W3C

CSS Fragmentation Module Level 3

Breaking the Web, one fragment at a time

Editor's Draft 15 January 2014

This version:
http://dev.w3.org/csswg/css3-break/
Latest version:
http://www.w3.org/TR/css3-break/
Editor's Draft:
http://dev.w3.org/csswg/css3-break/
Previous version:
http://www.w3.org/TR/2012/WD-css3-break-20120228/
Editors:
Rossen Atanassov, Microsoft Corporation,
Elika J. Etemad, Mozilla Corporation
Issues List:
http://wiki.csswg.org/spec/css3-break
Feedback:
www-style@w3.org with subject line “[css-break] … message topic …” (archives)

Abstract

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.

Status of this document

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.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-break” in the subject, preferably like this: “[css3-break] …summary of comment…

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

See the Changes section for significant changes since the previous draft.

Table of contents

1. Introduction

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.

1.1. Module Interactions

This module replaces and extends the pagination controls defined in [CSS21] section 13.3 and in [CSS3PAGE].

1.2. Values

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 ‘initial’ 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.

2. Fragmentation Model and Terminology

fragmentation container (fragmentainer)
A box—such as a page box, column box, or region—that contains a portion (or all) of a fragmented flow. Fragmentainers can be pre-defined, or generated as needed. When breakable content overflows a fragmentainer in the block dimension, instead of overflowing it breaks into the next container in its fragmentation context.
fragmentation context
An ordered series of fragmentainers, such as created by a multi-column element, a chain of CSS regions, or a paged media display. A given fragmentation context can only have one block flow direction across all its fragmentainers.
fragmented flow
A content flow that is being laid out in a fragmentation context. The fragmented flow consists of the content of a (possibly anonymous) box called the fragmentation root.
fragmentation direction
The block flow direction of the fragmentation context, i.e. the direction in which content is fragmented. (In this level of CSS, content only fragments in one dimension.)
fragmentation
The process of splitting a content flow across the fragmentainers that form a fragmentation context.
box fragment or fragment
The portion of a box that belongs to exactly one fragmentainer. A box in continuous flow always consists of only one fragment. A box in a fragmented flow consists of one or more fragments.
remaining fragmentainer extent
The remaining extent in the fragmentainer available to a given element, i.e. between the end of preceding content in fragmentainer and the edge of the fragmentainer.

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].

2.1. Parallel Fragmentation Flows

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.

2.2. Nested Fragmentation Flows

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.

3. Controlling Breaks

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 ‘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’ properties

Name: break-before, break-after
Value: auto | avoid | always | any | avoid-page | page | left | right | recto | verso | avoid-column | column | avoid-region | region
Initial: auto
Applies to: block-level elements, table row groups, table rows (but see prose)
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value

These properties specify page/column/region break behavior before/after the generated box. The forced break valuesalways’, ‘any’, ‘left’, ‘right’, ‘recto’, ‘verso’, ‘page’, ‘column’ and ‘region’ create a forced break in the flow while the avoid break valuesavoid’, ‘avoid-page’, ‘avoid-column’ and ‘avoid-region’ indicate that content should be kept together.

Since breaks are only allowed between siblings, not between a box and its container (see Possible Break Points), a ‘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 ‘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.

Generic Break Values

These values have an effect regardless of the type of fragmented context containing the flow.

auto
Neither force nor forbid a break before/after the principal box.
avoid
Avoid a break before/after the principal box.
always
Always force a break (of all possible types, through all fragmentation contexts) before/after the principal box.
any
Force a break of any type, whichever is the type of the deepest fragmentation context, before/after the principal box.

Page Break Values

These values only have an effect in paginated contexts; if the flow is not paginated, they have no effect.

avoid-page
Avoid a page break before/after the principal box.
page
Always force a page break before/after the principal box.
left
Force one or two page breaks before/after the principal box so that the next page is formatted as a left page.
right
Force one or two page breaks before/after the principal box so that the next page is formatted as a right page.
recto
Force one or two page breaks before/after the principal box so that the next page is formatted as either a left page or a right page, whichever is second (according to the page progression) in a page spread.
verso
Force one or two page breaks before/after the principal box so that the next page is formatted as either a left page or a right page, whichever is first (according to the page progression) in a page spread.

