CSS Overflow Module Level 3 (original) (raw)

Abstract

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This module contains the features of CSS relating to new mechanisms of overflow handling in visual media (e.g., screen or paper). In interactive media, it describes features that allow the overflow from a fixed size container to be handled by pagination (displaying one page at a time). It also describes features, applying to all visual media, that allow the contents of an element to be spread across multiple fragments, allowing the contents to flow across multiple regions or to have different styles for different fragments.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The (archived) public mailing list www-style@w3.org (seeinstructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css-overflow-3” in the subject, preferably like this: “[css-overflow-3] _…summary of comment…_”

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

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

This is the first public working draft of css-overflow-3.

Table of contents

• 1. Introduction
• 2. Overflow properties
• 3. Scrolling and hidden overflow
• 4. Paginated overflow
• 5. Fragment overflow
  • 5.1. Fragment styling
    * 5.1.1.The ::nth-fragment() pseudo-element
    * 5.1.2.Styling of fragments
    * 5.1.3.Styling inside fragments
  • 5.2. The ‘max-lines’ property
• 6. Overflow in static media
• 7. Conformance
  • 7.1. Module interactions
  • 7.2. Values
  • 7.3. Document conventions
  • 7.4. Conformance classes
  • 7.5. Partial implementations
  • 7.6. Experimental implementations
  • 7.7. Non-experimental implementations
  • 7.8. CR exit criteria
• Acknowledgments
• References
  • Normative references
  • Other references
• Index
• Property index

1. Introduction

In CSS Level 1 [CSS1], placing more content than would fit inside an element with a specified size was generally an authoring error. Doing so caused the content to extend outside the bounds of the element, which would likely cause that content to overlap with other elements.

CSS Level 2 [CSS21] introduced the ‘overflow’ property, which allows authors to have overflow be handled by scrolling, which means it is no longer an authoring error. It also allows authors to specify that overflow is handled by clipping, which makes sense when the author's intent is that the content not be shown.

However, scrolling is not the only way to present large amounts of content, and may even not be the optimal way. After all, the codex replaced the scroll as the common format for large written works because of its advantages.

This specification introduces a mechanism for Web pages to specify that an element of a page should handle overflow through pagination rather than through scrolling.

This specification also extends the concept of overflow in another direction. Instead of requiring that authors specify a single area into which the content of an element must flow, this specification allows authors to specify multiple fragments, each with their own dimensions and styles, so that the content of the element can flow from one to the next, using as many as needed to place the content without overflowing.

In both of these cases, implementations must break the content in the block-progression dimension. Implementations must do this is described in the CSS Fragmentation Module [CSS3-BREAK].

2. Overflow properties

The ‘overflow-x’ property specifies the handling of overflow in the horizontal direction (i.e., overflow from the left and right sides of the box), and the ‘overflow-y’ property specifies the handling of overflow in the vertical direction (i.e., overflow from the top and bottom sides of the box)

The ‘overflow’ property is a shorthand property that sets the specified values of both ‘overflow-x’ and ‘overflow-y’ to the value specified for ‘overflow’.

The values of these properties are:

visible

There is no special handling of overflow, that is, it may be rendered outside the block container.

hidden

scroll

auto

These values are collectively the scrolling values; they are defined in the section on scrolling and hidden overflow.

paged-x

paged-y

paged-x-controls

paged-y-controls

fragments

These values are collectively the fragmenting values; they are defined in the sections on paginated overflow and fragment overflow.

The computed values of ‘overflow-x’ and ‘overflow-y’ are determined from the cascaded values [CSS3CASCADE] based on the following rules:

1. If one or both of the cascaded values are fragmenting values, then:
   1. If one of the cascaded values is one of the fragmenting values and the other is not, then the computed values are the same as the cascaded values.
   2. If both of the cascaded values are fragmenting values, then:
      1. for horizontal writing mode [CSS3-WRITING-MODES], the computed value for ‘overflow-y’ is the cascaded value and the computed value for ‘overflow-x’ is ‘hidden’, or
      2. for vertical writing mode [CSS3-WRITING-MODES], the computed value for ‘overflow-x’ is the cascaded value and the computed value for ‘overflow-y’ is ‘hidden’.
2. Otherwise, if one cascaded values is one of the scrolling values and the other is ‘visible’, then computed values are the cascaded values with ‘visible’ changed to ‘hidden’.
3. Otherwise, the computed values are as specified.

