flow mechanism, which
   can be combined with positioning schemes as defined by other CSS modules
   such as the Multi-Column Module <a href="#CSS3COL"
   rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a> or the Grid Layout Module
   <a href="#CSS3-GRID-LAYOUT"
   rel=biblioentry>[CSS3-GRID-LAYOUT]<!--{{CSS3-GRID-LAYOUT}}--></a> to
   position the regions where content flows.

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

  <p>This is a public copy of the editors' draft. It is provided for
   discussion only and may change at any moment. Its publication here does
   not imply endorsement of its contents by W3C. Don't cite this document
   other than as work in progress.

  <p>The (<a
   href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
   mailing list <a
   href="mailto:www-style@w3.org?Subject=%5Bcss3-regions%5D%20PUT%20SUBJECT%20HERE">
   www-style@w3.org</a> (see <a
   href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
   discussion of this specification. When sending e-mail, please put the text
   &#8220;css3-regions&#8221; in the subject, preferably like this:
   &#8220;[<!---->css3-regions<!---->] <em>&hellip;summary of
   comment&hellip;</em>&#8221;

  <p>This document was produced by the <a href="/Style/CSS/members">CSS
   Working Group</a> (part of the <a href="/Style/">Style Activity</a>).

  <p>This document was produced by a group operating under the <a
   href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
   Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
   rel=disclosure>public list of any patent disclosures</a> made in
   connection with the deliverables of the group; that page also includes
   instructions for disclosing a patent. An individual who has actual
   knowledge of a patent which the individual believes contains <a
   href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
   Claim(s)</a> must disclose the information in accordance with <a
   href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
   W3C Patent Policy</a>.</p>
  <!--end-status-->

  <h2 class="no-num no-toc" id=contents>Table of contents</h2>
  <!--begin-toc-->

  <ul class=toc>
   <li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
    <ul class=toc>
     <li><a href="#named-flows-and-regions"><span class=secno>1.1.
      </span>Named flows and regions</a>

     <li><a href="#region-styling"><span class=secno>1.2. </span>Regions
      Styling</a>
    </ul>

   <li><a href="#css-regions-model"><span class=secno>2. </span>CSS Regions
    Model</a>
    <ul class=toc>
     <li><a href="#named-flows"><span class=secno>2.1. </span>Named flows</a>


     <li><a href="#regions"><span class=secno>2.2. </span>Regions</a>

     <li><a href="#flow-breaking-rules"><span class=secno>2.3. </span>Flow
      breaking rules</a>

     <li><a href="#formatting-model-considerations"><span class=secno>2.4.
      </span>The Visual Formatting Model and Flows</a>
      <ul class=toc>
       <li><a href="#normal-flow"><span class=secno>2.4.1. </span>Normal
        flow</a>

       <li><a href="#positioned-flow"><span class=secno>2.4.2.
        </span>Positioned flow</a>

       <li><a href="#column-flow"><span class=secno>2.4.3. </span>Column
        flow</a>

       <li><a href="#generated-flow"><span class=secno>2.4.4.
        </span>Generated flow</a>
      </ul>

     <li><a href="#relation-to-document-events"><span class=secno>2.5.
      </span>Relation to document events</a>
    </ul>

   <li><a href="#regions-properties"><span class=secno>3. </span>Properties
    and Rules</a>
    <ul class=toc>
     <li><a href="#the-flow-property"><span class=secno>3.1. </span>The
      &lsquo;<code class=property>flow</code>&rsquo; property</a>

     <li><a href="#the-content-property"><span class=secno>3.2. </span>The
      &lsquo;<code class=property>content</code>&rsquo; property</a>

     <li><a href="#the-content-order-property"><span class=secno>3.3.
      </span>The &lsquo;<code class=property>content-order</code>&rsquo;
      property</a>

     <li><a href="#region-flow-break-properties-break-befor"><span
      class=secno>3.4. </span>Region flow break properties: &lsquo;<code
      class=property>break-before</code>&rsquo;, &lsquo;<code
      class=property>break-after</code>&rsquo;, &lsquo;<code
      class=property>break-inside</code>&rsquo;</a>

