2012年12月27日木曜日

CSS Regions Module Level 3 Editor's Draft 29 November 2012

http://dev.w3.org/csswg/css3-regions/

This version:
http://www.w3.org/csswg/css3-regions
Latest version:
http://www.w3.org/TR/css3-regions/
Previous version:
http://www.w3.org/TR/2011/WD-css3-regions-20111129/
Editors:
Vincent Hardy, Adobe Systems, Inc., 
Alex Mogilevsky, Microsoft, 
Alan Stearns, Adobe Systems, Inc., 
Issues List:
Bugzilla Bugs for CSS regions
Discussion:
www-style@w3.org with subject line "[css3-regions] message topic"

Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

Abstract

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.

Status of this document

Note This document uses an experimental style sheet. We welcome your feedback on the styles at site-comments@w3.org.

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.

The (archived) public mailing list www-style@w3.org (see instructions) 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] …summary of comment…"

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures 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 Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This draft is related to the drafts about Multi-column Layout [CSS3COL], Grid Layout [CSS3GRID], Flexible Box Layout [CSS3-FLEXBOX], and Template Layout [CSS3LAYOUT].

Table of contents

1. Introduction

This section is non-normative.

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.

Issue-15733

Should the region specification define a mechanism to create blocks that can be regions with CSS syntax?

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 named flows. Each named flow 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 named flow are taken out of the normal visual formatting and rendered in a chain of CSS Regions.

1.1. Named flows and region chains

Consider the layout illustrated in figure 1.

multiple chained regions which should receive content from a flow

Layout requiring sophisticated content flow

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.

Figure 2 shows an example of the intended visual rendering of the content.

Sample rendering showing a single thread of text flowing through a chain of regions

Sample rendering for desired layout

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.

The following example illustrates how the content of an <article> element becomes a named flow and how boxes marked with 'region1', 'region2', 'region3' and 'region4' IDs become CSS Regions that consume the 'article_flow' content.

<style>    article {      flow-into: article_flow;    }      #region1, #region2, #region3, #region4 {      flow-from: article_flow;    }  </style>

The 'article_flow' value on the 'flow-into' property directs the article element to the 'article_flow'named flow. Setting the 'flow-from' property on block containers to 'article_flow' makes them CSS Regions and associates the resulting region chain with the named flow: the flow is 'poured' into the region chain.

See Appendix A for sample code using grid layout that implements this example

Issue-16858

Should creation of regions from elements be disallowed?

Note 

CSS Regions are independent from layout

Any of the CSS layout facilities can be used to create, position and size boxes that can become CSS Regions. Using a grid layout [CSS3-GRID-LAYOUT] is just an example. The example could use a flexible box layout [CSS3-FLEXBOX] instead.

The CSS regions specification does not define a layout mechanism and is meant to integrate with existing and future CSS layout facilities.

CSS Regions do not have to be elements

The CSS regions module is independent of the layout of boxes and the mechanism used to create them. For simplicity, our example code in Appendix A uses elements to define the boxes.

While the example uses elements for CSS Regions (since [CSS3-GRID-LAYOUT] requires elements to create grid items) it is important to note that this is not required. CSS Regions can be pseudo-elements, such as '::before' and '::after'. 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 'flow-from' property. The CSS page template module (see [CSS3-PAGE-TEMPLATE]) proposes another mechanism to create stylable boxes that can become CSS Regions with the @slot syntax.

1.2. CSS Region names and styling

Issue-16859

Reconsider using @rule for region styling

Content can be styled depending on the CSS Region it flows into. It is an extension of pseudo-element styling such as ::first-line 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.

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

This design can be expressed as shown below.

<style>    @region #region1 {        p {            color: #0C3D5F;            font-weight: bold;        }  </style>

The @region #region1 rule limits its selectors to content flowing into #region1. The following figure shows how the rendering changes if we apply styling specific to #region1. Note how less text fits into this box now that the 'font-weight' is bold instead of normal.

Illustrate how changing region styling affects the flow of content.

Different rendering with a different region styling

2. CSS regions concepts

2.1. Regions

A CSS Region is a block container that has an associated named flow (see the 'flow-from' property).

2.2. Region chain

A region chain is the sequence of regions that are associated with a named flow. CSS Regions in a region chain receive content from the named flow following their order in the chain. CSS Regions are organized into a region chain according to the document order.

2.3. Named flows

A named flow is the ordered sequence of elements associated with a flow with a given identifier. Elements in a named flow are ordered according to the document order.

Elements are placed into a named flow with the 'flow-into' property. The elements in a named flow are laid out in the region chain that is associated with this named flow.

Content from a named flow is broken up between regions according to the regions flow breaking rules.

A named flow 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.

2.4. Regions flow breaking rules

Breaking a named flow 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 named flow content whose dynamic generation is not in the scope of this specification.

Each CSS Region in turn consumes content from its associated named flow. The named flow content is positioned in the current region until a natural or forced region break occurs, at which point the next region in the region chain becomes the current region. If there are no more regions 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 region in the chain.

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

3. Properties and rules

3.1. The 'flow-into' property

Issue-16527

[Shadow]: getFlowByName and shadow DOM

The 'flow-into' property can place an element into a named flow. Elements that belong to the same flow are laid out in the region chain associated with that flow.

Name: flow-into
Value: <ident> | none | inherit
Initial: none
Applies to: All elements, but not pseudo-elements such as ::first-line, ::first-letter, ::beforeor ::after.
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified

none
The element is not moved to a named flow and normal CSS processing takes place.
<ident>
The element is taken out of its parent's flow and placed into the flow with the name '<ident>'. The element is said to have a specified flow. The values 'none', 'inherit', 'default', 'auto' and 'initial' are invalid flow names.

A named flow 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 named flow, the elements in thenamed flow are not rendered: they do not generate boxes and are not displayed.

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

If an element has the same specified flow as one of its ancestors, it becomes the next sibling of its closest ancestor with the same specified flow.

The 'flow-into' property does not affect the CSS cascade and inheritance for the elements on which it is specified. The 'flow-into' property affects the visual formatting of elements placed into a named flowand of the region chain laying out content from a named flow.

