CSS Regions Module Level 1

W3C Working Draft 28 May 2013

This version:
Latest version:
Previous version:
Vincent Hardy, Adobe Systems, Inc.,
Rossen Atanassov, Microsoft Corporation,
Alan Stearns, Adobe Systems, Inc.,
Authors and former editors:
Alex Mogilevsky, Microsoft Corporation,
Issues List:
Bugzilla Bugs for CSS regions
www-style@w3.org with subject line “[css-regions] … message topic …” (archives)


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. Content in a named flow is 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 an <article> element is placed in 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 and custom elements that implements this example


Should creation of regions from elements be disallowed?


HTML Templates in example code

All of the examples use Custom Elements and HTML Templates to contain the elements used to create CSS Regions. This is meant to show one way of separating content markup from layout. In each example, the body element becomes a custom element and a copy of the template gets inserted into the body element's Shadow DOM.


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. For instance, the code in Appendix A uses a grid layout [CSS3-GRID-LAYOUT] as an example. This 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 mechanisms used to create them.

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

The only requirement for box to become a CSS Region is that the ‘flow-from’ property applies to the box.

1.2. CSS Region names and 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.

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

The ::region() functional pseudo-element takes a p selector as an argument in this example. The color and font-weight declarations will apply to any fragments of paragraphs that are displayed in #region1. The following figure shows how the rendering changes if we apply this 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 content associated with a flow with a given identifier. Contents in a named flow are ordered according to the document order.

Content is placed into a named flow with the ‘flow-into’ property. The content in a named flow is laid out in the region chain that is associated with this named flow using the ‘flow-from’ property.

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

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

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 Region Styling

3.1. The ‘flow-into’ property


[Shadow]: getFlowByName and shadow DOM

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

Name: flow-into
Value: none | <ident> [element|content]?
Initial: none
Applies to: All elements, but not pseudo-elements 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.
If the keyword ‘element’ is present or neither keyword is present, then the element is taken out of its parent's flow and placed into the flow with the name ‘<ident>’. If the keyword ‘content’ is present, then only the element's contents are placed into the named flow. The element or content is said to have a specified flow. The values ‘none’, ‘inherit’, ‘default’, ‘auto’ and ‘initial’ are invalid flow names.

The ‘flow-into’ property does not affect the CSS cascade and inheritance for the elements on which it is specified. The ‘flow-into’ property does not affect the DOM [DOM] position of an element or its contents. The ‘flow-into’ property affects the visual formatting of elements or contents placed into a named flow and of the region chain laying out content from a named flow.

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

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

Content in a named flow is sequenced in document order. The visual formatting model uses the relationships between content in the named flow as input, rather than the contents’ position in the DOM.

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 principal writing mode for the entire flow. The writing mode on subsequent regions is ignored.


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.


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}
Then the ‘span-content’ named flow contents will contain this:
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.

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’ (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 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.

A CSS Region’s document children are not visually formatted unless they are directed to a named flow with 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].


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:

    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.

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.

The flex and grid display values do not result in block containers (they are defined as flex containers and grid elements, respectively). So ‘flow-from’ combined with those display values does not result in a CSS Region.

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.


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.

Fragments rendering separately is also relevant to elements that might normally be rendered as a unit (for example, an element with its own stacking context, or with transparency). Each fragment of these elements is separately contained in the stacking context created by the CSS Region, so each fragment of these elements is rendered separately.

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.


For example, styling like this:

#id {
    flow-into: foolish;
    flow-from: foolish;

would move the #id element to a ‘foolishnamed flow, and try to make the #id element a CSS Region for the ‘foolishnamed flow. The ‘foolishnamed flow would then contain its own region, creating a cycle. So the #id element does not become a CSS Region.

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.

A CSS Region can be part of the contents of a separate named flow, as long as there are no cycles broken by the Cycle Detection described above. This case is a nested region context, which has an effect on the Visual Formatting Steps described below.

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


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:

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-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.

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 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.

<element name="two-region-example" extends="body">
      article {
        flow-into: article-flow;
      #region-1, #region-2 {
        flow-from: article-flow;
        region-fragment: break; /* or auto */
        overflow: visible; /* or hidden */
    <div id="region-1"></div>
    <div id="region-2"></div>

<body is="two-region-example">
flow-into: "article"region-1 and region-2 region-fragment: auto
regions receiving the flow content result if region-overflow is set to 'break' regions receiving the flow content
region-fragment: breakregion-fragment: auto
result if region-overflow is set to 'break' regions receiving the flow content

Different values for the region-fragment 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() functional pseudo-element

A ::region() functional pseudo-element can be appended to a selector that matches a CSS Region, and takes a selector as an argument. This allows style declarations to be applied to fragments of named flow content flowing through particular regions.

<region selector>::region(<content selector>) {
    ... CSS styling declarations ...

When the ::region() pseudo-element is appended to a selector that matches one or more CSS Regions, this creates a ‘flow fragment’ selector. The flow fragment selector specifies which range of elements in the flow can be matched by the selector argument. The selector argument can match elements in the range(s) (see [DOM]) of the named flow that are displayed in the selected region(s).

Elements that are fully or partially in the flow fragment range may match a selector argument. However, the style declarations only apply to the fragment of the element that is displayed in the corresponding region(s).