     <li><a href="#the-region-style-rule"><span class=secno>3.5. </span>The
      region-style rule</a>
    </ul>

   <li><a href="#om_regions"><span class=secno>4. </span>CSSOM View and CSS
    Regions</a>

   <li><a href="#integration"><span class=secno>5. </span>Integration with
    other specifications</a>
    <ul class=toc>
     <li><a href="#region-requirements"><span class=secno>5.1. </span>Region
      requirements</a>

     <li><a href="#integration-examples"><span class=secno>5.2.
      </span>Integration Examples</a>
      <ul class=toc>
       <li><a href="#css-grid-layout"><span class=secno>5.2.1. </span>CSS
        Grid Layout</a>

       <li><a href="#css-multi-column-layout"><span class=secno>5.2.2.
        </span>CSS Multi-Column Layout</a>

       <li><a href="#css-flexible-box-layout-module"><span class=secno>5.2.3.
        </span>CSS Flexible Box Layout Module</a>

       <li><a href="#css-visual-formatting"><span class=secno>5.2.4.
        </span>CSS Visual Formatting</a>
      </ul>
    </ul>

   <li><a href="#relation-to-other-specs"><span class=secno>6.
    </span>Relation to other specifications</a>

   <li><a href="#conformance"><span class=secno>7. </span>Conformance</a>

   <li class=no-num><a href="#acknowledgments">Acknowledgments</a>

   <li class=no-num><a href="#references">References</a>
    <ul class=toc>
     <li class=no-num><a href="#normative-references">Normative
      references</a>

     <li class=no-num><a href="#other-references">Other references</a>
    </ul>

   <li class=no-num><a href="#index">Index</a>

   <li class=no-num><a href="#property-index">Property index</a>
  </ul>
  <!--end-toc-->

  <h2 id=introduction><span class=secno>1. </span>Introduction</h2>

  <p>Capturing the complex layouts of a typical magazine, newspaper, or
   textbook requires capabilities beyond those available in existing CSS
   modules. This is the purpose of the CSS Regions module.

  <p>&nbsp;

  <p>The CSS Regions module can be seen as an extension of the concept of
   multi-column elements. With CSS Multi-column layout <a href="#CSS3COL"
   rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>, columns share the same
   dimensions and define column boxes organized in rows. Content flows from
   one column to the next.

  <p>&nbsp;

  <p>The multi-column model is an example of flowing content from one area to
   another, where the areas are the multi-column element's column boxes and
   the flow is made of the multi-column element's children.

  <p>&nbsp;

  <p>However, for more complex layouts, content needs to flow from one area
   of the page to the next without limitation of the areas' sizes and
   positions. These arbitrary areas are the target of specific content flows.
   In this document these areas are called regions, and the content flows are
   called named flows. Regions are based on the rectangular geometry of the
   CSS box model. Elements in a named flow are taken out of the normal visual
   formatting and rendered in a chain of regions.

  <h3 id=named-flows-and-regions><span class=secno>1.1. </span>Named flows
   and regions</h3>

  <p>Consider the layout illustrated in figure 1.

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

  <p>The designer's intent is to position an image in region &lsquo;<code
   class=property>A</code>&rsquo; and to flow an article's text from region
   &lsquo;<code class=css>1</code>&rsquo;, to region &lsquo;<code
   class=css>2</code>&rsquo;, &lsquo;<code class=css>3</code>&rsquo; and
   &lsquo;<code class=css>4</code>&rsquo;.

  <p>The following code snippet shows the content to flow between the regions
   1, 2, 3 and 4.

  <pre>
&lt;div <em>id="article"</em>&gt;
&lt;h1&gt;Introduction&lt;/h1&gt;
    &lt;p&gt;This is an example ...&lt;/p&gt;

&lt;h1&gt;More Details&lt;/h1&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;/div&gt;

</pre>

  <p>CSS layout facilities can position and size regions as needed. However,
   there is no existing mechanism to associate the content with the regions
   so that content flows as intended. Figure 2 shows an example of the
   intended visual rendering of the content.

