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 .
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.
1.1. Module InteractionsThis module replaces and extends the pagination controls defined in [CSS2] section 13.3 and in [CSS3PAGE].
1.2. Value DefinitionsThis specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.
2. Fragmentation Model and TerminologyEach 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 [CSS2]/[CSS3TEXT].
A box can be broken into multiple fragments also due to bidi reordering of text (see Applying the Bidirectional Reordering Algorithm in CSS Writing Modes) or higher-level display type box splitting, e.g. block-in-inline splitting (see CSS2§9.2) or column-spanner-in-block splitting (see CSS Multi-column Layout). The division into box fragments in these cases does not depend on layout (sizing/positioning of content).
2.1. Parallel Fragmentation FlowsWhen multiple formatting contexts are laid out parallel to each other, fragmentation is performed independently in each formatting context. For example, if an element is floated, then a forced break inside the float will not affect the content outside the float (except insofar as it may increase the height of the float). UAs may (but are not required to) adjust the placement of unforced breaks in parallel formatting contexts to visually balance such side-by-side content, but must not do so to match a forced break.
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 and follows the normal fragmentation rules. Although overflowing content doesn’t affect the size of the fragmentation root box, it does increase the length of the fragmented flow, spilling into or generating additional fragmentainers as necessary.
2.2. Nested Fragmentation FlowsBreaking a fragmentainer F effectively splits the fragmentainer into two fragmentainers (F1 and F2). The only difference is that, with regards to the content of fragmentainer F, 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.
For example, if a region box is broken at a page boundary, then the content of the region will be affected by a page break at that point (but not by a region break).
Note that when a multi-column container breaks across pages, it generates a new row of columns on the next page for the rest of its content, so that a page break within a multi-column container is always both a page break and a column break.
3. Controlling BreaksThe 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 break-after property of the preceding element, and the break-before property of the following element. A page/column/region break opportunity between line boxes is under the influence of the containing block’s break-inside, 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.
3.1. Breaks Between Boxes: the break-before and break-after propertiesThese properties specify page/column/region break behavior before/after the generated box. The forced break values left, right, recto, verso, page, column and region create a forced break in the flow while the avoid break values avoid, avoid-page, avoid-column and avoid-region indicate that content should be kept together.
Values for break-before and break-after are 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. User agents must not apply these properties to absolutely-positioned boxes.
Generic Break ValuesThese 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.
Since breaks are only allowed between siblings, not between a box and its container (see Possible Break Points), break values applied to children at the start/end of a parent are propagated to the parent, where they can take effect.
Specifically—except in layout modes which define more specific rules to account for reordering and parallel layout (e.g. in flex layout [CSS-FLEXBOX-1] or grid layout [CSS-GRID-1])—a break-before value on a first in-flow child box is propagated to its container. Likewise a break-after value on a last in-flow 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.
Break propagation does not affect computed values; it is part of interpreting the elements’ computed values for layout.
3.2. Breaks Within Boxes: the break-inside propertyThis property specifies page/column/region break behavior within the element’s principal box. Values have the following meanings:
The orphans property specifies the minimum number of line boxes in a block container that must be left in a fragment before a fragmentation 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.
TestsOnly 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.
3.4. Page Break Aliases: the page-break-before, page-break-after, and page-break-inside propertiesFor compatibility with CSS Level 2, UAs that conform to [CSS2] must alias the page-break-before, page-break-after, and page-break-inside properties to break-before, break-after, and break-inside by treating the page-break-* properties as legacy shorthands for the break-* properties with the following value mappings:
Shorthand (page-break-*) Values Longhand (break-*) Values auto | left | right | avoid auto | left | right | avoid always page 4. Rules for BreakingA 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.
To guarantee progress, fragmentainers are assumed to have a minimum block size of 1px regardless of their used size.
4.1. Possible Break PointsFragmentation 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.
Other layout models may add breakpoints to the above classes. For example, [CSS-FLEXBOX-1] adds certain points within a flex formatting context to classes A and C.
Some content is not fragmentable, for example many types of replaced elements [CSS2] (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. Any forced breaks within such boxes therefore cannot split the box, and must therefore also be ignored by the box’s own fragmentation context.
In addition to any content which is not generally fragmentable, UAs may consider as monolithic any elements with overflow set to auto or scroll and any elements with overflow: hidden and a non-auto logical height (and no specified maximum logical height).
Since line boxes contain no possible break points, inline-block and inline-table boxes (and other inline-level display types that establish an independent formatting context) may also be considered monolithic: that is, in the cases where a single line box is too large to fit within its fragmentainer even by itself and the UA chooses to split the line box, it may fragment such boxes or it may treat them as monolithic.
4.2. Types of BreaksThere 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 [CSS2] [CSS3TEXT].
4.3. Forced BreaksA 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 break-before 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 left, right, recto, and/or verso are combined, the value specified on the latest element in the flow wins.
A forced 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.
4.4. Unforced BreaksWhile 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:
If the above doesn’t provide enough break points to keep content from overflowing the fragmentainer, then rule 3 is dropped to provide more break points.
If that still does not lead to sufficient break points, then rules 1, 2 and 4 are dropped in order to find additional breakpoints. In this case the UA may use the avoids that are in effect at those points to weigh the appropriateness of the new breakpoints; however, this specification does not suggest a precise algorithm.
If even that does not lead to sufficient break points, cloned margins/border/padding at the block-end side are truncated; and if more room is still needed, cloned margins/border/padding are truncated at the block-start side as well.
