CSS Overflow Module Level 3

Editor’s Draft,

This version:
https://drafts.csswg.org/css-overflow/
Latest published version:
https://www.w3.org/TR/css-overflow-3/
Previous Versions:
https://www.w3.org/TR/2016/WD-css-overflow-3-20160531/
https://www.w3.org/TR/2013/WD-css-overflow-3-20130418/
Issue Tracking:
Inline In Spec
GitHub Issues
Editors:
L. David Baron (Mozilla)
Florian Rivoal (On behalf of Bloomberg)
Change Log:
from 27 January 2015 to the present
from 28 March 2013 to 27 January 2015
from 31 July 2012 to 27 March 2013

Abstract

This module contains the features of CSS relating to scrollable overflow handling in visual media.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

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.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-overflow” in the title, preferably like this: “[css-overflow] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

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 document is governed by the 1 September 2015 W3C Process Document.

This update trims away most of the experimental new ideas for handling overflow that were described in the previous Working Draft. These ideas are not abandoned; they are merely deferred until Level 4. Level 3 is focused solely on completing a spec for the existing, shipped overflow features; work will resume on fragmented overflow and other fun things once this is completed.

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.

This specification introduces the long-standing de-facto overflow-x and overflow-y properties, and defines overflow handling more fully.

[Something something max-lines.]

2. Types of overflow

CSS uses the term overflow to describe the contents of a box that extend outside one of that box’s edges (i.e., its content edge, padding edge, border edge, or margin edge). The term might be interpreted as elements or features that cause this overflow, the non-rectangular region occupied by these features, or, more commonly, the minimal rectangle that bounds that region. A box’s overflow is computed based on the layout and styling of the box itself and of all descendants whose containing block chain includes the box.

In most cases, overflow can be computed for any box from the bounds and properties of that box itself, plus the overflow of each of its children. However, this is not always the case; for example, when transform-style: preserve-3d [CSS3-TRANSFORMS] is used on some of the children, any of their descendants with transform-style: preserve-3d must also be examined.

There are two different types of overflow, which are used for different purposes by the UA:

2.1. Ink overflow

The ink overflow of a box is the part of that box and its contents that creates a visual effect outside of the box’s border box. Ink overflow is the overflow of painting effects defined to not affect layout or otherwise extend the scrollable overflow region, such as box shadows, border images, text decoration, overhanging glyphs (with negative side bearings, or with ascenders/descenders extending outside the em box), outlines, etc.

Since some effects in CSS (for example, the blurs in text-shadow [CSS3TEXT] and box-shadow [CSS3BG]) do not define what visual extent they cover, the extent of the ink overflow is undefined.

The ink overflow region is the non-rectangular area occupied by the ink overflow, and the ink overflow rectangle is the minimal rectangle whose axis is aligned to the box’s axes and contains the ink overflow region. Note that the ink overflow rectangle is a rectangle in the box’s coordinate system, but might be non-rectangular in other coordinate systems due to transforms [CSS3-TRANSFORMS].

2.2. Scrollable overflow

The scrollable overflow of a box is the set of things extending outside of that box’s padding edge for which a scrolling mechanism needs to be provided.

The scrollable overflow region is the non-rectangular region occupied by the scrollable overflow, and the scrollable overflow rectangle is the minimal rectangle whose axis is aligned to the box’s axes and contains the scrollable overflow region.

The scrollable overflow region is the union of:

The UA may additionally include the margin areas of boxes for which it is the containing block. The conditions under which such margin areas are included is undefined in this level. This needs further testing and investigation; is therefore deferred in this draft.

Note: The scrollable overflow rectangle is always a rectangle in the box’s own coordinate system, but might be non-rectangular in other coordinate systems due to transforms [CSS3-TRANSFORMS]. This means scrollbars can sometimes appear when not actually necessary.

3. Scrollable Overflow: the overflow-x, overflow-y, and overflow properties

These properties specify whether a box’s content (including any ink overflow) is clipped to its padding edge, and if so, whether it is a scroll container that allows the user to scroll clipped parts of its scrollable overflow region into view. The visual viewport of the scroll container (through which the scrollable overflow region can be viewed) coincides with its padding box, and is called the scrollport.

Name: overflow-x, overflow-y
Value: visible | hidden | clip | scroll | auto
Initial: visible
Applies to: block containers [CSS21], flex containers [CSS3-FLEXBOX], and grid containers [CSS3-GRID-LAYOUT]
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified, except with visible computing to auto if one of overflow-x or overflow-y is not visible
Canonical order: per grammar
Animatable: no

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).