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

  <p>Since the CSS Regions module is independent of the layout of regions and
   the mechanism used to create them, the following assumes there is a CSS
   selector for the regions and, for the purpose of the example, the
   selectors for regions 1, 2, 3 and 4 are showing as
   &lt;<code>region1_sel</code>&gt;, &lt;<code>region2_sel</code>&gt;,
   &lt;<code>region3_sel</code>&gt; and &lt;<code>region4_sel</code>&gt;
   respectively. Such a selector could be an id selector (e.g.,
   <code>#region_1</code>) or a grid cell selector (e.g.,
   <code>#myGrid::grid-cell([cell-name])</code>) if using the CSS Grid Layout
   module, for example.

  <div class=example>
   <pre>
&lt;style&gt;

#article {
    <strong>flow: article_flow;</strong>
}

&lt;region1_sel&gt;, &lt;region2_sel&gt;,
&lt;region3_sel&gt;, &lt;region4_sel&gt; {
    <strong>content: from(article_flow);</strong>
}
&lt;/style&gt;
</pre>
  </div>

  <p>The above stylesheet directs the <code>#article</code> element to a
   named flow called &lsquo;<code class=css>article_flow</code>&rsquo; by
   setting the &lsquo;<a href="#flow2"><code
   class=property>flow</code></a>&rsquo; property. Then, content is "poured"
   from that named flow into the desired regions by setting the regions'
   &lsquo;<a href="#content-from"><code
   class=property>content</code></a>&rsquo; property to
   <code>from(article_flow)</code>.

  <h3 id=region-styling><span class=secno>1.2. </span>Regions Styling</h3>

  <p>Region styling allows content to be styled depending on the region it
   flows into. It is a form of context-based styling, similar to <a
   href="#CSSMEDIAQUERIES">Media Queries</a> <a href="#MEDIAQ"
   rel=biblioentry>[MEDIAQ]<!--{{MEDIAQ}}--></a> which enable or disable
   selectors depending on the rendering context. With region styling,
   additional selectors may apply depending on the region into which content
   flows.

  <p>In our example, the designer wants to make text that falls into region 1
   larger, bold and dark blue. In addition, &lt;h1&gt; headers should be
   run-ins and crimson.

  <p>This design can be expressed with region styling as shown below.

  <div class=example>
   <pre>
&lt;style&gt;
/*
 * Default article styling.
 */
#article {
    color:#777;
    text-align: justify;
}

#article h1 {
    border-left: 1px solid #777;
    padding-left: 2ex;
}

/*
 * Additional styling to apply to content when it falls into
 * region_1
 */
<strong>@region_style &lt;region1_sel&gt; {</strong>
    #article {
        font-weight: bold;
        color: #0C3D5F;
        font-size: larger;
    }

    #article h1 {
        color: crimson;
        display: run-in;
        border: none;
        padding: 0px;
    }
<strong>}</strong>
&lt;/style&gt;
</pre>
  </div>

  <p>The &lsquo;<code class=css>@region_style</code>&rsquo; rule for region 1
   limits its selectors to content flowing into region 1. The following
   figure shows how the rendering changes if we do not increase the font size
   nor make it bold for content flowing into region 1. As more content can be
   fitted, more content is subject to the contextual selectors, resulting in
   more dark blue text into region 1.

  <div class=figure> <img alt="Illustrate how changing region styling affects
   the flow of content." src="images/region-styling.png" width=450>
   <p class=caption>Different rendering with a different region styling</p>
  </div>

  <h2 id=css-regions-model><span class=secno>2. </span>CSS Regions Model</h2>

  <h3 id=named-flows><span class=secno>2.1. </span>Named flows</h3>

  <p>In the <a href="http://www.w3.org/TR/CSS21/visuren.html">CSS formatting
   model</a>, elements can be in the <a href="#normal-flow-def"><em>normal
   flow</em></a> or out of the <a href="#normal-flow-def"><em>(normal)
   flow</em></a>. Boxes generated by elements in the normal flow are subject
   to their container box's normal layout scheme. Boxes generated by elements
   out of the normal flow are subject to a different layout scheme. For
   example, absolutely positioned elements are subject to absolute
   positioning into their <a
   href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">containing
   block</a>. This can be described by saying that the absolutely positioned
   elements are part of a special flow (called &lsquo;<a
   href="#positioned-flow-def"><code class=css>positioned
   flow</code></a>&rsquo;) which is subject to a special layout by its
   container box (i.e., its container box positions it into the containing
   block's box).

