CSS Containment Module Level 1

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-contain-1/
Latest published version:
https://www.w3.org/TR/css-contain-1/
Previous Versions:
Test Suite:
http://test.csswg.org/suites/css-contain-1_dev/nightly-unstable/
Editors:
Tab Atkins (Google)
Florian Rivoal (On behalf of Bloomberg)
Issue Tracking:
GitHub Issues
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This CSS module describes the contain property, which indicates that the element’s subtree is independent of the rest of the page. This enables heavy optimizations by user agents when used well.

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

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 https://www.w3.org/TR/.

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.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-contain” in the title, like this: “[css-contain] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 15 September 2020 W3C Process Document.

This document was produced by a group operating under the 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.

A Disposition of Comments and an Implementation Report are available.

1. Introduction

Efficiently rendering a website relies on the User Agent being able to detect what parts of the page are being displayed, which parts might affect the currently-displayed section, and what can be ignored.

There are various heuristics that can be used to guess when a given sub-tree is independent of the rest of the page in some manner, but they’re fragile, so innocuous changes to a page may inadvertently make it fail such heuristic tests, causing rendering to fall into a slow code path. There are also many things that would be good to isolate which are difficult or impossible to detect in a heuristic manner.

To alleviate these problems and allow strong, predictable isolation of a subtree from the rest of the page, this specification defines a contain property.

1.1. Value Definitions

This 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. Strong Containment: the contain property

Name: contain
Value: none | strict | content | [ size || layout || paint ]
Initial: none
Applies to: See below
Inherited: no
Percentages: n/a
Computed value: specified keyword(s), except strict and content (see prose)
Canonical order: per grammar
Animation type: not animatable

User Agents are expected to support this property on all media, including non-visual ones.

The contain property allows an author to indicate that an element and its contents are, as much as possible, independent of the rest of the document tree. This allows user agents to utilize much stronger optimizations when rendering a page using contain properly, and allows authors to be confident that their page won’t accidentally fall into a slow code path due to an innocuous change.

none
This value indicates that the property has no effect. The element renders as normal, with no containment effects applied.
strict
This value computes to size layout paint, and thus turns on all forms of containment for the element.
content
This value computes to layout paint, and thus turns on all forms of containment except size containment for the element.

Note: contain: content is reasonably "safe" to apply widely; its effects are fairly minor in practice, and most content won’t run afoul of its restrictions. However, because it doesn’t apply size containment, the element can still respond to the size of its contents, which can cause layout-invalidation to percolate further up the tree than desired. Use contain: strict when possible, to gain as much containment as you can.

size
The value turns on size containment for the element. This ensures that the containment box can be laid out without needing to examine its descendants.
layout
This value turns on layout containment for the element. This ensures that the containment box is totally opaque for layout purposes; nothing outside can affect its internal layout, and vice versa.
paint
This value turns on paint containment for the element. This ensures that the descendants of the [=paint containment box|containment box] don’t display outside its bounds, so if an element is off-screen or otherwise not visible, its descendants are also guaranteed to be not visible.

This property generally applies to all elements (including CSS Pseudo-Elements 4 §4.1 Generated Content Pseudo-elements: ::before and ::after), although some types of containment have no effect on some elements, as detailed in § 3 Types of Containment. In addition, in the case of [SVG2], the contain property only applies to svg elements that have an associated CSS layout box.

contain is useful when used widely on a page, particularly when a page contains a lot of "widgets" which are all independent.

For example, assume a micropost social network had markup something like this:

<body>
  <aside>...</aside>
  <section>
    <h2>Messages</h2>
    <article>
      Lol, check out this dog: images.example.com/jsK3jkl
    </article>
    <article>
      I had a ham sandwich today. #goodtimes
    </article>
    <article>
      I have political opinions that you need to hear!
    </article></section>
</body>

There are probably a lot of messages displayed on the site, but each is independent and won’t affect anything else on the site. As such, each can be marked with contain: content to communicate this to the user agent, so it can optimize the page and skip a lot of computation for messages that are off-screen. If the size of each message is known ahead of time, contain: strict can be applied to communicate further restrictions.

3. Types of Containment

There are several varieties of containment that an element can be subject to, restricting the effects that its descendants can have on the rest of the page in various ways. Containment enables much more powerful optimizations by user agents, and helps authors compose their page out of functional units, as it limits how widely a given change can affect a document.

Specification authors introducing new properties or mechanisms need to consider whether and how the various types of containment affect what they are introducing, and include in their specification any effect not described here.

