[css3-regions] message topic
"
Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
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.
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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.
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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].
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.
Consider the layout illustrated in figure 1.
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 for desired layout
There is no existing mechanism in CSS to associate the content with these boxes arranged in this manner so that content flows as intended. The CSS regions module properties provide that mechanism.
The following example illustrates how the content of an
<article>
element becomes a named flow and how boxes marked with ‘region1
’, ‘region2
’, ‘region3
’ and ‘region4
’ IDs become CSS Regions that consume the
‘article_flow
’ content.
<style> article { flow-into: article_flow; } #region1, #region2, #region3, #region4 { flow-from: article_flow; } </style>
The ‘article_flow
’ value on the
‘flow-into
’ property directs the
article
element to the ‘article_flow
’ named
flow. Setting the ‘flow-from
’ property on block containers to
‘article_flow
’ makes them CSS Regions
and associates the resulting region chain with the named flow: the flow is ‘poured
’ into the region chain.
See Appendix A for sample code using grid layout that implements this example
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.
Reconsider using @rule for region styling
Content can be styled depending on the CSS Region it flows into. It is
an extension of pseudo-element styling such as ::first-line
which applies a particular style to a
fragment of content. With CSS Region styling, additional selectors may
apply depending on the CSS Region into which content flows.
In our example, the designer wants to make text flowing into #region1 dark blue and bold.
This design can be expressed as shown below.
<style> @region #region1 { p { color: #0C3D5F; font-weight: bold; } </style>
The @region #region1
rule limits its selectors to
content flowing into #region1
. The following figure
shows how the rendering changes if we apply styling specific to
#region1
. Note how less text fits into this box now that the
‘font-weight
’ is bold instead of
normal.
Different rendering with a different region styling
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).
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.
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.
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]).
flow-into
’
propertyCreating 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 ‘ |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | as specified |
<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-content
’ named
flow. However, the
table > * {flow-into: table-content} ...
selector will move all immediate children of all table elements into the ‘table-content’ named flow (which may be useful as it will usually result, if the immediate children are rows, in merging rows of multiple tables), but the
table * {flow-into: table-content}
selector will move all descendants of table elements into the
‘table-content’ named flow, transforming
the element tree into a flat list in order of opening tags (which is
probably not intentional). This will make all the descendants of table
elements siblings in the named flow. Having
the descendants become siblings in the named
flow results in a different processing (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.
flow-from
’
propertyThe ‘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 |
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. No display when combining flow-from and flow-into is bad
A CSS Region‘s 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:
<style> .article{ flow-into: thread; } .region{ flow-from: thread; } </style> <html> <body> <div class=region>div content</div> </body> </html>There is no element matching the
.article
selector and
therefore no content in the thread
flow. However, the block
container matching the .region
selector is still associated with that 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.
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:
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].
region-overflow:nobreak
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.
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.
<style> article { flow-into: "article"; } #region_1, #region_2 { flow-from: article; region-overflow: break; /* or none */ overflow: visible; /* or hidden */ } </style> <article>...</article> <div id="region_1"></div> <div id="region_2"></div>
‘flow-into: "article" ’
| region_1 and region_2
| ‘region-overflow: auto ’‘ overflow:visible ’
|
‘region-overflow: break ’
| ‘region-overflow: auto ’‘ overflow:hidden ’
| |
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]).
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
word-spacing
’
letter-spacing
’
text-decoration
’
text-transform
’
line-height
’
text-shadow
’ property
box-shadow
’ property
box-decoration-break
’ property
width
’ property
In the following example, the named flow
‘article_flow
’ flows through
‘region_1
’ and ‘region_2
’.
<style> #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 {...} } </style> <div id="div_1"> <p id="p_1">...</p> <p id="p_2">...</p> </div> <div id="region1"></div> <div id="region2"></div>
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].
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
’.A multi-column
[CSS3COL] element
can have an assigned named
flow. The element becomes part of the region chain for
that named flow, and flows its content fragments through columns according
to the multi-column specification [CSS3COL]. In particular, when
computing the flow
fragment height of a multi-column element that is associated with a named flow, the ‘column-fill
’ [CSS3COL] property is honored to
balance the fragments of content that would flow through its columns.
The following example:
<style> #multi-col { column-count: 2; flow-from: article; height: 6em; column-gap: 1em; } #remainder { flow-from: article; height: auto; } </style> <article>...</article> <div id="multicol"></div> <div id="remainder"></div>
is equivalent in rendering to, for example:
<style> #flex { display: flex; flex-pack: justify; height: 6em; } #flex > div { flow-from: article; width: calc(50% - 0.5em); } #remainder { flow-from: article; height: auto; } </style> <article>...</article> <div id="flex"> <div /> <div /> </div> <div id="remainder"></div>
Overflow of multicol regions is mostly handled according to the same rules as other CSS Regions. If the remainder of the named flow does not fit in the multicol region, then the rest of the content flows into the remaining region chain. However, if a multicol region is the last region in a region chain, then the multicol region must follow the overflow column rules [CSS3COL].