  <p>&nbsp;

  <p>In both cases, there is a notion of flow containing a sequence of
   elements and there is a notion of <a
   href="http://www.w3.org/Style/Group/css2-src/visuren.html#block-boxes">(block)
   container box</a> into which the elements flow.

  <p>&nbsp;

  <p>The CSS Regions module generalizes the concept of flow by adding the
   concept of a <em>named flow</em>. This module lets authors explicitly
   place elements into a <em>named flow</em>.

  <p>&nbsp;

  <p>With this model, all elements are moved to a flow as part of the visual
   formatting. That flow may be the normal flow, a named flow or a positioned
   flow, for example. A flow gets formatted visually when associated with one
   or several elements' container box(es). When an flow is associated with
   container boxes, the boxes generated by the flow's elements are laid out
   according to the container box's layout scheme and the flow is subject to
   the <a href="#flow-breaking-rules">&lsquo;<code class=css>flow breaking
   rules</code>&rsquo;</a> described below.

  <p>A container's layout scheme is the strategy used by a container to
   position the boxes generated by its children and itself. Examples are the
   normal layout (block and inline formatting) (<a href="#CSS21"
   rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>), table layout (<a
   href="#CSS21" rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>), the
   multi-column layout (<a href="#CSS3COL"
   rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>) or the grid layout (<a
   href="#CSS3-GRID-LAYOUT"
   rel=biblioentry>[CSS3-GRID-LAYOUT]<!--{{CSS3-GRID-LAYOUT}}--></a>).

  <h3 id=regions><span class=secno>2.2. </span>Regions</h3>

  <p class=index id=region title=region>A region is an element that generates
   a <a href="http://www.w3.org/TR/CSS21/visuren.html#block-boxes">block
   container box</a> and has an associated <em>named flow</em> (see the
   &lsquo;<a href="#content-from"><code
   class=property>content</code></a>&rsquo; property).

  <div class=issue> Could inline elements also be regions?</div>

  <h3 id=flow-breaking-rules><span class=secno>2.3. </span>Flow breaking
   rules</h3>

  <p>This section describes how container boxes consume content from a flow.
   The previous sections described how a flow may be referenced by one or
   more regions. The regions are the flow consumers and are ordered in a
   chain (see the &lsquo;<a href="#content-order"><code
   class=property>content-order</code></a>&rsquo; property). The current flow
   consumer is initially the first one in the chain.

  <p>&nbsp;

  <p><strong>Positioning recursion</strong> -- while there are remaining
   boxes in the flow, the current flow consumer is allowed to take more boxes
   from the flow and position them. If the consumer is unable to position
   additional boxes without overflowing, then one of two things happen:

  <ol>
   <li>If the flow consumer is the last in the chain, then it should proceed
    to the end of the flow and position all remaining generated boxes, even
    if they overflow.

   <li>Otherwise, the next flow consumer becomes the current one and a new
    iteration of the positioning recursion starts.
  </ol>

  <div class=issue> The above describes the simple case. It should explain
   how breaks influence this recursion. It should also discuss how boxes are
   split in general.</div>

  <div class=issue> Should the model provide a way to <a
   href="http://www.w3.org/TR/css3-multicol/#column-fill">balance</a>
   regions, as in the <a href="#CSS3COL"
   rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a> module?</div>

  <div class=issue> Is there a way to allow regions to be generated when
   content that does not fit into the last region remains. This would be
   analogous to the way the additional columns are generated when a
   multi-column box <a
   href="http://www.w3.org/TR/css3-multicol/#pagination-and-overflow-outside-multicol">
   overflows</a> (see <a href="#CSS3COL"
   rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>).</div>