Only a limited list of properties apply to a ::region() functional pseudo-element:

  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

In the following example, the named flowarticle-flow’ flows through ‘region-1’ and ‘region-2’.

<element name="x-region-style-example" extends="body">
      #div-1 {
        flow-into: article-flow;

      #region-1, #region-2 {
        flow-from: article-flow;

      /* region styling */
      #region-1::region(p) {
        margin-right: 5em;

    <div id="region-1"></div>
    <div id="region-2"></div>

<body is="x-region-style-example">
  <div id="div-1">
      <p id="p-1">...</p>
      <p id="p-2">...</p>
Example showing how a named flow content fits into regions to illustrate region styling.

The region styling applies to flow content that fits in ‘region-1’. The selector argument 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 the pseudo-element.

All of the selectors in a ::region() functional pseudo-element contribute to its specificity. So the specificity of the ::region() pseudo-element in the example above would combine the id selector's specificity with the specificity of the type selector in the argument, resulting in a specificity of 101.

Selectors that match a given element or element fragment (as described 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 to a region chain. The element becomes part of the region chain for the associated 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:

<element name="x-multicol-region" extends="body">
      article {
        flow-into: article-flow;
      #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>

<body is="x-multicol-region">

is equivalent in rendering to, for example:

<element name="x-single-col-regions" extends="body">
      article {
        flow-into: article-flow;
      #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>
<body is="x-single-col-regions">

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.


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 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 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 if index 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 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 contents that were moved to the named flow by the flow-into property but not any 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 :

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.

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.

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:

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-fragment’ 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 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.
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 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 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 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 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. 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 getComputedStyle works 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. Named flow events

NamedFlow objects are EventTargets which dispatch the following events for their respective triggers. These events are asynchronous, and fire at the end of the regions visual formatting steps.

The regionfragmentchange event is dispatched on any change to a named flow's fragmentation through its region chain, including changes to any overset fragment.

Type regionfragmentchange
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 fragmentation has changed.

The regionoversetchange event is dispatched if any of the regionOverset values change in a named flow's region chain, including when regions are added or removed from the chain.

Type regionoversetchange
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 region chain has regionOverset values that have changed.

6.4. Clarifications on pre-existing APIs

6.4.1. 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().

6.4.2. offsetTop, offsetLeft, offsetWidth, offsetHeight and offsetParent

The computation of the offset attributes for elements laid out in a named flow follow the specification [CSSOM]. For the purpose of these algorithms, the first CSS layout box associated with an element laid out in a named flow is the first box generated for the first region the element is laid out into. In the offsetParent algorithm, the nearest ancestor search skips from the topmost named flow elements directly to the body element.

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 (RFCB)

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 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 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.

In a nested region context, this phase will trigger the beginning of Step 1 for any inner named flows whose regions are contained in the outer named flow. All of Step 1 for inner flows must recursively complete before Step 1 for an outer flow completes.

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.

In a nested region context, this step will trigger Step 2 for inner named flows whose regions are contained in the outer named flow. Fragmentation of the inner regions may result as they are laid out in the outer region chain. Once Step 3 for an outer named flow is complete, Step 3 for the inner named flows recursively begins.

Once Step 3 for a named flow is complete, The conditions for the named flow events are checked, and if the triggers are met the events dispatch at this point.

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.

<element name="x-formatting-example" extends="body">
      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;

      #rB {
          float: left;
          width: 15em;
          max-height: 150px;

      #rC {
          float: right;
          width: 12em;

      #main-flow {
          padding: 0em 1em 0em 1em;

    <div id="rA"></div>
    <div id="rB"></div>
    <div id="rC"></div>

<body is="x-formatting-example">
        <p style="break-after:region;">I am not a ... </p>

    <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 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 ‘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.


Regions need a way to respond to events in their content fragment

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

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS 2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

12. Changes

12.1. Changes from August 28th 2012 version

12.2. Changes from May 3rd 2012 version

12.3. Older Changes

Older changelogs are archived on the CSSWG wiki


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.

    <title>Regions Intro Example</title>
    <link rel="components" href="x-fancy-layout.html">

     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 defined in the 
     layout template. All the presentation 
     is confined to the layout template file.
    <p>This is an example ...</p>
    <h2>More Details</h2>
    <p>This illustrates ...</p>
    <p>Then, the example ...</p>
    <p>Finally, this ...</p>
  <div id="boxA"></div>

     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">
      #grid {
        width: 80vw;
        height: 110vh;
        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;
    <div id="grid">
      <div id="region1"></div>
      <div id="region2"></div>
      <div id="region3"></div>
    <div id="region4"></div>

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.

Similarly, the last region breaks out of the grid in this example merely to show that regions in a region chain do not need to share a single parent.


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. 23 August 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-break-20120823/
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/
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; et al. DOM4. 6 December 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-dom-20121206/
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/
Ian Hickson. HTML5. 17 December 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-html5-20121217/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
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; 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/
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/
Tab Atkins Jr.; Elika J. Etemad; Rossen Atanassov. CSS Grid Layout. 2 April 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-grid-layout-20130402/
Koji Ishii. CSS Line Grid Module. Proposal for a CSS module. (Retrieved 26 October 2011) URL: http://dev.w3.org/csswg/css-line-grid/
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/
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 November 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-layout-20111129/
Melinda Grant; et al. CSS Paged Media Module Level 3. 14 March 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-page-20130314/


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 none | <ident> [element|content]? 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