This module describes the fragmentation model that partitions a flow into pages, columns, or regions. It builds on the Page model module and introduces and defines the fragmentation model. It adds functionality for pagination, breaking variable fragment size and orientation, widows and orphans.
This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as work in progress.
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See the Changes section for significant changes since the previous draft.
break-before’ and ‘
page-break-after’, and ‘
This section is not normative.
In paged media (e.g., paper, transparencies, photo album pages, pages displayed on computer screens as printed output simulations), as opposed to continuous media, the content of the document is split into one or more discrete display surfaces. In order to avoid awkward breaks (such as halfway through a line of text), the layout engine must be able to shift around content that would fall across the page break. This process is called pagination.
In CSS, in addition to paged media, certain layout features such as regions [CSS3-REGIONS] and multi-column layout [CSS3COL] create a similarly fragmented environment. The generic term for breaking content across containers is fragmentation. This module explains how content breaks across fragmentation containers (fragmentainers) such as pages and columns and how such breaks can be controlled by the author.
This module replaces and extends the pagination controls defined in [CSS21] section 13.3 and in [CSS3PAGE].
This specification follows the CSS property
definition conventions from [CSS21]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS
modules may expand the definitions of these value types: for example [CSS3VAL], when
combined with this module, adds the ‘
value to the properties defined here.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.
Each fragmentation break (hereafter, break) ends layout of the fragmented box in the current fragmentainer and causes the remaining content to be laid out in the next fragmentainer, in some cases causing a new fragmentainer to be generated to hold the deferred content.
Breaking inline content into lines is another form of fragmentation, and similarly creates box fragments when it breaks inline boxes across line boxes. However, inline breaking is not covered here; see [CSS21]/[CSS3TEXT].
When multiple flows are laid out parallel to each other, fragmentation is performed independently in each flow. For example, if an element is floated, then a forced break inside the float will not affect the content wrapping outside the float (except insofar as it may increase the height of the float).
The following are examples of parallel flows whose contents will fragment independently:
Content overflowing the content edge of a fixed-size box is considered parallel to the content after the fixed-size box. Such content fragments within the context of the box's fragmented flow following normal fragmentation rules, increasing the length of the fragmented flow as necessary.
Breaking a fragmentainer F effectively splits the fragmentainer into two fragmentainers (F1 and F2). The only difference is that the type of break between the two pieces F1 and F2 is the type of break created by the fragmentation context that split F, not the type of break normally created by F’s own fragmentation context.
The following sections explain how breaks are controlled in a fragmented flow. A page/column/region
break opportunity between two boxes is under the influence of the
containing block's ‘
break-inside’ property, the ‘
property of the preceding element, and the ‘
property of the following element. A page/column/region break opportunity
between line boxes is under the influence of the containing block's ‘
widows’, and ‘
orphans’ properties. A
fragmentation break can be allowed, forced, or discouraged depending on
the values of these properties. A forced break overrides any break
restrictions acting at that break point. In the case of forced page
breaks, the author can also specify on which page (left or right)
the subsequent content should resume.
See the section on rules for breaking for the exact rules on how these properties affect fragmentation.
break-before’ and ‘
|Value:||auto | avoid | always | any | avoid-page | page | left | right | recto | verso | avoid-column | column | avoid-region | region|
|Applies to:||block-level elements, table row groups, table rows (but see prose)|
|Computed value:||specified value|
These properties specify page/column/region break behavior before/after
the generated box. The forced break
column’ and ‘
a forced break in the flow while the avoid break values ‘
avoid-column’ and ‘
avoid-region’ indicate that content should be kept
Since breaks are only allowed between siblings, not between a box and
its container (see Possible Break Points),
break-before’ value on a first-child box is
propagated to its container. Likewise a ‘
break-after’ value on a last-child box is
propagated to its container. (Conflicting values combine as defined below.) This propagation
stops before it breaks through the nearest matching fragmentation context.
Values for ‘
break-before’ and ‘
defined in the sub-sections below. User Agents must apply these properties
to boxes in the normal flow of the fragmentation root. User agents
should also apply these properties to floated boxes whose containing block
is in the normal flow of the root fragmented element. User agents may also
apply these properties to other boxes.
These values have an effect regardless of the type of fragmented context containing the flow.
These values only have an effect in paginated contexts; if the flow is not paginated, they have no effect.
These values only have an effect in multi-column contexts; if the flow is not within a multi-column context, they have no effect.