Name: overflow
Value: visible | hidden | clip | scroll | auto
Initial: see individual properties
Applies to: block containers [CSS21], flex containers [CSS3-FLEXBOX], and grid containers [CSS3-GRID-LAYOUT]
Inherited: no
Percentages: N/A
Media: visual
Computed value: see individual properties
Canonical order: per grammar
Animatable: no

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.

Values have the following meanings:

visible
There is no special handling of overflow, that is, it may be rendered outside the box. The box is not a scroll container.
hidden
This value indicates that the box’s content is clipped to its padding box and that no scrolling user interface should be provided by the UA to view the content outside the clipping region. However, the content may still be scrolled programatically, for example using the mechanisms defined in [CSSOM-VIEW], and the box is therefore still a scroll container.
clip
Like hidden, this value indicates that the box’s content is clipped to its padding box and that no scrolling user interface should be provided by the UA to view the content outside the clipping region. In addition, unlike overflow: hidden which still allows programmatic scrolling, overflow: clip forbids scrolling entirely, through any mechanism, and therefore the box is not a scroll container.
scroll
This value indicates that the content is clipped to the padding box, but can be scrolled into view (and therefore the box is a scroll container). Furthermore, if the user agent uses a scrolling mechanism that is visible on the screen (such as a scroll bar or a panner), that mechanism should be displayed whether or not any of its content is clipped. This avoids any problem with scrollbars appearing and disappearing in a dynamic environment. When this value is specified and the target medium is print, overflowing content may be printed.
auto
This value indicates that the box’s content is clipped to the padding box, but can be scrolled into view (and therefore the box is a scroll container). However, if the user agent uses a scrolling mechanism that is visible on the screen (such as a scroll bar or a panner), that mechanism should only be displayed if there is overflow.

If the computed value of overflow on a block box is not visible, it creates a new block formatting context for its content.

3.1. Overflow Value Propagation

UAs must apply the overflow property set on the root element to the viewport. For HTML UAs, if this would result in the viewport having visible overflow, the UA must instead apply the overflow property set on the body element to the viewport. In either case, the used value of overflow for the element from which the value is propagated must evaluate to visible.

If this application would result in the viewport having visible overflow, the UA must instead treat the viewport as having auto overflow.

3.2. Scrollbars and Layout

In the case of a scrollbar being placed on an edge of the element’s box, it should be inserted between the inner border edge and the outer padding edge. Any space taken up by the scrollbars should be taken out of (subtracted from the dimensions of) the containing block formed by the element with the scrollbars.

import examples from [CSS3-BOX].

3.3. Scrolling Origin, Direction, and Restriction

The initial scroll position, that is, the initial position of the box’s scrollable overflow region with respect to its border box, prior to any user or programmatic scrolling that changes it, is dependent on the box’s writing mode, and is by default the block-start/inline-start edge of the box’s padding edge. However, the align-content and justify-content properties [CSS-ALIGN-3] can be used to change this.

Due to Web-compatibility constraints (caused by authors exploiting legacy bugs to surreptitiously hide content from visual readers but not search engines and/or speech output), UAs must clip the scrollable overflow region of scroll containers on the block-start and inline-start sides of the box (thereby behaving as if they had no scrollable overflow on that side).

The viewport uses the principal writing mode for these calculations.

[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?

4. Limiting Number of Visible Text Lines: the max-lines property

Name: max-lines
Value: none | <integer>
Initial: none
Applies to: all non-inline elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value
Canonical order: per grammar
Animatable: as integer

Add back max-lines, so we can kill the unspecified WebKit feature that does this poorly.

5. Overflow in static media

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.

6. Privacy and Security Considerations

This specification introduces no new privacy or security concerns.

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.

Conformance

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.

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.