  <h3 id=formatting-model-considerations><span class=secno>2.4. </span>The
   Visual Formatting Model and Flows</h3>

  <p>This section describes the flows to which elements formatted according
   to the CSS <a href="http://www.w3.org/TR/CSS21/visuren.html">Visual
   Formatting Model</a> and other layout modules (such as Multi-Column Layout
   <a href="#CSS3COL" rel=biblioentry>[CSS3COL]<!--{{CSS3COL}}--></a>) belong
   to.

  <p>In particular, this section describes the &lsquo;<a
   href="#generated-flow-def"><code class=css>generated
   flow</code></a>&rsquo; where generated content of pseudo-elements is
   placed and several <dfn id=auto-flow title="auto flows">&lsquo;<code
   class=css>auto flows</code>&rsquo;</dfn> where children of content
   elements are automatically placed.

  <h4 id=normal-flow><span class=secno>2.4.1. </span>Normal flow</h4>

  <p>In the CSS formatting model, elements are by default placed in the <dfn
   id=normal-flow-def>normal flow</dfn> of their container. Also by default,
   a container element gets its content from its <a
   href="#normal-flow-def"><em>normal flow</em></a>. This means that by
   default, a container element will visually format its children elements
   and will be the only container associated with its normal flow.

  <p>Note that floats and relatively positioned elements, in this model, are
   part of the same flow of content and flow into the same container but are
   positioned in different ways.

  <p>The <a href="#normal-flow-def"><em>normal flow</em></a> is one of the <a
   href="#auto-flow"><em>auto flows</em></a>.

  <h4 id=positioned-flow><span class=secno>2.4.2. </span>Positioned flow</h4>

  <p>An absolutely positioned element is placed into the <dfn
   id=positioned-flow-def>positioned flow</dfn> of its container. The
   container positions this element into its <em>containing block</em>.

  <p>If a container has children in the <a href="#normal-flow-def"><em>normal
   flow</em></a> and in the <a href="#positioned-flow-def"><em>positioned
   flow</em></a>, it applies different positioning schemes to each flow.

  <p>The <a href="#positioned-flow-def"><em>positioned flow</em></a> is one
   of the <a href="#auto-flow"><em>auto flows</em></a>.

  <h4 id=column-flow><span class=secno>2.4.3. </span>Column flow</h4>

  <p>Children of the multi-column element are placed in the element's <dfn
   id=column-flow-def>column flow</dfn>. The column boxes are then implicitly
   associated with the element's &lsquo;<a href="#column-flow-def"><code
   class=css>column flow</code></a>&rsquo;. In this model, multiple regions
   (column boxes) get their content from a single flow (the element's
   &lsquo;<a href="#column-flow-def"><code class=css>column
   flow</code></a>&rsquo;, which is the content of the element).

  <p>The <a href="#column-flow-def"><em>column flow</em></a> is one of the <a
   href="#auto-flow"><em>auto flows</em></a>.

  <div class=issue> Need to add a <dfn id=grid-cell-flow>grid cell flow</dfn>
   section.</div>

  <h4 id=generated-flow><span class=secno>2.4.4. </span>Generated flow</h4>

  <p>In the CSS formatting model, when the <code>::before</code> or
   <code>::after</code> pseudo-elements have their content property set to
   one of <code>&lt;string&gt;</code>, <code>&lt;uri&gt;</code>,
   <code>&lt;counter&gt;</code>, <code>attr(&lt;identifier&gt;)</code>,
   <code>open-quote</code>, <code>close-quote</code>,
   <code>no-open-quote</code> or <code>no-close-quote</code>, they create a
   <dfn id=generated-flow-def>generated flow</dfn> which they format
   visually.