The edges of the first CSS region in a region chain associated with a named flow establish the rectangle that is the containing block used for absolutely positioned elements in the named flow 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 named flow - they are still positioned relative to the viewport or page area even if they have been redirected to a named flow.

The first region defines the writing mode for the entire flow. The writing mode on subsequent regions is ignored.

Elements in a named flow 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.

Note 

The 'flow-into' property moves an element into the flow and the interplay with selectors should be considered carefully.

For example,

table {flow-into: table-content}

will move all tables in the 'table-content' named flow. However, the

table > * {flow-into: table-content} ...

selector will move all immediate children of all table elements into the 'table-content' named flow(which may be useful as it will usually result, if the immediate children are rows, in merging rows of multiple tables), but the

table * {flow-into: table-content}

selector will move all descendants of table elements into the 'table-content' named flow, 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 named flow. Having the descendants become siblings in the named flow results in a different processing (seeCSS 2.1's anonymous table objects). This note illustrates how authors must exercise caution when choosing a particular selector for setting the 'flow-into' property to avoid unintended results.

Note 

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:

span {flow-into: span-content}  <span>one</span>  <span>two</span>
Then the 'span-content' named flow contents will contain this:
<span>one</span><span>two</span>
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.
Note 

A future CSS Regions module level will define a mechanism for creating a named flow from an external resource.

3.2. The 'flow-from' property

The 'flow-from' property makes a block container a region and associates it with a named flow.

Name: flow-from
Value: <ident> | none | inherit
Initial: none
Applies to: Non-replaced block containers. 
This might be expanded in future versions of the specification to allow other types of containers to receive flow content.
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified

none
The block container is not a region.
<ident>
If the 'content' property computes to something else than 'normal' (or 'none' for a pseudo-element), the block container does not become a CSS Region. If the 'content' property computes to 'normal' (or 'none' for a pseudo-element), then the block container becomes a CSS Region and is ordered in aregion chain according to its document order. The content from the flow with the <ident> name will be broken into fragments and visually formatted in the principal boxes of the regions in the region chain. 
If there is no flow with name <ident>, then the block container does not format any content visually.

A CSS Region's document children are not visually formatted unless they are directed to a named flowwith an associated region chain.

Block container pseudo-elements where the value of 'flow-from' computes to an <ident> and the value of 'content' computes to 'none' are generated as CSS Regions, which is an update to the behavior described in [CSS21].

Note 

A block container becomes a CSS Region when its 'flow-from' property is set to a valid <ident>value, even if there is no content contributing to the referenced flow. For example:

<style>    .article{      flow-into: thread;    }    .region{      flow-from: thread;    }  </style>  <html>    <body>      <div class=region>div content</div>    </body>  </html>
There is no element matching the .article selector and therefore no content in the thread flow. However, the block container matching the .region selector is still associated with that emptynamed flow and, consequently, its children are not formatted visually.
Note 

At the time of this note-writing, the display values that always result in a non-replaced block container include block, inline-block, table-cell, table-caption, and list-item. All of these display values work as regions with non-replaced elements.

Issue-15827

Specify behavior of stacking contexts that are split between regions

CSS Regions create a new stacking context. CSS Regions establish a new block formatting Context. Exclusions (see [CSS3-EXCLUSIONS]) potentially impact the content laid out in region chains, just as for non-regions.

Note 

With region chains, 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 CSS Region creates a new stacking context, it is clear that each fragment is rendered separately and their rendering order follows the regular CSS rendering model.

See the regions visual formatting details section for a description of how 'width' and 'height' values are resolved for CSS Region boxes.

3.2.1. Cycle Detection

Named flows containing elements where the value of 'flow-from' computes to an <ident> can produce nonsensical circular relationships, such as a named flow containing CSS Regions in its own region chain. These relationships can be easily and reliably detected and resolved, however, by keeping track of a dependency graph and using common cycle-detection algorithms.

The dependency graph consists of edges such that:

If the graph contains a cycle, any elements where the value of 'flow-from' computes to an <ident>participating in the cycle do not become CSS Regions.

3.2.2. Nested fragmentation contexts

A CSS Region that contains a fragment of a named flow 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 CSS Region are determined using the standard fragmentation rules. 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 CSS Region, if that is desired.

3.3. Region flow break properties: 'break-before', 'break-after', 'break-inside'

Note 

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.

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.

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 region chain, if there is one.

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.

Name: break-before
Value: auto | always | avoid | left | right | page | column | region | avoid-page | avoid-column | avoid-region
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value
Name: break-after
Value: auto | always | avoid | left | right | page | column | region | avoid-page | avoid-column | avoid-region
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value
Name: break-inside
Value: auto | avoid | avoid-page | avoid-column | avoid-region
Initial: auto
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value

These properties describe page, column and region break behavior before/after/inside the generated box. These values are normatively defined in [CSS3COL]:

This specification adds the following new values:

region
Always force a region break before (after) the generated box.
avoid-region
Avoid a region break before (after, inside) the generated box.

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 [CSS21].

3.4. The region-fragment property

Name: region-fragment
Value: auto | break
Initial: auto
Applies to: CSS Regions
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value

The 'region-fragment' property controls the behavior of the last region associated with a named flow.

auto
Content flows as it would in a regular content box. If the content exceeds the container box, it is subject to the overflow property's computed value on the CSS Region. Region breaks must be ignored on the last region.
break

If the content fits within the CSS Region, then this property has no effect. If the content does not fit within the CSS Region, the content breaks as if flow was going to continue in a subsequent region. See the breaking rules section. A forced region break takes precedence over a natural break point.

Flow content that follows the last break in the last region is not rendered.

The 'region-fragment' property does not influence the size of the region it applies to.

The following code sample illustrates the usage of the 'region-fragment' property.

<style>  article {    flow-into: "article";  }    #region_1, #region_2 {    flow-from: article;    region-fragment: break; /* or auto */    overflow: visible; /* or hidden */  }    </style>    <article>...</article>    <div id="region_1"></div>  <div id="region_2"></div>
'flow-into: "article"' region_1 and region_2 'region-fragment: auto'
'overflow:visible'
regions receiving the flow content result if region-overflow is set to 'break' regions receiving the flow content
'region-fragment: break' 'region-fragment: auto'
'overflow:hidden'
result if region-overflow is set to 'break' regions receiving the flow content

Different values for the region-fragment property

Note 

The 'overflow' property is honored on a region: if region content overflows, such as the borders of elements on the last line, the 'overflow' property controls the visibility of the overflowing content. See the 'overflow' property definition ([CSS21]).