These values only have an effect in multi-region contexts; if the flow is not linked across multiple regions, these values have no effect.
|Value:||auto | avoid | avoid-page | avoid-column | avoid-region|
|Applies to:||elements in the normal flow that that establish formatting contexts, or are block containers, table row groups, or table rows|
|Computed value:||specified value|
This property specifies page/column/region break behavior within the element's principal box. Values have the following meanings:
|Applies to:||block containers|
|Computed value:||specified value|
property specifies the minimum number of line boxes in a block container
that must be left in a fragment before a
fragment break. The ‘
widows’ property specifies the minimum number
of line boxes of a block container that must be left in a fragment after a break. Examples of how they
are used to control fragmentation breaks are given below.
Only positive integers are allowed as values of ‘
orphans’ and ‘
widows’. Negative values
and zero are invalid and must cause the declaration to be ignored.
If a block contains fewer lines than the value of ‘
widows’ or ‘
orphans’, the rule
simply becomes that all lines in the block must be kept together.
page-break-after’, and ‘
For compatibility with CSS Level 2, UAs that
conform to [CSS21] must alias the ‘
page-break-after’, and ‘
page-break-inside’ properties to ‘
break-inside’ by treating the ‘
page-break-*’ properties as shorthands for the ‘
break-*’ properties with the following value mappings:
|Shorthand (‘||Longhand (‘|
A fragmented flow may be broken across fragmentainers at a number of possible break points. In the case of forced breaks, the UA is required to break the flow at that point. In the case of unforced breaks, the UA has to choose among the possible breaks that are allowed.
Some content is not fragmentable, for example many types of replaced
elements [CSS21] (such as images or video),
scrollable elements, or a single line of text content. Such content is
considered monolithic: it contains no possible
break points. In addition to any content which is not fragmentable, UAs
may consider as monolithic any elements
overflow’ set to ‘
auto’ or ‘
scroll’ and any
elements with ‘
overflow: hidden’ and a
auto’ logical height
(and no specified maximum logical height).
To guarantee progress, fragmentainers are assumend to have a minimum extent of 1px regardless of their used size.
Fragmentation splits boxes in the block flow dimension. In block-and-inline flow, breaks may occur at the following places:
There is no inherent prioritization among these classes of break points. However, individual break points may be prioritized or de-prioritized by using the breaking controls.
The UA is not required to fragment the contents
of monolithic elements, and may instead
either slice the element's graphical representation as necessary to
fragment it or treat its box as unbreakable and overflow the
fragmentainer. In both cases it must treat the element as having ‘
break-inside: avoid’, i.e. only slice or overflow at
the fragmentainer edge if there are no possible
break points on the fragmentainer.
When paginating, if there are no possible break points below the top of the page, and not all the content fits, the UA may break anywhere in order to avoid losing content off the edge of the page.
Other layout models may add breakpoints to the above classes. For example, [CSS3-FLEXBOX] adds certain points within a flex formatting context to classes A and C.
There are different types of breaks in CSS, defined based on the type of fragmentainers they span:
A fifth type of break is the line break, which is a break between two line boxes. These are not covered in this specification; see [CSS21] [CSS3TEXT].
A forced break is one explicitly indicated
by the style sheet author. A forced
break occurs at a class A break point
if, among the ‘
break-after’ properties specified on or
propagated to the earlier sibling box and the ‘
properties specified on or propagated to the later sibling box there is at
least one with a forced break value. (Thus a forced break
value effectively overrides any avoid break value that also
applies at that break point.)
When multiple forced break
values apply to a single break point, they combine such that all
types of break are honored. When ‘
verso’ are combined, the value
specified on the latest element in the flow wins.
A page break must also occur at a class A break point if the last line box above this
margin and the first one below it do not have the same value for ‘
page’. See [CSS3PAGE]
When a forced break occurs, it forces ensuing content into the next fragmentainer of the type associated with the break, breaking through as many fragmentation contexts as necessary until the specified break types are all satisfied. If the forced break is not contained within a matching type of fragmentation context, then the forced break has no effect.
