CSS Regions Module Level 3

W3C Working Draft 3 May 2012

This version:
Latest version:
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Vincent Hardy, Adobe Systems, Inc.,
Alex Mogilevsky, Microsoft,
Alan Stearns, Adobe Systems, Inc.,
Issues List:
Bugzilla Bugs for CSS regions
www-style@w3.org with subject line "[css3-regions] message topic"


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

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than 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.


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.

  article {
    flow-into: article_flow;

  #region1, #region2, #region3, #region4 {
    flow-from: article_flow;

The ‘article_flow’ value on the ‘flow-into’ property directs the article element to the ‘article_flownamed 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


Should creation of regions from elements be disallowed?


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


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.

  @region #region1 {
      p {
          color: #0C3D5F;
          font-weight: bold;

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


Is a mechanism to auto-generate regions necessary in order to support reusable style sheets?


Explain how regions and pages interact. How can regions be placed onto certain pages, and how can they be positiond wrt. pages.

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.

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-overflow’ 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


Creating a named flow from external resource


flow-into and anonymous blocks or fix-ups


[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: any element.

The ‘flow-into’ property does not apply to any pseudo-element such as ::first-line, ::first-letter, ::before or ::after.

Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
The element is not moved to a named flow and normal CSS processing takes place.
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 the named 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 a sibling of its ancestor for the purpose of layout in the region chain laying out content from that 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 flow and of the region chain laying out content from a named flow.


Describe how containing blocks are used for element fragments

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.

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 ‘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-contentnamed 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 (see CSS 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.

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
The block container is not a region.
If the ‘content’ property computes to something else than ‘normal’, the block container does not become a CSS Region. If the ‘content’ property computes to ‘normal’, then the block container becomes a CSS Region and is ordered in a region 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.
Likewise, if the block container is part of the flow with name <ident>, then the block container does not format any content visually.

No display when combining flow-from and flow-into is bad

A CSS Regions document children are not visually formatted unless they are directed to a named flow with an associated region chain.


An 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:

    flow-into: thread;
    flow-from: thread;
    <div class=region>div content</div>
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 empty named flow and, consequently, its children are not formatted visually.

Should regions be non-breakable?


Should regions not create a new stacking context?


Specify behavior of stacking contexts that are split between regions


Should re-introduce region order

CSS Regions create a new stacking context. CSS Regions establish a new block formatting Context.


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.

Floats or other exclusions (see [CSS3-EXCLUSIONS]) potentially impact the content laid out in region chains, just as for non-regions.

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

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

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:

Always force a region break before (after) the generated box.
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-overflow property



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

Should we have region-overflow:nobreak and have ‘region-overflow’ apply to all regions, not just the last?


Should we rename region-overflow to flow-overset?

The ‘region-overflow’ property controls the behavior of the last region associated with a named flow.

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.

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, if any is not rendered.

The ‘region-overflow’ property does not influence the size of the region it applies to.

The following code sample illustrates the usage of the ‘region-overflow’ property.

article {
  flow-into: "article";

#region_1, #region_2 {
  flow-from: article;
  region-overflow: break; /* or none */
  overflow: visible; /* or hidden */



<div id="region_1"></div>
<div id="region_2"></div>
flow-into: "article"region_1 and region_2 region-overflow: auto
regions receiving the flow content result if region-overflow is set to 'break' regions receiving the flow content
region-overflow: breakregion-overflow: auto
result if region-overflow is set to 'break' regions receiving the flow content

Different values for the region-overflow property


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).


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:


List of region style properties

  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. border properties
  11. rounded corner properties
  12. border images properties
  13. margin properties
  14. padding properties
  15. text-shadow’ property
  16. box-shadow’ property
  17. box-decoration-break’ property
  18. width’ property

In the following example, the named flowarticle_flow’ flows through ‘region_1’ and ‘region_2’.

    #div_1 {
        flow-into: article_flow;
    #region1, #region2 {
        flow-from: article_flow;

    /* region style "RSA" */
    @region #region1, #region2 {
        div {...}
        p {...}
    /* region style "RSB" */
    @region #region1 {
        p {...}

<div id="div_1">
    <p id="p_1">...</p>
    <p id="p_2">...</p>

<div id="region1"></div>
<div id="region2"></div>
Illustration showing how a named flow content fits into regions to illustrate the @region styling.