3.1. Size Containment

If the element does not generate a principal box (as is the case with display: contents or display: none), or its inner display type is table, or its principal box is an internal table box, or an internal ruby box, or a non-atomic inline-level box, size containment has no effect. Otherwise, giving an element size containment makes its principal box a size containment box and has the following effects:

Note: Internal table boxes, which do not include table captions, are excluded, because the table layout algorithm does not allow boxes to become smaller than their inflow content. Sizing a table cell as if it was empty and then layout out its content inside without changing the size is effectively an undefined operation. Manually setting the width or height properties to 0 cannot make it smaller than its content. This concern does not apply to table captions, which are perfectly capable of having a fixed size that is independent of their content.

  1. When calculating the size of the size containment box, including when computing its intrinsic size, it must be treated as having no contents.

    Note: Even when the element’s sizing properties are auto this does not necessarily make the element zero-sized: properties set on the element itself, such as the columns property or the grid property, continue to be taken into account.

    Then, its contents must then be laid out into the size containment box's resolved size normally (including any pseudo-elements), i.e. size containment has no effect in this phase.

    Note: size containment does not suppress baseline alignment. See layout containment for that.

    Replaced elements must be treated as having an intrinsic width and height of 0.

  2. Size containment boxes must be treated as if they had no intrinsic aspect ratio.

    Note: Size containment only suppresses the intrinsic aspect ratio, not preferred aspect ratio, so properties like aspect-ratio which affect that preferred aspect ratio directly are honored.

    Given the following markup and style, the image would be sized to 100px by 100px, as the aspect ratio set by the aspect-ratio property takes effect.
    img {
      width: 100px;
      aspect-ratio: 1/1;
      contain: size;
    }
    <img src="https://www.example.com/300x100.jpg">
    

    If the aspect-ratio property had not been declared, the image would have been 100px by 0px, as its intrinsic aspect ratio is suppressed, and its intrinsic height is treated as 0.

  3. Size containment boxes are monolithic (See CSS Fragmentation 3 §4.1 Possible Break Points).

By itself, size containment does not offer much optimization opportunity. Its primary benefit on its own is that tools which want to lay out the containment box's contents based on the containment box's size (such as a JS library implementing the "container query" concept) can do so without fear of "infinite loops", where having a child’s size respond to the size of the containment box causes the containment box's size to change as well, possibly triggering further changes in how the child sizes itself and possibly thus more changes to the containment box's size, ad infinitum.

When paired with layout containment, though, possible optimizations that can be enabled include (but are not limited to):

  1. When the style or contents of a descendant of the containment box is changed, calculating what part of the DOM tree is "dirtied" and might need to be re-laid out can stop at the containment box.

  2. When laying out the page, if the containment box is off-screen or obscured, the layout of its contents can be delayed or done at a lower priority.

3.2. Layout Containment

If the element does not generate a principal box (as is the case with display values of contents or none), or its principal box is an internal table box other than table-cell, or an internal ruby box, or a non-atomic inline-level box, layout containment has no effect. Otherwise, giving an element layout containment makes its principal box a layout containment box and has the following effects:

  1. The layout containment box establishes an independent formatting context.

  2. If at least one fragmentation container of a fragmentation context has layout containment, or if at least one fragmentation container of a fragmentation context is a descendant of layout containment box and at least one subsequent fragmentation container of the same fragmentation context is not a descendant of that same element with layout containment, then the first layout containment box which is either a fragmentation container itself or is an ancestor of a fragmentation container must “trap” the remainder of the fragmented flow: fragmentation must not continue past the layout containment boundary, and the last fragmentation container within the first layout containment boundary is treated as if it is the last fragmentation container in its fragmentation context.

    If subsequent fragmentation containers in the fragmentation context are only generated when more content remains in the fragmented flow, then they are not generated. If they would exist regardless, they remain part of the fragmentation context, but do not receive any content from the fragmented flow.

    Note: At the time of writing, no stable specification is affected by this point. Only specifications that would enable some (but not all) fragmentation containers of a fragmentation context to be layout-contained (or descendants of a layout contained element) are concerned. This is not the case of [CSS-PAGE-3] nor of [CSS-MULTICOL-1]. This requirement is nonetheless included because several mechanisms that would make this a possibility have been considered (e.g.: [CSS-REGIONS-1], ::nth-fragment(), a hypothetical selector for individual columns of a multicol…), and the guarantees that layout containment is intended to offer would not be realized if such mechanisms did not abide by this rule. [CSS-REGIONS-1] has details over how layout containment affects regions.