While breaking controls can force breaks, they can also discourage them. An unforced break is one that is inserted automatically by the UA in order to prevent content from overflowing the fragmentainer. The following rules control whether unforced breaking at a possible break point is allowed:
break-after’ and ‘
break-before’ values applicable to this break point allow it, which is when at least one of them forces a break or when all of them are ‘
auto’ and a common ancestor of all the elements has a ‘
break-inside’ value of ‘
avoid’, then breaking here is not allowed.
orphans’ or more, and the number of line boxes between the break and the end of the box is the value of ‘
widows’ or more.
break-inside’ property of all ancestors is ‘
If the above doesn't provide enough break points to keep content from
overflowing the fragmentainer, then
rules 1, 2 and 4 are dropped in order to find additional breakpoints. In
this case the UA may use the ‘
avoid’s that are
in effect at those points to weigh the appropriateness of the new
breakpoints; however, this specification does not suggest a precise
If that still does not lead to sufficient break points, rule 3 is dropped as well, to find still more break points.
While CSS3 requires that a fragmented flow must break at allowed break points in order to avoid overflowing the fragmentainers in its fragmentation context, it does not define whether content breaks at a particular allowed break. However, it is recommended that user agents observe the following guidelines (while recognizing that they are sometimes contradictory):
Suppose, for example, that the style sheet contains ‘
orphans : 4’, ‘
widows : 2’,
and there is space for 20 lines (line boxes) available at the bottom of
the current page, and the next block in normal flow is considered for
widows’ constraint, and so the second fragment must contain at least two line boxes; likewise the first fragment must contain at least four line boxes.
orphans’ constraint), so it must move as a block to the next page.
The sizing terminology used in this section is defined in [CSS3-SIZING].
When a flow is fragmented into varying-size fragmentainers, the following rules are observed for adapting layout:
box-decoration-break: clone’, if specified, wraps the fragment with the box's margin in addition to its padding and border.
First page +----------------------------------+ |########### ......... ############| |# # ..... # #| |# left # ....... # right #| |# float # ......... # float #| |# # ........ # #| |# # ......... # #| |# # ........ # #| |#,,,,,,,,,# ......... #,,,,,,,,,,#| +----------------------------------+ Second page +-----------------+ |#`````````# .... | |# left # ... | |# float # .... | |# cont. # .... | |########### ... | |.... #``````````#| |... # right #| |.... # float #| |.... # cont. #| |.... ############| |............... | |....... |
Since document order of elements doesn't change during fragmentation, fragments are processed following the same rules that apply to continuous media. In particular, the order of floats is preserved across all fragments and follows the same rules as defined in CSS 2.1 9.5.
Below are listed (informatively) some implications of these rules:
Here is an example that shows the use of percentage-based progress:
Suppose we have an absolutely-positioned element that is positioned
top: calc(150% + 30px)’ and has ‘
height: calc(100% - 10px)’. If it is placed into a
paginated context with a first page height of 400px, a second page of
200px, and a third page of 600px, its layout progresses as follows:
When an unforced break occurs between block-level boxes, any margins adjoining the break truncate to the remaining fragmentainer extent. When a forced break occurs there, adjoining margins before the break are truncated, but margins after the break are preserved.
When a box breaks, its content box extends to fill any remaining fragmentainer
extent (leaving room for any margins/borders/padding applied by
box-decoration-break: clone’) before the
content resumes on the next fragmentainer. (A fragmentation break that pushes
content to the next fragmentainer
effectively increases the extent of a box's contents.)
The extra extent contributed by fragmenting the box (i.e. the distance from the break point to the edge of the fragmentainer) contributes progress towards any specified limits on the box's extent.
+-----------------+ +-----------------+ | ............... | | ............... | | .............. | | .............. | | ............... | | ............... | | .............. | | .............. | | ............ | | ............ | | ############### | | ############### | | # this box # | | # this box # | | # is # | | # is # | << auto-height block | # fragmented # | | # fragmented # | | # # | | # # | <-+ | # # | | # # | | remaining | # # | | # # | | blank | # # | | ############### | | space +-----------------+ +-----------------+ <-+
|Value:||slice | clone|
|Applies to:||all elements|
|Computed value:||as specified|
When a break (page/column/region/line) splits a box,
box-decoration-break’ property controls
Values have the following meanings:
border-radius’ and ‘
border-image’ and ‘
box-shadow’, if any, are applied to each fragment independently. The background is drawn independently in each fragment of the element. A no-repeat background image will thus be rendered once in each fragment of the element.