3.5. The @region rule

An '@region' rule contains style declarations specific to particular regions.

@region <selector> {      ... CSS styling rules ...  }

The '@region' rule consists of the keyword '@region' followed by a selector and a block of style rules. The '@region' rule and the selector constitute the region's 'flow fragment' 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 [DOM]) from the region's flow that flows in the selected region(s).

Issue-15713

Model for styling element fragments

Elements that are fully or partially in the 'flow fragment' 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.

Only a limited list of properties can be set in a region style rule:

  1. font properties
  2. color property
  3. opacity property
  4. background property
  5. 'word-spacing'
  6. 'letter-spacing'
  7. 'text-decoration'
  8. 'text-transform'
  9. 'line-height'
  10. alignment and justification properties
  11. border properties
  12. rounded corner properties
  13. border images properties
  14. margin properties
  15. padding properties
  16. 'text-shadow' property
  17. 'box-shadow' property
  18. 'box-decoration-break' property
  19. 'width' property
Issue-19965

Rework region styling example to use color changes

In the following example, the named flow 'article_flow' flows through 'region_1' and 'region_2'.

<style>      #div_1 {          flow-into: article_flow;      }            #region_1, #region_2 {          flow-from: article_flow;      }        /* region style "RSA" */      @region #region_1, #region_2 {          div {...}          p {...}      }            /* region style "RSB" */      @region #region_1 {          p {...}      }            </style>    <div id="div_1">      <p id="p_1">...</p>      <p id="p_2">...</p>  </div>    <div id="region_1"></div>  <div id="region_2"></div>
Illustration showing how a named flow content fits into regions to illustrate the @region styling.
  •  div div_1
  •  paragraph p_1
  •  paragraph p_2
  •  range of flow that fits into region_1
  •  range of flow that fits into region_2

The region style 'RSA' applies to flow content that is laid out in either 'region_1' or 'region_2'.

The first rule set 'div {...}' applies to all <div> elements that fit partially or fully into 'region_1' or 'region_2'. div_1 is split between 'region_1' and 'region_2' and gets the style from this style rule.

The second rule set 'p {...}' applies to all <p> elements that fit into 'region_1' or 'region_2'. In our example, both p_1 and p_2 are selected.

The region style 'RSB' applies to flow content that fits in 'region_1'.

The first rule set 'p {...}' matches p_1 and p_2 because these paragraphs flow into 'region_1'. Only the fragment of p_2 that flows into region_1 is styled with this rule.

Issue-15734

@region and specificity

The specificity of the selectors in a '@region' rule is calculated as defined in the CSS Selectors module (see [SELECT]). In other words, the '@region' 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 '@media' rules declaration blocks (see [MEDIAQ]), where the rule does not influence the selectors' specificity.

Consequently, selectors that match a given element (as describe above), participate in the CSS Cascading order as defined in [CSS21].

Region styling does not apply to nested regions. For example, if a region 'A' receives content from a flow that contains region 'B', the content that flows into 'B' does not receive the region styling specified for region 'A'.

4. Multi-column regions

A multi-column [CSS3COL] element can have an assigned named flow. 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 [CSS3COL]. In particular, when computing the flow fragment height of a multi-column element that is associated with a named flow, the 'column-fill' [CSS3COL] property is honored to balance the fragments of content that would flow through its columns.

The following example:

<style>         #multi-col {         column-count: 2;         flow-from: article;         height: 6em;         column-gap: 1em;      }          #remainder {         flow-from: article;         height: auto;     }  </style>    <article>...</article>  <div id="multicol"></div>  <div id="remainder"></div>

is equivalent in rendering to, for example:

<style>         #flex {         display: flex;         flex-pack: justify;         height: 6em;     }     #flex > div {         flow-from: article;         width: calc(50% - 0.5em);     }       #remainder {         flow-from: article;         height: auto;     }  </style>    <article>...</article>  <div id="flex">     <div />     <div />  </div>  <div id="remainder"></div>

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 region chain, then the multicol region must follow the overflow column rules [CSS3COL].

5. Pseudo-elements

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 'continued below' 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.

The '::before' and '::after' pseudo-elements (see [SELECT]) let content authors mark the beginning and end of a region with such markers.

5.1. Processing model

The '::before' content is laid out in the region prior to any other content coming from the flow. Note that it is possible to set the '::before' pseudo-element's 'display' property to 'run-in' to align it with the content's inline boxes.

The '::after' 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 '::after' content. Accommodating means that the '::after' content is laid out without overflowing the region. The 'display' property of the '::after' content can be set to 'run-in' to align with the region's content's inline boxes. In that case, the '::after' content aligns with the last inline box of the previous element in the flow that has some visual rendering in the region and can accommodate for the '::after' box.

If there is not enough room to accommodate the ::before content, the '::after' content after removing all flow fragment content, or a combination of the two, then the ::before and/or ::after content overflows that region.

6. CSSOM

Issue-15679

elementFromPoint and CSS regions

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.

6.1. The NamedFlow interface

The following APIs allow scripts to reference a NamedFlow object representation of a named flow.

An additional method on the Document interface provide access to named flows.

partial interface Document {    NamedFlowCollection getNamedFlows();  };

The getNamedFlows() method on the Document interface returns a static snapshot of all the currentnamed flows in the document.

The namedFlows collection must include all named flows that are currently in the CREATED state. The list must not include named flows that are in the NULL state.

The NamedFlowCollection interface provides a list of current NamedFlow instances in the document. The collection is a snapshot of the data.

interface NamedFlowCollection {    readonly attribute unsigned long length;    [IndexGetter] NamedFlow? item (unsigned long index);    [NameGetter] NamedFlow? namedItem (DOMString name);  };

The length attribute returns the number of items in the collection.