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.


@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:

   #multi-col {
       column-count: 2;
       flow-from: article;
       height: 6em;
       column-gap: 1em; 
   #remainder {
       flow-from: article;
       height: auto;

<div id="multicol"></div>
<div id="remainder"></div>

is equivalent in rendering to, for example:

   #flex {
       display: flex;
       flex-pack: justify;
       height: 6em;
   #flex > div {
       flow-from: article;
       width: calc(50% - 0.5em);

   #remainder {
       flow-from: article;
       height: auto;

<div id="flex">
   <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


add functionality for having textual description like "continued here" or "continued on page x" to regions.

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 becomes the last inline box of the previous element in the flow that has some visual rendering in the region and can accommodate for the ‘::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.



elementFromPoint and CSS regions


Terms in WebIDL snippets inconsistently linked

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

A named flow is created when it becomes referenced by the ‘flow-into’ and/or ‘flow-from’ computed values. The following APIs allow scripts to reference a NamedFlow object representation of a named flow.

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

partial interface Document {
  readonly attribute NamedFlowCollection namedFlows;
  NamedFlow? getFlowByName(DOMString name);

The namedFlows attribute on the Document interface returns the live list of all the current named 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 getFlowByName method on the Document interface provides access to the document's NamedFlow instances. The return value depends on the named flow state:

The NamedFlowCollection interface provides a list of current NamedFlow instances in the document. The collection is live and methods operate on the underlying data, not a snapshot of the data.

interface NamedFlowCollection {
  readonly attribute unsigned long length;
  getter NamedFlow? item (unsigned long index);

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

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

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 implements 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. The attribute also returns true if there is no associated region chain. Otherwise, it returns false.

The getRegions() method returns the sequence of regions in the region chain associated with the named 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 regionOverset attribute set to empty. If all regions have the regionOverset attribute set to fit or overset, the value for firstEmptyRegionIndex 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 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. Note that the returned values is a static sequence in document order.

The named flow states are :

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

If a script holds a reference to a NamedFlow instance that has entered the NULL state, its attributes and methods should behave as follows:


Should we have a NamedFlow.getRegionsByFlowRanges?

6.2. The Region interface

The Region interface provides an abstraction for Elements, pseudo-elements or other CSS constructs (such as slots [CSS3-PAGE-TEMPLATE]) which can be regions.

interface Region {
  readonly attribute DOMString regionOverset;
  readonly attribute DOMString flowFrom;
  sequence<Range>? getRegionFlowRanges();
  CSSStyleDeclaration getComputedStyle(Element elt);
  CSSStyleDeclaration getComputedStyle(Element elt, DOMString pseudoElt);

The regionOverset attribute returns one of the following values:

The region is the last one in the region chain and not able to fit the remaining content from the named flow. Note that the region's overflow property value can be used to control the visibility of the overflowing content and the ‘region-overflow’ property controls whether or not fragmentation happens on the content that overflows the last region.
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 is 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).
All content from the named flow was fitted in prior regions.

Note that if there is no content in the named flow, all regions associated with that named flow should have their regionOverset 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 flowFrom attribute returns the name of the named flow this region is associated with.

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 and startOffset 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, the startContainer 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 getComputedStyle methods on the Region interface work the same as the getComputedStyle [CSSOM] methods on the Window interface [HTML5] with the following exceptions. For the Region interface 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.

A Region instance may represent and 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.

If a Region instance is no longer a region, accessing its attributes (regionOverset and flowFrom) or invoking its getRegionFlowRanges method throws a DOMException with the INVALID_ACCESS_ERR error code.

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;
The rule is a CSSRegionStyleRule
interface CSSRegionStyleRule : CSSRule {
  attribute DOMString selectorText;
  readonly attribute CSSRuleList cssRules;
  unsigned long insertRule(DOMString rule,  unsigned long index)
  void deleteRule(unsigned long index)

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:

6.4. Region flow layout events

NamedFlow objects are Event Targets which dispatch regionLayoutUpdate events when there is a possible layout change of their named flow fragment. Note that the event is asynchronous.


regionLayoutUpdate needs to be async

The remaining issue in this bug is when the event gets fired.
Type regionLayoutUpdate
Interface UIEvent (see [DOM-LEVEL-3-EVENTS])
Sync / Async Async
Bubbles Yes
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 their named 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 principles described in that specification.