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.
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.
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.
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:
NULL
: the user agent must return
null
.
CREATED
: the user agent must return a valid NamedFlow
instance.
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 :
NULL
: the named flow
is not referenced by any ‘flow-into
’ or ‘flow-from
’
computed value.
CREATED
: the named
flow is referenced by at least one ‘flow-into
’ or ‘flow-from
’
computed value.
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:
name
attribute returns its value as normal.
overset
attribute returns true
.
getRegions
method returns an empty sequence.
firstEmptyRegionIndex
attribute returns -1
.
getContent()
method returns an empty NodeList
instance.
getRegionsByContentNode()
method retuns an
empty sequence.
Should we have a NamedFlow.getRegionsByFlowRanges?
The Region
interface provides an abstraction for Element
s, 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:
overset
’
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.
fit
’
empty
’
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.
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; };
interface CSSRegionStyleRule : CSSRule { attribute DOMString selectorText; readonly attribute CSSRuleList cssRules; unsigned long insertRule(DOMString rule, unsigned long index) raises(DOMException); void deleteRule(unsigned long index) raises(DOMException); };
The selectorText
attribute gets and sets the associated
selector as defined in section 6.4.3
of [CSSOM].
The cssRules
attribute must return a CSSRuleList
[CSSOM] object for
the list of CSSStyleRules
specified in the region style block.
The insertRule(rule, index)
method inserts a CSSStyleRule
into the region style block before the
specified index. If the index is equal to the length of the CSSRuleList
the CSSStyleRule
is inserted at the end of the list.
Possible exceptions:
HIERARCHY_REQUEST_ERR
: Raised if the rule to insert
is not a CSSStyleRule
.
INDEX_SIZE_ERR
: Raised if the specified index is not
a valid insertion point.
SYNTAX_ERR
: Raised if the specified rule has a
syntax error and is unparsable.
The deleteRule(index)
method deletes the CSSStyleRule
in the CSSRuleList
at the specified index.
Possible exception:
INDEX_SIZE_ERR
: Raised if the specified index does
not correspond to a rule in the region style block.
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
Type | regionLayoutUpdate
|
---|---|
Interface | UIEvent
(see [DOM-LEVEL-3-EVENTS])
|
Sync / Async | Async |
Bubbles | Yes |
Target | NamedFlow
|
Cancelable | Yes |
Default action | none |
Context info |
|
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()
.
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.
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.
A region box lays out the following boxes:
::before
and ::after
pseudo-elements, if any.
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.
width
’ resolutionAt 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]).
Formatting documents that contain named flows laid out in regions is a three-step process:
Regions visual formatting steps
Conceptually, resolving the flow fragment height is a two phase process.
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.
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 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.
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.
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.
This section is non-normative.
This example considers a document where content flows between three regions, and region boxes are intermixed with the normal document content.
<style> article { flow-into: article; } #rA, #rB, #rC { flow-from: article; height: auto; margin: 1em 1em 0em 1em; padding: 1em; border: 3px solid #46A4E9; } #rA { width: auto; height: auto; } #rB { float: left; width: 15em; height: auto; max-height: 150px; } #rC { float: right; width: 12em; height: auto; } #main-flow { padding: 0em 1em 0em 1em; } </style> <body> <article> <p style="region-break-after:always;">I am not a ... </p> <p>...</p> </article> <div id="rA"></div> <div id="rB"></div> <div id="rC"></div> <div id="main-flow"> <p>Lorem ipsum dolor ...</p> </div> </body>
The following sections and figures illustrate the intermediate results for the visual formatting process. In the following, we call RFCB-A, RFCB-B and RFCB-C the RFCBs for regions rA, rB and rC respectively.
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.
auto
’ ‘width
’, its own used ‘width
’ is stretched to fit the
<body>
content box.
rB
content box.
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
In this second phase of Step 1, the named flow is laid out in regions and the height of each fragment falling in each RFCB is computed.
The user agent lays out as much of the flow into an area with RFCB-A's
used ‘width
’. rA's ‘height
’ computes to ‘auto
’ and there is no vertical maximum height
for RFCA's ‘height
’. However,
because there is a break after the first paragraph in the ‘article
’ named
flow
, only this first paragraph is laid out in RFCB-A and FH-A
(the flow fragment height for RFCB-A) is resolved by laying out this first
paragraph in the used ‘width
’.
At this point, the user agent lays out as much of the remaining flow as
possible in RFCB-B. Because rB's ‘max-height
’ computed value is ‘150px
’, the user agent only lays out the
‘article
’ named flow using
RFCB-B's used ‘width
’ until the
point where laying out additional content would cause RFCB-B to overflow
rB's box. The fragment height for RFCB-B is resolved: FH-B (150px
).