The item(index) method returns the NamedFlow instance at index index in the collection or undefined ifindex is out of range. An object collection implementing NamedFlowCollection supports indices in the range 0 ≤ index < collection.length.

The namedItem(name) method returns the NamedFlow instance in the collection whose name attribute matches the supplied name, or undefined if there is no match.

The NamedFlow interface offers a representation of a named flow instance.

The NamedFlow interface can be used for different purposes. For example, the getRegionsByContent()method can help navigate by bookmark: a script can find the regions that display a particular anchor and bring them to view.

Likewise, the interface allows authors to check if all content has been fitted into existing regions. If it has, the overset attribute would be false.

interface NamedFlow : EventTarget {    readonly attribute DOMString name;    readonly attribute boolean overset;    sequence<Region> getRegions();    readonly attribute integer firstEmptyRegionIndex;    NodeList getContent();    sequence<Region> getRegionsByContent(Node node);  };

The name attribute returns the name of the NamedFlow instance.

The overset attribute returns true if there are named flow fragments that do not fit in the associated region chain (including the case where there are named flow fragments but no regions in the region chain). Otherwise, it returns false.

The getRegions() method returns the sequence of regions in the region chain associated with thenamed flow. Note that the returned values is a static sequence in document order.

The firstEmptyRegionIndex is the index of the first region in the region chain with the regionOversetattribute set to empty. If all regions have the regionOverset attribute set to fit or overset, the value forfirstEmptyRegionIndex is -1. If there are no regions in the region chain, the value is -1 as well.

The getContent() method returns an ordered collection of nodes that constitute the named flow. The returned list is a static snapshot of the named flow content at the time the method is invoked. This list contains the elements that were moved to the named flow by their flow-into property but not their descendants (unless the descendants are themselves moved to the named flow).

The getRegionsByContent() method returns the sequence of regions that contain at least part of the target content node if it belongs to the named flow directly or one of its ancestors belongs to the named flow. Otherwise, the method returns an empty sequence. The returned value is a static sequence in document order.

The named flow states are :

  • NULL: the named flow contains no elements and has no region chain.
  • CREATED: the named flow either contains an element or has a region chain.

A NamedFlow object is live: it always represents the named flow with the corresponding name even if thatnamed flow has transitioned to the NULL state.

6.2. The Region interface

The Region interface is a supplemental interface which must be implemented by all objects ( Elements, pseudo-elements or other CSS constructs such as slots) in an implementation which can be CSS Regions.

[NoInterfaceObject]  interface Region {    readonly attribute DOMString regionOverset;    sequence<Range>? getRegionFlowRanges();    CSSStyleDeclaration getComputedRegionStyle(Element elt);    CSSStyleDeclaration getComputedRegionStyle(Element elt, DOMString pseudoElt);  };    Element implements Region;

The regionOverset attribute returns one of the following values:

'overset'
The region is the last one in the region chain and not able to fit the remaining content from thenamed flow. Note that the region's overflow property value can be used to control the visibility of the overflowing content and the 'region-fragment' property controls whether or not fragmentation happens on the content that overflows the last region.
'fit'
The region's flow fragment content fits into the region's content box. If the region is the last one in the region chain, it means that the content fits without overflowing. If the region is not the last one in the region chain, that means the named flow content may be further fitted in subsequent regions. In this last case, note that the named flow fragment may be empty (for example if the region is too small to accommodate any content). This value is returned if the region object is not (or no longer) a region.
'empty'
All content from the named flow was fitted in prior regions.

If there is no content in the named flow, all regions associated with that named flow should have theirregionOverset attribute return 'empty'. If there is content in the flow but that content does not generate any box for visual formatting, the 'overset' attribute on the first region in the region chain associated with the flow will return 'fit'.

The getRegionFlowRanges method returns an array of Range instances corresponding to fragment from the named flow 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 Range where the startContainer andstartOffset have the same values as the endContainer and endOffset and therefore the collapsed attribute on the Range is true. In that situation, if the region is the first in the region chain, thestartContainer is the first Node in the named flow and the startOffset is zero. If the region is the last region in the region chain (but not the first and only one), the startContainer and startOffset are the same values as the endContainer and endOffset on the previous region in the region chain. The method returns null if the region object is not (or no longer) a region.

The getComputedRegionStyle methods on the Region interface work the same as the getComputedStyle[CSSOM] methods on the Window interface [HTML5] with the following exceptions. For the Regioninterface the CSSStyleDeclaration returned must include the result of region styling. If the element is fragmented across region boundaries, the CSSStyleDeclaration returned must apply only to the fragment that flows through the CSS Region. The method returns null if the region object is not (or no longer) a region.

If the element is not contained in the CSS Region at all, the method returns the region styling that would apply to the element if it were contained in the CSS Region (following how getComputedStyleworks with elements not contained in the window's document).

A Region instance may represent an object that is no longer a region. This may happen for example if the 'flow-from' property on the corresponding pseudo-element, element or other construct becomes 'none' but a script is still holding a reference to the Region object.

6.3. The CSSRegionStyleRule interface

The CSSRegionStyleRule interface represents an '@region' rule in a CSS style sheet. This rule type is added to the CSSRule [CSSOM] interface.

partial interface CSSRule {    const unsigned short REGION_STYLE_RULE = 16;  };
REGION_STYLE_RULE
The rule is a CSSRegionStyleRule
interface CSSRegionStyleRule : CSSRule {    attribute DOMString selectorText;    readonly attribute CSSRuleList cssRules;    unsigned long insertRule(DOMString rule,  unsigned long index)      raises(DOMException);    void deleteRule(unsigned long index)      raises(DOMException);  };

The selectorText attribute gets and sets the associated selector as defined in section 6.4.3 of [CSSOM].

The cssRules attribute must return a CSSRuleList [CSSOM] object for the list of CSSStyleRules specified in the region style block.

The insertRule(rule, index) method inserts a CSSStyleRule into the region style block before the specified index. If the index is equal to the length of the CSSRuleList the CSSStyleRule is inserted at the end of the list.

Possible exceptions:

The deleteRule(index) method deletes the CSSStyleRule in the CSSRuleList at the specified index.