Column Break Values

These values only have an effect in multi-column contexts; if the flow is not within a multi-column context, they have no effect.

avoid-column
Avoid a column break before/after the principal box.
column
Always force a column break before/after the principal box.

Region Break Values

These values only have an effect in multi-region contexts; if the flow is not linked across multiple regions, these values have no effect.

avoid-region
Avoid a region break before/after the principal box.
region
Always force a region break before/after the principal box.

3.2. Breaks Within Boxes: the ‘break-inside’ property

Name: break-inside
Value: auto | avoid | avoid-page | avoid-column | avoid-region
Initial: auto
Applies to: elements in the normal flow that that establish formatting contexts, or are block containers, table row groups, or table rows
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value

This property specifies page/column/region break behavior within the element's principal box. Values have the following meanings:

auto
Impose no additional breaking constraints within the box.
avoid
Avoid breaks within the box.
avoid-page
Avoid a page break within the box.
avoid-column
Avoid a column break within the box.
avoid-region
Avoid a region break within the box.

3.3. Breaks Between Lines: ‘orphans’, ‘widows

Name: orphans, widows
Value: <integer>
Initial: 2
Applies to: block containers
Inherited: yes
Percentages: N/A
Media: visual
Computed value: specified value

The ‘orphans’ 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.

3.4. Page Break Aliases: the ‘page-break-before’, ‘page-break-after’, and ‘page-break-inside’ properties

For compatibility with CSS Level 2, UAs that conform to [CSS21] 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 shorthands for the ‘break-*’ properties with the following value mappings:

Shorthand (‘page-break-*’) Values Longhand (‘break-*’) Values
auto | left | right | avoidauto | left | right | avoid
alwayspage

4. Rules for Breaking

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 with ‘overflow’ set to ‘auto’ or ‘scroll’ and any elements with ‘overflow: hidden’ and a non-‘autological 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.

4.1. Possible Break Points

Fragmentation splits boxes in the block flow dimension. In block-and-inline flow, breaks may occur at the following places:

Class A
Between sibling boxes of the following types:
Block-parallel Fragmentation
When the block flow direction of the siblings' containing block is parallel to that of the fragmentation context: in-flow block-level boxes, a float and an immediately-adjacent in-flow or floated box, table row group boxes, table row boxes, multi-column column row boxes.
Block-perpendicular Fragmentation
When the block flow direction of the siblings' containing block is perpendicular to that of the fragmentation context: table column group boxes, table column boxes, multi-column column boxes.
Class B
Between line boxes inside a block container box.
Class C
Between the content edge of a block container box and the outer edges of its child content (margin edges of block-level children or line box edges for inline-level children) if there is a (non-zero) gap between them.

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.

4.2. Types of Breaks

There are different types of breaks in CSS, defined based on the type of fragmentainers they span:

page break
A break between two page boxes. [CSS3PAGE]
spread break
A break between two page boxes that are not associated with facing pages. A spread break is always also a page break. [CSS3PAGE]
column break
A break between two column boxes. Note that if the column boxes are on different pages, then the break is also a page break. Similarly, if the column boxes are in different regions, then the break is also a region break. [CSS3COL]
region break
A break between two regions. Note that if the region boxes are on different pages, then the break is also a page break. [CSS3-REGIONS]

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].

4.3. Forced Breaks

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 ‘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 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 Breaks

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:

Rule 1
A fragmented flow may break at a class A break point only if all the ‘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’.
Rule 2
However, if 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.
Rule 3
Breaking at a class B break point is allowed only if the number of line boxes between the break and the start of the enclosing block box is the value of ‘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.
Rule 4
Additionally, breaking at class B or class C break points is allowed only if the ‘break-inside’ property of all ancestors is ‘auto’.

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 algorithm.

If that still does not lead to sufficient break points, rule 3 is dropped as well, to find still more break points.

4.5. Optimizing Unforced Breaks

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 placement:

Now suppose that ‘orphans’ is ‘10’, ‘widows’ is ‘20’, and there are 8 lines available at the bottom of the current page:

5. Box Model for Breaking

The sizing terminology used in this section is defined in [CSS3-SIZING].

5.1. Breaking into Varying-size Fragmentainers

When a flow is fragmented into varying-size fragmentainers, the following rules are observed for adapting layout:

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:

5.2. Adjoining Margins at Breaks

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.