Are all 4 of the ‘paged-*’ values really needed?

When the fragmenting values are used, the overflow from the fragments themselves treats the fragmenting value as ‘hidden’. Is this the right behavior? Give example.

[CSS3-MARQUEE] describes an ‘overflow-style’ property, but it has not picked up implementation experience that the working group is aware of. Should this document treat ‘overflow-style’ as a defunct proposal, or should this document describe the ‘overflow-style’ property and attempt to revive it, despite that implementations have implemented ‘overflow-x’ and ‘overflow-y’ instead?

There are discussions about how overflow, overflow-style, overflow-x and overflow-y should work and interact with each other. Until consensus on this topic is reached, it is not completely clear which of these should be used for paged-x | paged-y | paged-x-controls | paged-y-controls | fragments

Move material from [CSS21] and [CSS3BOX] here.

4. Paginated overflow

overflow:paginate or overflow:pages (or paged-x, paged-y, paged-x-controls, paged-y-controls as [CSS3GCPM] has?)

Ability to display N pages at once rather than just one page at once?

The current implementation of paginated overflow uses the ‘overflow’/‘overflow-x’/‘overflow-y’ properties rather than the ‘overflow-style’ property as proposed in the [CSS3GCPM] draft (which also matches the [CSS3-MARQUEE] proposal). We should probably switch away from ‘overflow-style’, but that's not 100% clear.

5. Fragment overflow

This section introduces and defines the meaning of the new ‘fragments’ value of the ‘overflow’ property.

When the computed value of ‘overflow’ for an element is ‘fragments’, and implementations would otherwise have created a box for the element, then implementations must create a sequence of fragment boxes for that element. (It is possible for an element with ‘overflow: fragments’ to generate only one fragment box. However, if an element's computed ‘overflow’ is not ‘fragments’, then its box is not a fragment box.) Every fragment box is a fragmentation container, and any overflow that would cause that fragmentation container to fragment causes another fragment box created as a next sibling of the previous one. Or is it as though it's a next sibling of the element? Need to figure out exactly how this interacts with other box-level fixup. Additionally, if the fragment box is also a multi-column box (as defined in [CSS3COL] though it defines multi-column element) any content that would lead to the creation of overflow columns [CSS3COL] instead is flown into an additional fragment box. However, fragment boxes may themselves be broken (due to fragmentation in a fragmentation context outside of them, such as pages, columns, or other fragment boxes); such breaking leads to fragments of the same fragment box rather than multiple fragment boxes. (This matters because fragment boxes may be styled by their index; such breaking leads to multiple fragments of a fragment box with a single index. This design choice is so that breaking a fragment box across pages does not break the association of indices to particular pieces of content.) Should a forced break that breaks to an outer fragmentation context cause a new fragment of a single fragment box or a new fragment box? Should we find a term other than fragment box here to make this a little less confusing?

What if we want to be able to style the pieces of an element split within another type of fragmentation context? These rules prevent ever using ‘::nth-fragment()’ for that, despite that the name seems the most logical name for such a feature.

We should specify that ‘overflow: fragments’ does not apply to at least some table parts, and perhaps other elements as well. We need to determine exactly which ones.

This specification needs to say which type of fragmentation context is created so that it's clear which values of the ‘break’ property cause breaks within this context. We probably want ‘break: regions’ to apply.

This specification needs a processing model that will apply in cases where the layout containing the fragments has characteristics that use the intrinsic size of the fragments to change the amount of space available for them, such as [CSS3-GRID-LAYOUT]. There has already been some work on such a processing model in [CSS3-REGIONS], and the work done on a model there, and the editors of that specification, should inform what happens in this specification.

5.1. Fragment styling

5.1.1. The ::nth-fragment() pseudo-element

The ::nth-fragment() pseudo-element is a pseudo-element that describes some of the fragment boxes generated by an element. The argument to the pseudo-element takes the same syntax as the argument to the :nth-child() pseudo-class defined in [SELECT], and has the same meaning except that the number is relative to fragment boxes generated by the element instead of siblings of the element.

Selectors that allow addressing fragments by counting from the end rather than the start are intentionally not provided. Such selectors would interfere with determining the number of fragments.