Possible exception:

  • INDEX_SIZE_ERR: Raised if the specified index does not correspond to a rule in the region style block.

6.4. Region flow layout events

NamedFlow objects are EventTargets which dispatch regionlayoutupdate events when there is a possible layout change in their region chain. The event is asynchronous, and fires in the same step of the event loop [HTML5] as when MutationObservers [DOM] are invoked.

If region chain nesting occurs (the contents of a NamedFlow contains regions for a different NamedFlow) then the regionlayoutupdate event for the nested region(s) must be dispatched before theregionlayoutupdate event for the containing flow is dispatched.

Type regionlayoutupdate
Interface UIEvent (see [DOM-LEVEL-3-EVENTS])
Sync / Async Async
Bubbles No
Target NamedFlow
Cancelable Yes
Default action none
Context info
  • Event.target: NamedFlow whose layout may have changed

6.5. Clarifications on pre-existing APIs

6.5.1. getClientRects() and getBoundingClientRects()

The CSSOM View Module defines how user agents compute the bounding client rectangle for an element (getBoundingClientRect()) and its generated boxes (getClientRects()).

This definition applies to the (possibly) multiple boxes generated for an element in a named flow flowing through a region chain. The getClientRects() method returns the list of boxes generated for each of the element fragments laid out in different regions. The getBoundingClientRect() method operates as specified in the CSSOM View Module as well and is computed from the set of rectangles returned by getClientRects().

6.5.2. offsetTop, offsetLeft, offsetWidth and offsetWidth

The computation of the offset attributes for elements laid out in a named flow follow the specification[CSSOM]. For the purpose of the algorithm, the first CSS layout box associated with an element laid out in a named flow is the box generated for the first region the element is laid out into.

7. Regions visual formatting details

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 ([CSS3-FLEXBOX]). However, regions lay out a fragment of theirnamed flow instead of laying out descendant content as happens with other boxes.

This section describes the model for laying out regions and for laying out named flow content into regions. The descriptions in this section are biased towards a horizontal writing mode, using width for logical width (or measure) and height for logical height (or extent) as defined in the CSS Writing Modes Module [CSS3-WRITING-MODES]). To use this model in a vertical writing mode apply the principlesdescribed in that specification.

7.1. The Region Flow Content Box (RFCB)

A region box lays out the following boxes:

  • The boxes generated by its ::before and ::after pseudo-elements, if any.
  • The anonymous region flow content box (called RFCB in this document) which contains the fragment of the named flow that the region receives.
The ::before, RFCB and ::after boxes contained in the Region Box

The Region Flow Content Box (RFCB)

Laying out a region box follows the same processing rules as for any other block container box.

The RFCB is a block container box with a computed 'width' of 'auto' and a whose used 'height' is resolved as detailed below.

7.1.1. RFCB 'width' resolution

At various points in the visual formatting of documents containing regions, the used 'width' of RFCBs and regions need to be resolved. In all cases, the resolution is done following the rules for calculating widths and margins (see [CSS21]). Sometimes, resolving the used 'width' value requires measuring the content's min-content and max-content values (as defined in the CSS Writing Modes Module [CSS3-WRITING-MODES]). For an RFCB, these measures are made on the entire associated named flowcontent.

As a consequence, all RFCBs of regions associated with a given named flow share the same min-contentand max-content measures.

This approach is consistent with the box model for breaking ([CSS3-BREAK]).

7.2. Regions visual formatting steps

Formatting documents that contain named flows laid out in regions is a three-step process:

  • Step 1: RFCB flow fragment height resolution. In this step, the heights of fragments fitting in the regions' RFCBs are resolved.
  • Step 2: document and regions layout. In this step, the document content and regions are laid out. However, named flow content is not laid out in regions yet.
  • Step 3: named flow layout. In this step, the content of named flows is laid out in their respectiveregion chains.
visual representation of the three-step process

Regions visual formatting steps

7.2.1. Step 1: RFCB flow fragment height resolution

Conceptually, resolving the flow fragment height is a two phase process.

7.2.1.1. RFCB flow fragment height resolution, Phase 1

The document is laid out with a used height of zero for all RFCBs. In this phase, the content of named flows is not laid out in regions. This phase yields resolved position and sizes for all regions and their RFCBs in the document.

7.2.1.2. RFCB flow fragment height resolution, Phase 2

Named flows are laid out in regions. The user agent resolves the flow fragment height for the RFCBs using the remainder of the flow and accounting for fragmentation rules. This process accounts for constraints such as the 'height' or 'max-height' values, as described in the CSS 2.1 section on calculating heights and margins (see the Block-level non-replaced elements in normal flow when 'overflow' computes to 'visible' section and the complicated cases section). During this phase, generated content is laid out according to the rules described earlier in this document.

7.2.2. Step 2: region boxes layout

In this step, the document is laid out according to the normal CSS layout rules.

If a measure of the content is required to resolve the used 'width' of the region box, the value is resolved as described in the RFCB width resolution section.

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 flow fragment height resolved in Step 1 is used for the vertical content measure of the RFCB.

At the end of this step, regions are laid out and ready to receive content from their associated named flows.

7.2.3. Step 3: named flows layout

In this final step, the content of named flows is laid out in the regions' RFCBs along with the generated content boxes.

The used 'width' for RFCBs is resolved as described before.

The used 'height' 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 fragmentation rules and generated content boxes.

During this phase, generated content is laid out according to the rules described earlier in this document.

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.

7.3. Regions visual formatting: implementation note

The process for resolving an RFCB's 'height' and the three-step process used to lay out documents containing regions and named flows are conceptual descriptions of what the layout should yield and implementations should optimize to reduce the number of steps and phases necessary wherever possible.

7.4. Regions visual formatting example

This section is non-normative.

This example considers a document where content flows between three regions, and region boxes are intermixed with the normal document content.

<style>  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;  }    </style>         <body>      <article>          <p style="break-after:region;">I am not a ... </p>          <p>...</p>      </article>      <div id="rA"></div>      <div id="rB"></div>      <div id="rC"></div>        <div id="main-flow">          <p>Lorem ipsum dolor ...</p>      </div>  </body>

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 RFCBs for regions rA, rB and rC respectively.