  <h3 id=relation-to-document-events><span class=secno>2.5. </span>Relation
   to document events</h3>

  <p>The CSS Region module does not alter in any way the normal processing of
   events in the document tree. In particular, if an event occurs on an
   element that is part of a named flow, the <a
   href="http://www.w3.org/TR/DOM-Level-3-Events/#dom-event-architecture">event's
   bubble and capture phases</a> happen following the document tree order.

  <h2 id=regions-properties><span class=secno>3. </span>Properties and Rules</h2>

  <p>The main CSS Regions module properties are the ‘flow’ and &lsquo;<a
   href="#content-from"><code class=property>content</code></a>&rsquo;
   properties. The ‘flow’ property is places an element into a specific
   named flow or lets the <a href=auto-flows>automatic flow assignment</a>
   take place. The ‘content’ property binds a region with a flow. When
   multiple regions are bound to the same flow, the &lsquo;<a
   href="#content-order"><code class=property>content-order</code></a>&rsquo;
   property determines the order in which content flows into the sequence of
   regions. This sequences of regions is called a <dfn
   id=chain-of-regions>chain of regions</dfn>.

  <p>&nbsp;

  <p>The way in which the content is broken into segments that fit in a
   region can be controlled by the “break” properties. Finally, styling
   that is unique to the region can be specified by ‘region-style’ rules.

  <h3 id=the-flow-property><span class=secno>3.1. </span>The &lsquo;<a
   href="#flow2"><code class=property>flow</code></a>&rsquo; property</h3>

  <p>The ‘flow’ property places an element in
6D50
to a named flow or
   automatically places it in the appropriate flow (see the <a
   href="#formatting-model-considerations">Visual Formatting Model and
   Flows</a> section). Elements that belong to the same flow are laid out in
   the regions associated with that flow.

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

     <td><dfn id=flow2>flow</dfn>

    <tr>
     <th>Value:

     <td>&lt;name&gt;

    <tr>
     <th>Initial:

     <td>auto

    <tr>
     <th>Applies to:

     <td>any element

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

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

     <td>as specified
  </table>

  <dl>
   <dt>auto

   <dd>The element is automatically moved to the appropriate flow according
    to the <a href="#formatting_model_considerations">Visual Formatting Model
    and Flows</a> definitions.

   <dt>&lt;name&gt;

   <dd>The element is placed (appended) to the flow with that name. The
    element is said to have a <dfn id=specified-flow>specified flow</dfn>.
  </dl>

  <p>A named flow needs to be associated with one or more regions for its
   elements to be visually formatted. If no region is associated with a given
   named flow, the elements in the named flow are not rendered: they do not
   generate boxes and are not displayed.

  <p>The children of an element with a specified flow may themselves have a
   specified flow.

  <p>If an element has a the same specified flow value as one of its
   ancestors, it is moved out of its parent's normal flow and becomes a
   sibling of it's ancestor for the purpose of layout in the regions taking
   content from this flow.

  <p>The &lsquo;<a href="#flow2"><code class=property>flow</code></a>&rsquo;
   property does not affect the CSS cascade and inheritance for the elements
   on which it is specified. The flow property affects the visual formatting
   of elements associated to a named flow and of regions getting their
   content from a named flow.

  <p>The containing block for absolutely positioned descendants of an element
   with a specified flow is the region into which the element is rendered.

  <p>All the elements participating inside a named flow are rendered as
   children of an anonymous block that spans across all the regions
   associated with the specified named flow. The elements flowed inside the
   same named flow are taken in document order.

  <h3 id=the-content-property><span class=secno>3.2. </span>The &lsquo;<a
   href="#content-from"><code class=property>content</code></a>&rsquo;
   property</h3>

  <p>This specification extends the definition of the &lsquo;<a
   href="#content-from"><code class=property>content</code></a>&rsquo;
   property and makes it applicable to all block elements and
   pseudo-elements.