Finally, the user agent lays out the remainder of the flow in RFCB-C.
Because rC has no other constraints and no region breaks, the remaining
content is laid out in RFCB-C's used ‘width
’. This results in a resolved flow
fragment height: FH-C.
Step 1 - Phase 2: Measure flow fragments heights
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
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
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.
This specification is related to other specifications as described in the references section. In addition, it is related to the following specifications:
Use cases are described on this page.
content
’ property.
overflow
’ property applies to content flown
in regions from the break section, since this is covered in the section
on ‘region-overflow
’ already. See mailing
list feedback.
portion
’ of an element that falls into the
corresponding region and added an issue that the model for ‘partial
’ styling needs to be defined. See mailing
list feedback.
NodeList
returned by
getRegionsByContentNode
is live.
NamedFlow
interface. Added a NamedFlowCollection
interface and added a getNamedFlows
method on the
Document
interface, as per Bug
15828.
regionOverflow
’ to
‘regionOverset
’ to avoid confusion between
fitting a flow in regions and the concept of visual overflow that the
word ‘overflow
’ (and the
property) carry.
getFlowByName
.
Region
interface
to replace the supplemental Element interface
flowFrom
’ attribute on the Region
interface
NamedFlow
instead of region as
before. Was requested by Issue
15938 and required in the general effort to have the DOM APIs work
with non-element regions.
flow-into
’ on all
pseudo-elements because moving a ‘::before
’ element (for a example) to a named flow
does not seem useful and causes specification and implementation
complexity.
Regions visual formatting
details
’ section to better describe the model for resolving
auto sizing on regions.
content-order
’ a
<integer> and not a <float> following a working group resolution
none
’, ‘inherit
’ and ‘initial
’ from the possible identifier names
on the flow property following discussion
on the mailing list.
flow-into
’ property interacts with
object, embed and iframe elements.
default
’ from the
possible identifier names on the flow property because it may
get reserved.
auto
’ on ‘region-overflow
’ specifying that region
breaks must be ignored.
Document.flowWithName
’ to
‘Document.getFlowByName
’ since it is
not a property.
NamedFlow
’ instance is live.
undefined
’ string
value to the regionOverflow property on the Element interface extension.
Event
’ in the event name.
flow
’ to ‘flow-into
’
following working group resolution.
from-flow
’ to
‘flow-from
’ following a working group resolution.
content:
flow-from()
’ to block container box and added a note that
this might be expanded in the future, following a working group resolution.
flow
’ property
description, removed the required wrapper anonymous block as agreed on mailing
list discussion.
flow-into
’
property following a mailing
list discussion.
content:flow-from(<ident>)
’ with
‘flow-from: <ident>
’ following a
working group
resolution.
::before
’ and ‘::after
’ and explain how ‘display:run-in
’ works with regions.
region-order
’
property following implementation feedback.
auto
’ to the list of
invalid flow identifiers.
none
’ the initial value
for ‘flow-into
’ and aligned with ‘flow-from
’, as
explained in this email.
Also allowed the ‘inherit
’ value
on ‘flow-from
’ and ‘flow-into
’ (same
email).
content
’ property computes to normal,
following the resolution of Issue
22.
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.
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.
<style> #grid { width: 80vw; height: 60vw; grid-template: "aaa.d" "....d" "bbb.d" "....d" "ccc.d"; grid-rows: 52% 4% 20% 4% 20%; grid-columns: 30% 5% 30% 5% 30%; } #region1 { grid-cell: a; } #region2 { grid-cell: b; } #boxA { grid-cell: c; } #region3 { grid-cell: d; } #region4 { width: 80vw; } #region2 { column-count: 2; } /* * Creates the named flow */ article { flow-into: article_flow; } /* * Associate it with the intended CSS Regions. * This creates a region chain for the named flow. */ #region1, #region2, #region3, #region4 { flow-from: article_flow; } </style> <!-- The following code element is the content to flow through the region chain. --> <article> <h1>Introduction</h1> <p>This is an example ...</p> <h2>More Details</h2> <p>This illustrates ...</p> <p>Then, the example ...</p> <p>Finally, this ...</p> </article> <!-- 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> <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
cssRules
, 6.3.
deleteRule(index)
, 6.3.
getFlowByName
, 6.1.
getRegions()
, 6.1.
insertRule(rule, index)
, 6.3.
item(index)
, 6.1.
length
, 6.1.
name
, 6.1.
NamedFlow
, 6.1.
NamedFlowCollection
, 6.1.
namedFlows
,
6.1.
overset
,
6.1.
Region
interface, 6.2.
regionOverset
, 6.2.
selectorText
, 6.3.
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 |