7.4.1. Step 1 - Phase 1: Laying out RFCBs with used height of zero

Applying the rules for Step 1, Phase 1, the computed 'auto' 'width' values for the RFCBs are resolved to used values according to the normal CSS layout rules meaning they stretch to the width of their containing block's content box.

  1. RFCB-A: stretches to fit the rA content box.

    Since rA also has an 'auto' 'width', its own used 'width' is stretched to fit the <body> content box.

  2. RFCB-B: stretches to fit the rB content box.
  3. RFCB-C: stretches to fit the rC content box.

Also applying the rules for Step 1, Phase 1, the used values for the RFCBs 'height' properties are all zero.

Conceptually, this produces the layout illustrated below.

Step 1 - Phase 1: Layout RFCBs with used heights of 0

Step 1 - Phase 1: Layout RFCBs with used heights of 0

7.4.2. Step 1 - Phase 2: Layout flow to compute the RFCBs' flow fragments heights

In this second phase of Step 1, the named flow is laid out in regions and the height of each fragment falling in each RFCB is computed.

The user agent lays out as much of the flow into an area with RFCB-A's used 'width'. rA's 'height' computes to 'auto' and there is no vertical maximum height for RFCA's 'height'. However, because there is a break after the first paragraph in the 'article' named flow, 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 'width'.

At this point, the user agent lays out as much of the remaining flow as possible in RFCB-B. Because rB's 'max-height' computed value is '150px', the user agent only lays out the 'article' named flow using RFCB-B's used 'width' 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 (150px).

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 'width'. This results in a resolved flow fragment height: FH-C.

Step 1 - Phase 2: Measure flow fragments heights

Step 1 - Phase 2: Measure flow fragments heights

7.4.3. Step 2: Layout document and regions without named flows

The used 'width' of RFCB-A, RFCB-B and RFCB-C are resolved as in the previous step. However, the 'height' is resolved differently.

Resolving the 'height' of rA requires a content measure which is FH-A (the flow fragment height for RFCB-A).

The 'height' of rB results from first computing its content measure and then applying the rules for 'max-height'. Here, the vertical content measure evaluates to FH-B. After applying the rules for 'max-height' and accounting for margins, padding and borders, the used 'height' of rB is resolved to LH-B (150px).

The 'height' of rC's box results from calculating its content measure: FH-C becomes rC's used 'height'.

Step 2: Layout document and regions without named flows

Step 2: Layout document and regions without named flows

7.4.4. Step 3: named flows layout

In this final step, the article named flow is laid out in its region chain. The used 'width' for each of the RFCB is resolved as in step 1 above.

The used 'height' for the RFCB is a result of laying out the as much of the content in the region without overflowing its content box and following the fragmentation rules.

Because the computed 'width' of the RFCB has not changed and the fragmentation rules applied are the same as in Phase 1, Step 2, the used 'height' for RFCB-A, RFCB-B and RFCB-C are LH-A, LH-B and LH-C, respectively.

There may be situations where the used 'height' of RFCBs resolved in Step 3 are different from the flow fragment height computed in Step 1 Phase 2.

Step 3: Final result after laying out named flows in regions

Step 3: Final result after laying out named flows in regions

8. Relation to document events

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 named flow, the event's bubble and capture phases happen following the document tree order.

9. Relation to other specifications

This specification is related to other specifications as described in the references section. In addition, it is related to the following specifications:

  1. CSS Fragmentation Module Level 3 [CSS3-BREAK]. 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.
  2. CSS Pagination Templates Module Level 3 [CSS3-PAGE-TEMPLATE]. This module defines a syntax to define layout templates which can be used when paginating content. The page templates use regions.
  3. CSS Exclusions Module [CSS3-EXCLUSIONS]. 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.
  4. CSS Line Grid Module [CSS3-LINE-GRID]. 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.

10. Use Cases

Use cases are described on these pages.

11. Conformance

12. Changes

12.1. Changes from August 28th 2012 version

  • Changed Appendix A to use custom element layout.
  • Noted change in pseudo-element generation with flow-from.
  • Changed case of regionlayoutupdate to match other events in [DOM-LEVEL-3-EVENTS].
  • Added section on fragmenting the fragmenters.
  • Added section on handling circular flow-from and flow-into situations.
  • Added alignment and justification to region styling properties.
  • Added timing for regionLayoutUpdate event.
  • Clarified interaction between content and flow-from for pseudo-elements.
  • Changed NamedFlowCollection getters to return undefined when there is no NamedFlow.
  • Changed region-overflow property to region-fragment.

12.2. Changes from May 3rd 2012 version

  • Removed exceptions from the Region interface.
  • Changed NamedFlowCollection from live to a static snapshot.
  • Changed NamedFlow to inherit from EventTarget.
  • Removed flowFrom from Region interface and changed method name to getComputedRegionStyle().
  • Region interface is now a supplemental interface with the [NoInterfaceObject] extended attribute.
  • Added note for regionLayoutUpdate dispatching in nested flows.
  • Removed Document.getFlowByName() in favor of NamedFlowCollection.namedItem().
  • Changed to overset:false for NULL NamedFlow.
  • Changed regionLayoutUpdate to not bubble.