  <table class=propdef summary="content property definition">
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=content-from>content</dfn>

    <tr>
     <th>Value:

     <td>normal | none | from(&lt;name&gt;)| [ &lt;string&gt; | &lt;uri&gt; |
      &lt;counter&gt; | at
9E70
tr(&lt;identifier&gt;) | open-quote | close-quote |
      no-open-quote | no-close-quote ]+ | inherit

    <tr>
     <th>Initial:

     <td>normal

    <tr>
     <th>Applies to:

     <td>any block element

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

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

     <td>as specified
  </table>

  <dl>
   <dt><a id=x9 name=x9><span class=index-inst id=none
    title=none><strong>none</strong></span></a>

   <dd>For <code>::before</code> and <code>::after</code> pseudo-elements,
    the pseudo-element is not generated. For other elements, the element will
    not get any content for its visual formatting. If the children of the
    element are not directed to a flow referenced by another region, then
    they are not visually formatted. An element with its value set to
    &lsquo;<code class=property>none</code>&rsquo; is said to be <dfn
    id=disconnected>disconnected</dfn>

   <dt><strong>normal</strong>

   <dd>Computes to &lsquo;<code class=property>none</code>&rsquo; for the
    <code>::before</code> and <code>::after</code> pseudo-elements. For other
    elements, the element uses the flow assigned to its children following
    the <a href=auto-flows>automatic flow assignment for the Visual
    Formatting Model..</a>

   <dt><strong>&lt;string&gt;</strong>

   <dd>Text content (see the section on <a
    href="syndata.html#strings">strings</a>).

   <dt><strong>&lt;uri&gt;</strong>

   <dd>The value is a URI that designates an external resource (such as an
    image). If the user agent cannot display the resource it must either
    leave it out as if it were not specified or display some indication that
    the resource cannot be displayed.

   <dt><strong>&lt;counter&gt;</strong>

   <dd><a href="syndata.html#counter">Counters</a> may be specified with two
    different functions: &lsquo;<code class=css>counter()</code>&rsquo; or
    &lsquo;<code class=css>counters()</code>&rsquo;. The former has two
    forms: &lsquo;<code class=css>counter(<var>name</var>)</code>&rsquo; or
    &lsquo;<code class=css>counter(<var>name</var>,
    <var>style</var>)</code>&rsquo;. The generated text is the value of the
    innermost counter of the given name in scope at this pseudo-element; it
    is formatted in the indicated <a href="#counter-styles">style</a>
    (&lsquo;<code class=property>decimal</code>&rsquo; by default). The
    latter function also has two forms: &lsquo;<code
    class=css>counters(<var>name</var>, <var>string</var>)</code>&rsquo; or
    &lsquo;<code class=css>counters(<var>name</var>, <var>string</var>,
    <var>style</var>)</code>&rsquo;. The generated text is the value of all
    counters with the given name in scope at this pseudo-element, from
    outermost to innermost separated by the specified string. The counters
    are rendered in the indicated <a href="#counter-styles">style</a>
    (&lsquo;<code class=property>decimal</code>&rsquo; by default). See the
    section on <a href="#counters">automatic counters and numbering</a> for
    more information. The name must not be &lsquo;<code
    class=property>none</code>&rsquo;, &lsquo;<code
    class=property>inherit</code>&rsquo; or &lsquo;<code
    class=property>initial</code>&rsquo;. Such a name causes the declaration
    to be ignored.

   <dt><strong>open-quote</strong> and <strong>close-quote</strong>

   <dd>These values are replaced by the appropriate string from the <a
    class=noxref
    href="http://www.w3.org/TR/CSS21/generate.html#propdef-quotes"><span
    class=propinst-quotes><strong>&lsquo;<code
    class=property>quotes</code>&rsquo;</strong></span></a> property.

   <dt><strong>no-open-quote</strong> and <strong>no-close-quote</strong>

   <dd>Introduces no content, but increments (decrements) the level of
    nesting for quotes.

   <dt><strong>attr(X)</strong>

   <dd>This function returns as a string the value of attribute X for the
    subject of the selector. The string is not parsed by the CSS processor.
    If the subject of the selector does not have an attribute X, an empty
    string is returned. The case-sensitivity of attribute names depends on
    the document language.