Depending on future discussions, this ‘::nth-fragment(an+b)’ syntax may be replaced with the new ‘::fragment:nth(an+b)’ syntax.

5.1.2. Styling of fragments

Should this apply to fragment overflow only, or also to paginated overflow? (If it applies, then stricter property restrictions would be needed for paginated overflow.)

In the absence of rules with ‘::nth-fragment()’ pseudo-elements, the computed style for each fragment box is the computed style for the element for which the fragment box was created. However, the style for a fragment box is also influenced by rules whose selector's subject [SELECT] has an ‘::nth-fragment()’ pseudo-element, if the 1-based number of the fragment box matches that ‘::nth-fragment()’ pseudo-element and the selector (excluding the ‘::nth-fragment()’ pseudo-element) matches the element generating the fragments.

When determining the style of the fragment box, these rules that match the fragment pseudo-element cascade together with the rules that match the element, with the fragment pseudo-element adding the specificity of a pseudo-class to the specificity calculation. Does this need to be specified in the cascading module as well?

Styling an ‘::nth-fragment()’ pseudo-element with the ‘overflow’ property does take effect; if a fragment box has a computed value of ‘overflow’ other than ‘fragments’ then that fragment box is the last fragment. However, overriding ‘overflow’ on the first fragment does not cause the fragment box not to exist; whether there are fragment boxes at all is determined by the computed value of overflow for the element. Need to reword this to refer to the appropriate choice of ‘overflow-x’ or ‘overflow-y’, and then point to rule about the handling of the other one of ‘overflow-x’ or ‘overflow-y’.

Styling an ‘::nth-fragment()’ pseudo-element with the ‘content’ property has no effect; the computed value of ‘content’ for the fragment box remains the same as the computed value of content for the element.

Specifying ‘display: none’ for a fragment box causes the fragment box with that index not to be generated. However, in terms of the indices used for matching ‘::nth-fragment()’ pseudo-elements of later fragment boxes, it still counts as though it was generated. However, since it is not generated, it does not contain any content.

Specifying other values of ‘display’, ‘position’, or ‘float’ is permitted, but is not allowed to change the computed value of ‘display-inside’. (Since ‘overflow’, ‘overflow-x’, and ‘overflow-y’ only apply to block containers, flex containers, and grid containers the computed value of ‘display-inside’ is always ‘block’, ‘flex’ or ‘grid’. Need to specify exactly how this works, but it depends on having ‘display-inside’ and ‘display-outside’ specified.

To match the model for other pseudo-elements where the pseudo-elements live inside their corresponding element, declarations in ‘::nth-fragment()’ pseudo-elements override declarations in rules without the pseudo-element. The relative priority within such declarations is determined by normal cascading order (see [CSS21]).

Styles specified on ‘::nth-fragment()’ pseudo-elements do affect inheritance to content within the fragment box. In other words, the content within the fragment box must inherit from the fragment box's style (i.e., the pseudo-element style) rather than directly from the element. This means that elements split between fragment boxes may have different styles for different parts of the element.

This inheritance rule allows specifying styles indirectly (by using explicit ‘inherit’ or using default inheritance on properties that don't apply to ‘:first-letter’) that can't be specified directly (based on the rules in the next section). This is a problem. The restrictions that apply to styling inside fragments should also apply to inheritance from fragments.

5.1.3. Styling inside fragments

Should this apply to fragment overflow only, or also to paginated overflow, or even to pagination across pages?

The ‘::nth-fragment()’ pseudo-element can also be used to style content inside of a fragment box. Unlike the ‘::first-line’ and ‘::first-letter’ pseudo-elements, the ‘::nth-fragment()’ pseudo-element can be applied to parts of the selector other than the subject: in particular, it can match ancestors of the subject. However, the only CSS properties applied by rules with such selectors are those that apply to the ‘::first-letter’ pseudo-element.

To be more precise, when a rule's selector has ‘::nth-fragment()’ pseudo-elements attached to parts of the selector other than the subject, the declarations in that rule apply to a fragment (or pseudo-element thereof) when:

1. the declarations are for properties that apply to the ‘::first-letter’ pseudo-element,
2. the declarations would apply to that fragment (or pseudo-element thereof) had those ‘::nth-fragment()’ pseudo-elements been removed, with a particular association between each sequence of simple selectors and the element it matched, and
3. for each removed ‘::nth-fragment()’ pseudo-element, the fragment lives within a fragment box of the element associated in that association with the selector that the pseudo-element was attached to, and whose index matches the pseudo-element.