12.3. Changes from November 29th 2011 version

  • 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).
  • Removed divs with class set to "issue moved", "issue stale" and "issue resolved" since these divs where not displayed.
  • Minor updates to the alternate stylesheet.
  • Removed "This section is normative" paragraphs since in CSS specifications, sections are normative unless otherwise specified.
  • Removed "This section is informative" paragraphs since in CSS specifications notes are always informative.
  • Reworded the text about flow-into: <ident> and removed obsolete text about the interaction with the 'content' property.
  • Removed "this section is non-normative" from the "Regions Concepts" section.
  • 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 'overflow' property applies to content flown in regions from the break section, since this is covered in the section on 'region-overflow' already. See mailing list feedback.
  • Clarified that @region style rules only apply to the 'portion' of an element that falls into the corresponding region and added an issue that the model for 'partial' styling needs to be defined. See mailing list feedback.
  • Clarified that the NodeList returned by getRegionsByContentNode is live.
  • Added a name property to the NamedFlow interface. Added a NamedFlowCollection interface and added a getNamedFlows method on the Document interface, as per Bug 15828.
  • Modified wording about containing block resolution for absolutely positioned elements in a named flow.
  • Modified initial examples as per Bug 15131
  • Multiple editorial changes following mailing list review comments
  • Fixed DOM references to now point to the DOM TR
  • Fixed Web IDL issues as reported in Issue 15931
  • Added text to explain support for multi-column elements as required by Issue 15191 and Action 375.
  • Renamed 'regionOverflow' to 'regionOverset' to avoid confusion between fitting a flow in regions and the concept of visual overflow that the word 'overflow' (and the property) carry.
  • Reworked the partial document interface following the Issue 14948 on getFlowByName.
  • Updated the object model section as proposed on the wiki and in particular:
    • introduced a Region interface to replace the supplemental Element interface
    • Added a 'flowFrom' attribute on the Region interface
    • NamedFlow.getRegions() was added
    • Renamed getContentNode to getContent and getRegionsByContentNode to getRegionsByContent as per Isseu 15879
    • NamedFlow now returns static lists (see Issue 16286)
  • Modified region layout event to be dispatched on NamedFlow instead of region as before. Was requested by Issue 15938 and required in the general effort to have the DOM APIs work with non-element regions.
  • Changed paragraph on pseudo-elements to disallow 'flow-into' on all pseudo-elements because moving a '::before' element (for a example) to a named flow does not seem useful and causes specification and implementation complexity.
  • Added section about getClientRects(), getBoundingClientRect(), offsetWidth, offsetHeight, offsetTop and offsetLeft.
  • Added 'Regions visual formatting details' section to better describe the model for resolving auto sizing on regions.
  • Reworked the initial specification example.
  • Changed break and region-overflow properties to apply to visual media instead of paged.
  • Added opacity to region styling.
  • Added possibility of ::before content contributing to overflow.
  • Added CSSRegionStyleRule

12.4. Changes from June 09th 2011 version

  • Editorial changes (typos, rephrasings).
  • Made 'content-order' a <integer> and not a <float> following a working group resolution
  • Added Alex Mogilevsky as an editor
  • Flow names became <ident> instead of <string> following a working group resolution
  • Removed issue about possibly altering the DOM Events model for region events following a working group resolution
  • Made the "relation to document events" section informative following a working groupresolution
  • Removed issue in section "The NamedFlow interface" following the working group's resolutionto have both NamedFlow and the Element interface extension
  • Following a working group resolution:
    • 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.
    • Added NamedFlowUpdate
  • Excluded 'none', 'inherit' and 'initial' from the possible identifier names on the flow property following discussion on the mailing list.
  • Simplified integration discussion on multi-column layout and just state that since column boxes are not addressable by selectors, they cannot be regions.
  • Added specification of how the 'flow-into' property interacts with object, embed and iframe elements.
  • Excluded 'default' from the possible identifier names on the flow property because it may get reserved.
  • Added to the definition of 'auto' on 'region-overflow' specifying that region breaks must be ignored.
  • Renamed 'Document.flowWithName' to 'Document.getFlowByName' since it is not a property.
  • Added a note that a 'NamedFlow' instance is live.
  • Added an 'undefined' string value to the regionOverflow property on the Element interface extension.
  • Renamed NamedFlowEvent to regionLayoutUpdate to avoid having 'Event' in the event name.
  • Added description for special behavior of iframe/object/embed as flow source
  • Removed issue on copying content to a named flow instead of moving elements to named flow following working group resolution.
  • Removed issue on content:flow-from v.s., flow-from property following working groupresolution.
  • Renamed 'flow' to 'flow-into' following working group resolution.
  • Updated the css3-grid-align example following working group resolution that it should use <div> instead of grid-cell pseudo elements that were removed from the CSS Grid Layout specification.
  • Renamed 'from-flow' to 'flow-from' following a working group resolution.
  • Limited the applicability of 'content: flow-from()' to block container box and added a note that this might be expanded in the future, following a working group resolution.
  • Removed issue about API to find which region displays an element in a named flow sinceACTION-350 was created to add this API.
  • In the 'flow' property description, removed the required wrapper anonymous block as agreed on mailing list discussion.
  • Reworded the paragraph on how regions create a new stacking context, as per the mailing list discussion.
  • Reworked the "CSS regions Model" section to now be "CSS regions Layout". Moved the definition of a region as the first sub-section.
  • Removed the "Visual Formatting Model and Flows" section to match the new Regions Model (now CSS regions Layout) section.
  • 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.
  • Added note about why regions create a new stacking context following the discussion on themailing list.
  • Removed sentence about content:none making elements disconnected following the discussion on the mailing list.
  • Removed sentence about content:none making elements disconnected following the discussion on the mailing list.
  • Added the ::region-before and a ::region-after pseudo-elements.
  • Added note of caution when using selectors and the 'flow-into' property following a mailing list discussion.
  • 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.
  • 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.
  • Added a note that regions establish a new block formatting context.
  • Renamed content-order to region-order.
  • Added a note about overflowing content in regions (e.g., for content with borders).
  • 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.
  • Replaced 'content:flow-from(<ident>)' with 'flow-from: <ident>' following a working group resolution.
  • Added more specific wording about auto width and auto height, following ACTION 351.
  • Reworked section on region markers to now use '::before' and '::after' and explain how 'display:run-in' works with regions.
  • Modified the @region style rule to remove the ::region-lines pseudo-selector.
  • Removed the 'region-order' property following implementation feedback.
  • Specified that region-overflow does not influence a region's size.
  • Specified that the flow's writing mode is defined by the first region's writing mode followingmailing list discussion.
  • Made iframe, object and embed support of flow-into optional following mailing list discussion.
  • Clarified that flow content following the last break in the last region is not rendered, followingmailing list discussions.
  • Modified the rule for computing the width and height of a region when they are set to auto, following a mailing list discussion.
  • Added 'auto' to the list of invalid flow identifiers.
  • Made 'none' the initial value for 'flow-into' and aligned with 'flow-from', as explained in thisemail. Also allowed the 'inherit' value on 'flow-from' and 'flow-into' (same email).
  • Added note about nested region styling following a mailing list discussion.
  • Added additional DOM interface following Action 350.
  • Clarified that a region becomes a region only if its 'content' property computes to normal, following the resolution of Issue 22.
  • Removed text about special iframe behavior as a result of ACTION 376.
  • Made the selectors explicit in the initial section code examples, following discussion in San Jose, October 2011 face to face meeting.
  • Added section on use cases following ACTION-377.