7.1. The Region Flow Content Box (RFBC)

A region box lays out the following boxes:

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 computedwidth’ of ‘auto’ and a whose usedheight’ 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 flow content.

As a consequence, all RFCBs of regions associated with a given named flow share the same min-content and 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:

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. 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. 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.

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;

        <p style="region-break-after:always;">I am not a ... </p>
    <div id="rA"></div>
    <div id="rB"></div>
    <div id="rC"></div>

    <div id="main-flow">
        <p>Lorem ipsum dolor ...</p>

The following sections and figures illustrate the intermediate results for the visual formatting process. In the following, we call RFCB-A, RFCB-B and RFCB-C the 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 RFBCs with used heights of 0

Step 1 - Phase 1: Layout RFBCs 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 ‘articlenamed 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 this page.

11. Conformance

12. Changes

12.1. Changes from November 29th 2011 version

12.2. Changes from June 09th 2011 version


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, Andrei Bucur, Alexandru Chiculita, Phil Cupp, Arron Eicholz, John Jansen, Daniel Glazman, Arno Gourdol, David Hyatt, Brian Heuston, Ian Hickson, Jonathan Hoersch, Michael Jolson, Brad Kemper, Håkon Wium Lie, Markus Mielke, Robert O'Callahan, Edward O'Connor, Mihnea Ovidenie, Virgil Palanciuc, Olga Popiv, Christoph Päper, Anton Prowse, Peter Sorotokin, Christian Stockwell, Eugene Veselov, Boris Zbarsky, Stephen Zilles and the CSS Working Group members.

Appendix A. Example Code for Introduction

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

  #grid {
    width: 80vw;
    height: 60vw;
    grid-template: "aaa.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;

     The following code element is the content to flow 
     through the region chain. 

  <p>This is an example ...</p>
  <h2>More Details</h2>
  <p>This illustrates ...</p>
  <p>Then, the example ...</p>
  <p>Finally, this ...</p>

     For this example, we layout the regions with a grid.
     Regions could be pseudo-elements and could be laid out
     with another layout module (e.g., flexible boxes)
<div id="grid">
  <div id="region1"></div>
  <div id="region2"></div>
  <div id="boxA"></div>
  <div id="region3"></div>
<div id="region4"></div>

Note that a multi-column element is used for #region2, which is a bit gratuitous here (because grid cells could be used). The reason to use a multi-column element is to illustrate that regions can be multi-column.



Fix specification dependencies

Normative references

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
Rossen Atanassov; Elika J. Etemad. CSS Fragmentation Module Level 3. 28 February 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-break-20120228/
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/
Elika J. Etemad; Koji Ishii; Shinyu Murakami. CSS Writing Modes Module Level 3. 1 September 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-writing-modes-20110901/
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
Anne van Kesteren. CSSOM. 12 July 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-cssom-20110712/
Anne van Kesteren; Aryeh Gregor; Ms2ger. DOM4. 5 January 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-dom-20120105/
Doug Schepers; et al. Document Object Model (DOM) Level 3 Events Specification. 31 May 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-DOM-Level-3-Events-20110531
Ian Hickson. HTML5. 25 May 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-html5-20110525/
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

Vincent Hardy; Rossen Atanassov. CSS Exclusions and Shapes Module Level 3. 13 December 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-exclusions-20111213/
Tab Atkins Jr.; Alex Mogilevsky; L. David Baron. CSS Flexible Box Layout Module. 29 November 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-flexbox-20111129/
Alex Mogilevsky; et al. Grid Layout. 7 April 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-grid-layout-20110407
Koji Ishii. CSS Line Grid Module. Proposal for a CSS module. (Retrieved 26 October 2011) URL: http://dev.w3.org/csswg/css-line-grid/
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
Bert Bos; César Acebal. CSS Template Layout Module. 29 April 2010. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2010/WD-css3-layout-20100429
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
Håkon Wium Lie; et al. Media Queries. 27 July 2010. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2010/CR-css3-mediaqueries-20100727/


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 any element. The ‘flow-into’ property does not apply to any pseudo-element such as ::first-line, ::first-letter, ::before or ::after. no N/A visual
region-overflow auto | break auto CSS Regions no N/A visual