Finally, if there are no possible break points below the top of the fragmentainer, and not all the content fits, the UA may break anywhere in order to avoid losing content off the edge of the fragmentainer. In such cases, the UA may also fragment the contents of monolithic elements by slicing the element’s graphical representation. However, the UA must not break at the top of the page, i.e. it must place at least some content on each fragmentainer, so that each fragmentainer has a non-zero amount of content, in order to guarantee progress through the content.
4.5. Optimizing Unforced BreaksWhile 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 placement:
Now suppose that orphans is 10, widows is 20, and there are 8 lines available at the bottom of the current page:
Additionally, CSS imposes one requirement: a zero-sized box fragment, since it does not take up space, must appear on the earlier side of a fragmentation break if it is able to fit within the fragmentainer.
Note: A zero-sized box fragment will be pushed to the next fragmentainer if it is placed immediately after content that itself overflows the fragmentainer.
5. Box Model for BreakingThe sizing terminology used in this section is defined in [CSS3-SIZING].
5.1. Breaking into Varying-size FragmentainersWhen a flow is fragmented into varying-size fragmentainers, the following rules are observed for adapting layout:
Illustration of breaking in varying-size fragmentainers.
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 before or after a block-level box, any margins adjoining the break are truncated to zero. When a forced break occurs there, adjoining margins before the break are truncated, but margins after the break are preserved. Cloned margins are always truncated to zero on block-level boxes.
Note: CSS Fragmentation Level 4 will introduce control over margin truncation at breaks.
5.3. Splitting BoxesWhen 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 block size of a box’s contents.)
The extra block size 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 block size.
5.4. Fragmented Borders and Backgrounds: the box-decoration-break propertyWhen a break (page/column/region/line) splits a box, the box-decoration-break property controls
Values have the following meanings:
Note: Cloned margins are truncated on block-level boxes.
The effect is as though the element were rendered with no breaks present, and then sliced by the breaks afterward: no border and no padding are inserted at a break; no box-shadow is drawn at a broken edge; and backgrounds, border-radius, and the border-image are applied to the geometry of the whole box as if it were unbroken.
UAs should 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—and/or at display-type–imposed breaks—i.e. when a higher-level display type (such as a block-level box / column spanner) splits an incompatible ancestor (such as an inline box / block container). Otherwise such breaks must be handled as slice. See Applying the Bidirectional Reordering Algorithm in CSS Writing Modes, CSS2§9.2 Block-level elements and block boxes, and CSS Multi-column Layout §6 Spanning Columns.
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].
5.4.1. Joining Boxes for sliceFor box-decoration-break: 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...
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.
5.5. Transforms, Positioning, and PaginationFragmentation interacts with layout, and thus occurs before relative positioning [CSS2], 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. (The origin of an overflow-only fragment is determined as if that content were overflowing an empty box with zero margins/borders/padding at the start of the fragmentainer.) However, in order to reduce dataloss when printing, 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.
A fixed-height box spanning 2.5 pages with overflow content spanning to a total of 4 pages. The transform origin of each fragment is the center of its border box; the fragment without a border box assumes a zero-height box at the start of the overflow.
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 fragmentainers in the same fragmentation flow as the containing block.
UAs are not required to correctly position boxes that span a fragmentation break and whose block-start 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.
ChangesThe following significant changes were made since the 4 December 2018 Candidate Recommendation:
The following significant changes were made since the 14 January 2016 Candidate Recommendation:
by treating the page-break-* properties as shorthands legacy shorthands for the break-* properties
Applies to: block containers that establish a new inline formatting context
When an unforced break occurs between before or after a block-level box es , any margins adjoining the break truncate to the remaining fragmentainer extent before the break, and are truncated to zero after the break .
Since line boxes contain no possible break points, inline-block and inline-table boxes (and other inline-level display types that establish an independent formatting context) may also be considered monolithic that is, in the cases where a single line box is too large to fit within its fragmentainer even by itself and the UA chooses to split the line box, it may fragment such boxes or it may treat them as monolithic .
Additionally, CSS imposes one requirement: a zero-sized box fragment, since it does not take up space, must appear on the earlier side of a fragmentation break if it is able to fit within the fragmentainer.
A zero-sized box fragment can be pushed to the next fragmentainer if it is placed immediately after content that itself overflows the fragmentainer.
A box can be broken into multiple fragments also due to bidi reordering of text (see Applying the Bidirectional Reordering Algorithm in CSS Writing Modes) or higher-level display type box splitting, e.g. block-in-inline splitting (see CSS2§9.2) or column-spanner-in-block splitting (see CSS Multi-column Layout). The division into box fragments in these cases does not depend on layout (sizing/positioning of content).
UAs may should 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 —and/or at display-type–imposed breaks—i.e. when a higher-level display type (such as a block-level box / column spanner) splits an incompatible ancestor (such as an inline box / block container) . Otherwise such breaks must be handled as slice. See Applying the Bidirectional Reordering Algorithm in CSS Writing Modes, CSS2§9.2 Block-level elements and block boxes, and CSS Multi-column Layout §6 Spanning Columns.
A Disposition of Comments is available.
The following significant changes were made since the 29 January 2015 Working Draft:
A Disposition of Comments is available.
AcknowledgmentsThe 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.
Privacy and Security ConsiderationsThis specification introduces no new privacy or security considerations over CSS in general.
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:
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.
Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.
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.
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.
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.
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.3