No border and no padding are inserted at a break; no box-shadow is
drawn at a broken edge; ‘
does not apply to its corners; and backgrounds and the ‘
border-image’ are rendered for the whole box as
if the box were unbroken. The effect is as though the element were
rendered with no breaks present, and then sliced by the breaks
UAs may also apply ‘
box-decoration-break’ to control rendering at
bidi-imposed breaks, i.e. when bidi reordering causes an inline to split
into non-contiguous fragments. Otherwise such breaks are always handled as
For inline elements, which side of a fragment is considered
the broken edge is determined by the parent element's inline progression
direction. For example, if an inline element whose parent has ‘
direction: rtl’ breaks across two lines, the
left edge of the fragment on the first line will be the broken
edge. (Note in particular that neither the element's own ‘
direction’ nor its containing block's ‘
direction’ is used.) See [CSS3-WRITING-MODES].
box-decoration: slice’, backgrounds (and
border-image’) are drawn as if applied to
a composite box consisting of all of the box's fragments reassembled in
visual order. This theoretical assembly occurs after the element has been
laid out (including any justification, bidi reordering, page breaks,
etc.). To assemble the composite box...
direction’ is ‘
ltr’), the first fragment is the leftmost fragment on the first line and fragments from subsequent lines are put to the right of it. In a right-to-left containing block, the first fragment is the rightmost on the first line and subsequent fragments are put to the left of it.
If the box fragments have different widths (heights, if the fragments are joined horizontally), then each piece draws its portion of the background assuming that the whole element has the same width (height) as this piece. However, if the used height (width) of an image is derived from the width (height) of the box, then it is calculated using the widest fragment's width and maintained as a fixed size. This ensures that right-aligned images stay aligned to the right edge, left-aligned images stay aligned to the left edge, centered images stay centered, and stretched images cover the background area as intended while preserving continuity across fragments.
Fragmentation interacts with layout, and thus occurs before relative positioning [CSS21], transforms [CSS3-TRANSFORMS], and any other graphical effects. Such effects are applied per fragment: for example, rotation applied to a fragmented box will calculate a rotation origin for each fragment and independently rotate that fragment around its origin. However, the separation and transfer of page boxes should occur last; thus a transformed fragment that spans pages should be sliced at the page breaks and print in its entirety rather than being clipped by its originating page.
There's an issue of how to handle fragments consisting entirely of overflowing content. See discussion.
Absolute positioning affects layout and thus interacts with fragmentation. Both the coordinate system and absolutely-positioned boxes belonging to a containing block will fragment across pages in the same fragmentation flow as the containing block.
UAs are not required to correctly position boxes that span a fragmentation break and whose before edge position depends on where the box's content fragments. UAs with memory constraints that prevent them from manipulating an entire document in memory are not required to correctly position absolutely-positioned elements that end up on a previously-rendered page.
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 to CSS Fragmentation is defined for three conformance classes:
A style sheet is conformant to CSS Fragmentation if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.
A renderer is conformant to CSS Fragmentation if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Fragmentation 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 CSS Fragmentation 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.
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.
To avoid clashes with future CSS features, the CSS2.1 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.
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 firstname.lastname@example.org mailing list.
For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:
The specification will remain Candidate Recommendation for at least six months.
The following significant changes were made since the 29 August 2012 Working Draft:
box-decoration-break’ (moved from [CSS3BG]) and defined better handling of aspect-ratio-sized images in variable-sized fragments.
any’ value to ‘
break-before’ and ‘
The editors would like to thank Mihai Balan, Michael Day, Alex Mogilevsky, Shinyu Murakami, Florian Rivoal, and Alan Stearns for their contributions to this module. Special thanks go to the former [CSS3PAGE] editors Jim Bigelow (HP), Melinda Grant (HP), Håkon Wium Lie (Opera), and Jacob Refstrup (HP) for their contributions to this specification, which is a successor of their work there.
|box-decoration-break||slice | clone||slice||all elements||no||N/A||visual|
|break-after||auto | avoid | always | any | avoid-page | page | left | right | recto | verso | avoid-column | column | avoid-region | region||auto||block-level elements, table row groups, table rows (but see prose)||no||N/A||visual|
|break-before||auto | avoid | always | any | avoid-page | page | left | right | recto | verso | avoid-column | column | avoid-region | region||auto||block-level elements, table row groups, table rows (but see prose)||no||N/A||visual|
|break-inside||auto | avoid | avoid-page | avoid-column | avoid-region||auto||elements in the normal flow that that establish formatting contexts, or are block containers, table row groups, or table rows||no||N/A||visual|