Conformance classes

Conformance to this specification 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 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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-ALIGN-3]
Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 14 June 2016. WD. URL: https://drafts.csswg.org/css-align/
[CSS-BACKGROUNDS-3]
CSS Backgrounds and Borders Module Level 3 URL: https://drafts.csswg.org/css-backgrounds-3/
[CSS-CONTAINMENT-3]
CSS Containment Module Level 3 URL: https://drafts.csswg.org/css-containment-3/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 15 October 2015. WD. URL: http://dev.w3.org/csswg/css-display/
[CSS-MASKING-1]
Dirk Schulze; Brian Birtles; Tab Atkins Jr.. CSS Masking Module Level 1. 26 August 2014. CR. URL: http://dev.w3.org/fxtf/css-masking-1/
[CSS-TEXT-DECOR-3]
Elika Etemad; Koji Ishii. CSS Text Decoration Module Level 3. 1 August 2013. CR. URL: http://dev.w3.org/csswg/css-text-decor-3/
[CSS-VALUES]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 11 June 2015. CR. URL: http://dev.w3.org/csswg/css-values/
[CSS-WRITING-MODES-3]
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 15 December 2015. CR. URL: http://dev.w3.org/csswg/css-writing-modes-3/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS2
[CSS3-FLEXBOX]
Tab Atkins Jr.; Elika Etemad; Rossen Atanassov. CSS Flexible Box Layout Module Level 1. 26 May 2016. CR. URL: https://drafts.csswg.org/css-flexbox/
[CSS3-GRID-LAYOUT]
Tab Atkins Jr.; Elika Etemad; Rossen Atanassov. CSS Grid Layout Module Level 1. 19 May 2016. WD. URL: https://drafts.csswg.org/css-grid/
[CSS3-TRANSFORMS]
Simon Fraser; et al. CSS Transforms Module Level 1. 26 November 2013. WD. URL: http://dev.w3.org/csswg/css-transforms/
[HTML]
Ian Hickson. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[MEDIAQUERIES-4]
Florian Rivoal; Tab Atkins Jr.. Media Queries Level 4. 6 July 2016. WD. URL: https://drafts.csswg.org/mediaqueries-4/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Informative References

[CSS1]
Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. REC. URL: https://www.w3.org/TR/CSS1/
[CSS3-BOX]
Bert Bos. CSS basic box model. 9 August 2007. WD. URL: https://www.w3.org/TR/css3-box
[CSS3-MARQUEE]
Bert Bos. CSS Marquee Module Level 3. 14 October 2014. NOTE. URL: https://www.w3.org/TR/css3-marquee
[CSS3BG]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 9 September 2014. CR. URL: http://dev.w3.org/csswg/css3-background/
[CSS3TEXT]
Elika Etemad; Koji Ishii. CSS Text Module Level 3. 10 October 2013. LCWD. URL: http://dev.w3.org/csswg/css-text-3/
[CSSOM-VIEW]
Simon Pieters. CSSOM View Module. 17 March 2016. WD. URL: https://drafts.csswg.org/cssom-view/

Property Index

Name Value Initial Applies to Inh. %ages Media Ani­mat­able Canonical order Com­puted value
overflow-x visible | hidden | clip | scroll | auto visible block containers [CSS21], flex containers [CSS3-FLEXBOX], and grid containers [CSS3-GRID-LAYOUT] no N/A visual no per grammar as specified, except with visible computing to auto if one of overflow-x or overflow-y is not visible
overflow-y visible | hidden | clip | scroll | auto visible block containers [CSS21], flex containers [CSS3-FLEXBOX], and grid containers [CSS3-GRID-LAYOUT] no N/A visual no per grammar as specified, except with visible computing to auto if one of overflow-x or overflow-y is not visible
overflow visible | hidden | clip | scroll | auto see individual properties block containers [CSS21], flex containers [CSS3-FLEXBOX], and grid containers [CSS3-GRID-LAYOUT] no N/A visual no per grammar see individual properties
max-lines none | <integer> none all non-inline elements no N/A visual as integer per grammar specified value

Issues Index

There’s disagreement on the scrolling model. 2.1 apparently defined that you scrolled the *content* area; the content would overflow the content-box, and you would union that overflow with the content box to find the scrollable area. In particular, this means that the content would be offset by the start-sides padding, but if it overflowed, it would go right to the edge on the end sides. This is what Firefox and IE do.

At least some authors (and spec authors) instead have the mental model that the padding box is what’s scrollable, so when you scroll to the end of the overflow, there’s the right/bottom padding. Chrome/WebKit do this for the block axis, at least. They’re somewhat inconsistent for the inline axis; there’s something weird about how they handle lineboxes.

It seems that the block-axis padding is probably web-compatible to honor. It’s unclear that the inline-axis padding will be. Further experimentation is needed.

Is this description of handling transforms sufficiently accurate?
Need to evaluate compat of honoring or ignoring clip and clip-path.
It’s not yet clear if including the end-side padding in the scrollable layer is Web-compatible, so this clause is under investigation. It appears that Chrome and Safari include such padding in the block axis; and the behavior in the inline axis is not clear.
This needs further testing and investigation; is therefore deferred in this draft.
import examples from [CSS3-BOX].
[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?
Add back max-lines, so we can kill the unspecified WebKit feature that does this poorly.
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.