Acknowledgments

The editors are grateful to the CSS working group for their feedback and help with the genesis of this specification.

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:

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, Kang-Hao (Kenny) Lu, Mihai Maerean, Markus Mielke, Robert O'Callahan, Edward O'Connor, Mihnea Ovidenie, Virgil Palanciuc, Olga Popiv, Christoph Päper, Anton Prowse, Peter Sorotokin, Elliott Sprehn, Christian Stockwell, Eugene Veselov, Boris Zbarsky, Stephen Zilles and the CSS Working Group members.

Appendix A. Example Code for Introduction

The following is one possible way to code the example from the introduction. This code uses grid cells in a custom element to define, size and position the region areas but table layout, absolute positioning, or any other CSS layout facility could be used.

<head>      <title>Regions Intro Example</title>      <link rel="components" href="x-fancy-layout.html">  </head>    <style>    #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;    }        /*     * Creates the named flow      */    article {      flow-into: article_flow;    }      /*     * Associate it with the intended CSS Regions.      * This creates a region chain for the named flow.     */    #region1, #region2, #region3, #region4 {      flow-from: article_flow;    }  </style>    <!--        The body element is extended with the        layout template from the link above.  -->  <body is="x-fancy-layout">    <!--        The article element is the content to flow        through the region chain. In the content        markup, it's followed by the image the        region chain will flow around.  -->    <article>      <h1>Introduction</h1>      <p>This is an example ...</p>              <h2>More Details</h2>      <p>This illustrates ...</p>      <p>Then, the example ...</p>      <p>Finally, this ...</p>    </article>    <div id="boxA"></div>  </body>    <!--       In a separate file, we lay out the regions with a grid        inside a custom element template from Web Components.  -->  <element name="x-fancy-layout" extends="body">    <template>      <div id="grid">        <div id="region1"></div>        <div id="region2"></div>        <content></content>        <div id="region3"></div>      </div>      <div id="region4"></div>    </template>  </element>
Note 

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.

References

Normative references

[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607
[CSS3-BREAK]
Rossen Atanassov; Elika J. Etemad. CSS Fragmentation Module Level 3. 23 August 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-break-20120823/
[CSS3-WRITING-MODES]
Elika J. Etemad; Koji Ishii. CSS Writing Modes Module Level 3. 15 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-writing-modes-20121115/
[CSS3COL]
Håkon Wium Lie. CSS Multi-column Layout Module. 12 April 2011. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2011/CR-css3-multicol-20110412
[CSSOM]
Anne van Kesteren. CSSOM. 12 July 2011. W3C Working Draft. (Work in progress.) URL:http://www.w3.org/TR/2011/WD-cssom-20110712/
[DOM]
Anne van Kesteren; Aryeh Gregor; Ms2ger. DOM4. 5 April 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-dom-20120405/
[DOM-LEVEL-3-EVENTS]
Travis Leithead; et al. Document Object Model (DOM) Level 3 Events Specification. 6 September 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-DOM-Level-3-Events-20120906/
[HTML5]
Ian Hickson. HTML5. 25 May 2011. W3C Working Draft. (Work in progress.) URL:http://www.w3.org/TR/2011/WD-html5-20110525/
[SELECT]
Tantek Çelik; et al. Selectors Level 3. 29 September 2011. W3C Recommendation. URL:http://www.w3.org/TR/2011/REC-css3-selectors-20110929/

Other references

[CSS3-EXCLUSIONS]
Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Exclusions and Shapes Module Level 3. 3 May 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-exclusions-20120503/
[CSS3-FLEXBOX]
Tab Atkins Jr.; Elika J. Etemad; Alex Mogilevsky. CSS Flexible Box Layout Module. 18 September 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/
[CSS3-GRID-LAYOUT]
Alex Mogilevsky; et al. CSS Grid Layout. 6 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-grid-layout-20121106/
[CSS3-LINE-GRID]
Koji Ishii. CSS Line Grid Module. Proposal for a CSS module. (Retrieved 26 October 2011) URL:http://dev.w3.org/csswg/css-line-grid/
[CSS3-PAGE-TEMPLATE]
Alan Stearns. CSS Pagination Templates Module Level 3. Proposal for a CSS module. (Retrieved 4 April 2012) URL: http://dev.w3.org/csswg/css3-page-template/
[CSS3GRID]
Alex Mogilevsky; Markus Mielke. CSS Grid Positioning Module Level 3. 5 September 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-grid-20070905
[CSS3LAYOUT]
Bert Bos; César Acebal. CSS Template Layout Module. 29 November 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-layout-20111129/
[CSS3PAGE]
Håkon Wium Lie; Melinda Grant. CSS3 Module: Paged Media. 10 October 2006. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2006/WD-css3-page-20061010
[MEDIAQ]
Florian Rivoal. Media Queries. 19 June 2012. W3C Recommendation. URL:http://www.w3.org/TR/2012/REC-css3-mediaqueries-20120619/

Index

Property index

Property Values Initial Applies to Inh. Percentages Media
break-after auto | always | avoid | left | right | page | column | region | avoid-page | avoid-column | avoid-region auto block-level elements no N/A visual
break-before auto | always | avoid | left | right | page | column | region | avoid-page | avoid-column | avoid-region auto block-level elements no N/A visual
break-inside auto | avoid | avoid-page | avoid-column | avoid-region auto block-level elements no N/A visual
flow-from <ident> | none | inherit none Non-replaced block containers. This might be expanded in future versions of the specification to allow other types of containers to receive flow content. no N/A visual
flow-into <ident> | none | inherit none All elements, but not pseudo-elements such as ::first-line, ::first-letter, ::before or ::after. no N/A visual
region-fragment auto | break auto CSS Regions no N/A visual

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