5.2. The ‘max-lines’ property

Authors may wish to style the opening lines of an element with different styles by putting those opening lines in a separate fragment. However, since it may be difficult to predict the exact height occupied by those lines in order to restrict the first fragment to that height, this specification introduces a ‘max-lines’ property that forces a fragment to break after a specified number of lines. This forces a break after the given number of lines contained within the element or its descendants, as long as those lines are in the same block formatting context.

none

Breaks occur only as specified elsewhere.

In addition to any breaks specified elsewhere, a break is forced before any line that would exceed the given number of lines being placed inside the element (excluding lines that are in a different block formatting context from the block formatting context to which an unstyled child of the element would belong).

If there are multiple boundaries between this line and the previous, where exactly (in terms of element boundaries) is the break forced?

Only positive integers are accepted. Zero or negative integers are a parse error.

Should this apply to fragment overflow only, or also to pagination?

This specification should define useful behavior for all values of ‘overflow’ in static media (such as print). Current implementation behavior is quite poor and produces unexpected results when authors have not considered what will happen when the content they produce for interactive media is printed.

7. Conformance

7.1. Module interactions

This module extends the ‘overflow’ feature defined in [CSS21] section 11.1.1. It defines additional overflow handling mechanisms that implementations must implement as described in this module in order to conform to this module.

No properties in this module apply to the ::first-line or::first-letter pseudo-elements.

7.2. 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 [CSS3COLOR], when combined with this module, expands the definition of the value type as used in this specification.

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.

7.3. 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 withclass="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.

7.4. Conformance classes

Conformance to CSS Overflow Module Level 3 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 Overflow Module Level 3 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 CSS Overflow Module Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by CSS Overflow Module Level 3 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 Overflow Module Level 3 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.

7.5. 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.

7.6. 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.

7.7. 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.

7.8. CR exit criteria

[Change or remove the following CR exit criteria if the spec is not a module, but, e.g., a Note or a profile. This text was decided on 2008-06-04.]

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.

Acknowledgments

Thanks especially to the feedback from Rossen Atanassov, Bert Bos, Tantek Çelik, John Daggett, fantasai, Daniel Glazman, Vincent Hardy, Håkon Wium Lie, Peter Linss, Robert O'Callahan, Florian Rivoal, Alan Stearns, Steve Zilles, and all the rest of the www-style community.

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-BREAK]

Rossen Atanassov; Elika J. Etemad. CSS Fragmentation Module Level 3. 23 August 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-break-20120823/

[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-GRID-LAYOUT]

Alex Mogilevsky; et al. CSS Grid Layout. 6 November 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-grid-layout-20121106/

[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/

[CSS3CASCADE]

Håkon Wium Lie; Elika J. Etemad; Tab Atkins Jr. CSS Cascading and Inheritance Level 3. 3 January 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-cascade-20130103/

[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

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

[SELECT]

Tantek Çelik; et al. Selectors Level 3. 29 September 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-selectors-20110929/

Other references

[CSS1]

Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS1-20080411

[CSS3-MARQUEE]

Bert Bos. CSS Marquee Module Level 3. 5 December 2008. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2008/CR-css3-marquee-20081205

[CSS3-REGIONS]

Vincent Hardy; Alex Mogilevsky; Alan Stearns. CSS Regions Module Level 3. 23 August 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-regions-20120823/

[CSS3BOX]

Bert Bos. CSS basic box model. 9 August 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-box-20070809

[CSS3COLOR]

Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

[CSS3GCPM]

Håkon Wium Lie. CSS Generated Content for Paged Media Module. 29 November 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-css3-gcpm-20111129/

Index

• authoring tool, 7.4.
• auto, 2.
• fragment box, 5.
• fragmenting values, 2.
• fragments, 2.
• hidden, 2.
• max-lines, 5.2.
• overflow, 2.
• overflow-x, 2.
• overflow-y, 2.
• paged-x, 2.
• paged-x-controls, 2.
• paged-y, 2.
• paged-y-controls, 2.
• renderer, 7.4.
• scroll, 2.
• scrolling values, 2.
• style sheet
  • as conformance class, 7.4.
• visible, 2.

Property index