5.3. Splitting Boxes

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.

The diagrams below illustrate filling the remaining fragmentainer extent.

+-----------------+  +-----------------+
| ............... |  | ............... |
| ..............  |  | ..............  |
| ............... |  | ............... |
| ..............  |  | ..............  |
| ............    |  | ............    |
| ############### |  | ############### |
| #  this box   # |  | #  this box   # |
| #  is         # |  | #  is         # |  << auto-height block
| #  fragmented # |  | #  fragmented # |
| #             # |  | #             # |  <-+
| #             # |  | #             # |    | remaining
| #             # |  | #             # |    | blank
| #             # |  | ############### |    | space
+-----------------+  +-----------------+  <-+

5.4. Fragmented Borders and Backgrounds: the ‘box-decoration-break’ property

Name: box-decoration-break
Value: slice | clone
Initial: slice
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Animatable: no

When a break (page/column/region/line) splits a box, the ‘box-decoration-break’ property controls

Values have the following meanings:

clone
Each box fragment is independently wrapped with the border and padding. The ‘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.
slice

No border and no padding are inserted at a break; no box-shadow is drawn at a broken edge; ‘border-radius’ 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 afterward.

Illustration:       (1) a single box cut in two in between two lines of text by a page break and       (2) two boxes, one before and one after the page break,       both with a border all around and their own background image

Two possibilities for ‘box-decoration-break’: on the left, the value ‘slice’, on the right the value ‘clone’.

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 ‘slice’.

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 ‘slice

For ‘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...

For boxes broken across lines
First, fragments on the same line are connected in visual order. Then, fragments on subsequent lines are ordered according to the element's inline base direction and aligned on the element's dominant baseline. For example, in a left-to-right containing block (‘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.
For boxes broken across columns
Fragments are connected as if the column boxes were glued together in the block flow direction of the multi-column element.
For boxes broken across pages
Fragments are connected as if page content areas were glued together in the block flow direction of the root element.
For boxes broken across regions
Fragments are connected as if region content areas were glued together in the block flow direction of the principal writing mode of the region chain.

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 Pagination

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.

6. Conformance

6.1. Document Conventions

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

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

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

This is an example of an informative example.

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

Note, this is an informative note.

6.2. Conformance Classes

Conformance to CSS Fragmentation is defined for three conformance classes:

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

A style sheet is conformant to 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.

6.3. Partial Implementations

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

6.4. Experimental Implementations

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.

6.5. Non-Experimental Implementations

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

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

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

6.6. CR Exit Criteria

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:

independent
each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
interoperable
passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.
implementation
a user agent which:
  1. implements the specification.
  2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
  3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Changes

The following significant changes were made since the 29 August 2012 Working Draft:

Acknowledgments

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.

References

Normative references

[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607
[CSS3-REGIONS]
Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Regions Module Level 1. 28 May 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-regions-20130528/
[CSS3-TRANSFORMS]
Simon Fraser; et al. CSS Transforms Module Level 1. 26 November 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css-transforms-1-20131126/
[CSS3BG]
Bert Bos; Elika J. Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 24 July 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-background-20120724/
[CSS3COL]
Håkon Wium Lie. CSS Multi-column Layout Module. 12 April 2011. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2011/CR-css3-multicol-20110412
[CSS3PAGE]
Melinda Grant; et al. CSS Paged Media Module Level 3. 14 March 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-page-20130314/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

Other references

[CSS3-FLEXBOX]
Tab Atkins Jr.; Elika J. Etemad; Alex Mogilevsky. CSS Flexible Box Layout Module. 18 September 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/
[CSS3-SIZING]
Tab Atkins Jr.; Elika J. Etemad. CSS Intrinsic & Extrinsic Sizing Module Level 3. 27 September 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-sizing-20120927/
[CSS3-WRITING-MODES]
Elika J. Etemad; Koji Ishii. CSS Writing Modes Module Level 3. 15 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-writing-modes-20121115/
[CSS3TEXT]
Elika J. Etemad; Koji Ishii. CSS Text Module Level 3. 13 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-text-20121113/
[CSS3VAL]
Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 30 July 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-css3-values-20130730/

Index

Property index

Property Values Initial Applies to Inh. Percentages Media
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
orphans <integer> 2 block containers yes N/A visual
widows <integer> 2 block containers yes N/A visual