   <dt><dfn id=from-name>from(&lt;name&gt;)</dfn>

   <dd>The element will visually format the available elements in the named
    flow with name &lt;name&gt;. If there is no flow with name &lt;name&gt;,
    then the element is disconnected and does not format any children
    visually. Multiple elements may be associated with the same named flow.
    See the &lsquo;<a href="#content-order"><code
    class=property>content-order</code></a>&rsquo; property discussion to
    understand how content segments are associated with different regions.
  </dl>

  <div class=issue>
   <p>Should we consider adding the ability to do the following:</p>

   <ul>
    <li>Support repeatable content. For example, have a repeatable heading at
     the start of each region where content flows.

    <li>Support flow breaking marks. It might be very useful to support a
     <em>mark</em> (such as <code>...</code>) at the region breaking points
     to indicate that content has been broken out and is continuing on a
     different region.
   </ul>
  </div>

  <div class=issue> Should the &lsquo;<a href="#content-from"><code
   class=property>content</code></a>&rsquo; property be limited to block
   elements or should it be allowed on all elements?</div>

  <h3 id=the-content-order-property><span class=secno>3.3. </span>The
   &lsquo;<a href="#content-order"><code
   class=property>content-order</code></a>&rsquo; property</h3>

  <p>Defines the ordering of the chain of regions into which content flows.
   If region A and region B are in the same chain of regions and the
   ‘content-order’ of region A is lower than that of region B, then
   region A will precede region B in that chain: content will flow into
   region A before it flows into region B.

  <table class=propdef summary="content-order property definition">
   <tbody>
    <tr>
     <th>Name:

     <td><dfn id=content-order>content-order</dfn>

    <tr>
     <th>Value:

     <td>&lt;float&gt;

    <tr>
     <th>Initial:

     <td>0

    <tr>
     <th>Applies to:

     <td>any block container

    <tr>
     <th>Inherited:

     <td>no

    <tr>
     <th>Percentages:

     <td>N/A

    <tr>
     <th>Media:

     <td>visual

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

     <td>as specified
  </table>

  <dl>
   <dt>&lt;float&gt;

   <dd>Specifies the region's priority in the chain it belongs to, i.e.., the
    chain of regions associated with the same content flow. Negative values
    are allowed.
  </dl>

  <p>If two or more regions with the same &lsquo;<code class=css>from(
   &lt;name&gt;)</code>&rsquo; value for their &lsquo;<a
   href="#content-from"><code class=property>content</code></a>&rsquo;
   property, they are first sorted according to their &lsquo;<a
   href="#content-order"><code class=property>content-order</code></a>&rsquo;
   value. If multiple regions in have the same &lsquo;<a
   href="#content-order"><code class=property>content-order</code></a>&rsquo;
   value, they are sorted according to the document order.

  <div class=issue> CSS Modules should define the document order for pseudo
   elements. For example, the [CSSGRID] specification should define the
   document order for the &lsquo;<code class=css>::grid-cell</code>&rsquo;
   pseudo elements (e.g., row-major sorting or column major sorting of
   cells).</div>

  <h3 id=region-flow-break-properties-break-befor><span class=secno>3.4.
   </span>Region flow break properties: &lsquo;<a href="#break-before"><code
   class=property>break-before</code></a>&rsquo;, &lsquo;<a
   href="#break-after"><code class=property>break-after</code></a>&rsquo;,
   &lsquo;<a href="#break-inside"><code
   class=property>break-inside</code></a>&rsquo;</h3>

  <p>When content is laid out in multiple regions, the user agent must
   determine where content breaks occur. The problem of breaking content into
   segments 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 named flow to be visually formatted in the following
   region, in the order defined by the &lsquo;<a href="#content-order"><code
   class=property>content-order</code></a>&rsquo; computed values. Note that
   there is no region break in the last region associated to a named flow.

  <p>The following extends the &lsquo;<a href="#break-before"><code
   class=property>break-before</code></a>&rsquo;, &lsquo;<a
   href="#break-after"><code class=property>break-after</code></a>&rsquo; and
   &lsquo;<a href="#break-inside"><code
   class=property>break-inside</code></a>&rsquo; 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>