    <article>Lorem ipsum…</article>
    <div id=a></div>
    <aside>
      <div id=b></div>
      <div id=c></div>
    </aside>
    <aside>
      <div id=d></div>
      <div id=e></div>
    </aside>
    <div id=f></div>
    
    article {flow-into: foo;}
    #a, #b, #c, #d, #e, #f {flow-from: foo;}
    aside {contain: layout}
    

    In this [CSS-REGIONS-1] example, content can flow from #a to #b, from #b to #c. However as #c is the last fragment container in the first layout containment box it traps all the remaining content, and nothing gets flowed into #d, #e, or #f.

  3. If the computed value of the overflow property is either visible or clip or a combination thereof, any overflow must be treated as ink overflow.

  4. The layout containment box acts as a containing block for absolutely positioned and fixed positioned descendants.

  5. The layout containment box creates a stacking context.

  6. Forced breaks are allowed within layout containment boxes but do not propagate to the parent as otherwise described in CSS Fragmentation 3 §3.1 Breaks Between Boxes: the break-before and break-after properties.

    Note: This introduces the previously non-existent possibility that forced breaks may occur between a box and its container (See CSS Fragmentation 3 §4.1 Possible Break Points).

  7. For the purpose of the vertical-align property, or any other property whose effects need to relate the position of the layout containment box's baseline to something other than its descendants, the containment box is treated as having no baseline.

Possible optimizations that can be enabled by layout containment include (but are not limited to):

  1. When laying out the page, the contents of separate containment boxes can be laid out in parallel, as they’re guaranteed not to affect each other.

  2. When laying out the page, if the [=layout containment box|containment box] is off-screen or obscured and the layout of the visible parts of the screen do not depend on the size of the containment box (for example, if the containment box is near the end of a block container, and you’re viewing the beginning of the block container), the layout of the [=layout containment box|containment box]' contents can be delayed or done at a lower priority.

    (When paired with size containment, this optimization can be applied more liberally.)

3.3. Paint Containment

If the element does not generate a principal box (as is the case with display values of contents or none), or its principal box is an internal table box other than table-cell, or an internal ruby box, or a non-atomic inline-level box, paint containment has no effect. Otherwise, giving an element paint containment makes its principal box a paint containment box and has the following effects:

  1. The contents of the element including any ink or scrollable overflow must be clipped to the padding edge of the paint containment box, taking corner clipping into account. This does not include the creation of any mechanism to access or indicate the presence of the clipped content; nor does it inhibit the creation of any such mechanism through other properties, such as overflow, resize, or text-overflow.

    Note: The next level of this specification [CSS-CONTAIN-2] refines this effect to apply to the overflow clip edge rather than the padding edge, in order to take the new overflow-clip-margin property into account. For implementations that do not support overflow-clip-margin, the effect is identical.

    Note: The behavior is described in this paragraph is equivalent to changing overflow-x: visible into overflow-x: clip and overflow-y: visible into overflow-y: clip at used value time, while leaving other values of overflow-x and overflow-y unchanged.

  2. The paint containment box acts as a containing block for absolutely positioned and fixed positioned descendants.

  3. The paint containment box creates a stacking context.

  4. The paint containment box establishes an independent formatting context.

Possible optimizations that can be enabled by paint containment include (but are not limited to):
  1. If the containment box is off-screen or obscured, the UA can directly skip trying to paint its contents, as they’re guaranteed to be off-screen/obscured as well.

  2. Unless the clipped content is made accessible via a separate mechanism such as the overflow, resize, or text-overflow properties, the UA can reserve "canvas" space for the box exactly the box’s size. (In similar, scrollable, situations, like overflow: hidden, it’s possible to scroll to the currently-clipped content, so UAs often predictively overpaint somewhat so there’s something to see as soon as the scroll happens, rather than a frame later.)

  3. Because they are guaranteed to be stacking contexts, scrolling elements can be painted into a single GPU layer.

4. Privacy and Security Considerations

This specification introduces no new privacy or security considerations.

Like any other CSS specification, it affects the rendering of the document, but does not introduce any special ability to present content in a misleading way that was not previously available through other CSS modules and that isn’t inherent to the act of formatting the document.

The TAG has developed a self-review questionnaire to help editors and Working Groups evaluate the risks introduced by their specifications. Answers are provided below.

Does this specification deal with personally-identifiable information?
No.
Does this specification deal with high-value data?
No.
Does this specification introduce new state for an origin that persists across browsing sessions?
No.
Does this specification expose persistent, cross-origin state to the web?
No.
Does this specification expose any other data to an origin that it doesn’t currently have access to?
No.
Does this specification enable new script execution/loading mechanisms?
No.
Does this specification allow an origin access to a user’s location?
No.
Does this specification allow an origin access to sensors on a user’s device?
No.
Does this specification allow an origin access to aspects of a user’s local computing environment?
No.
Does this specification allow an origin access to other devices?
No.
Does this specification allow an origin some measure of control over a user agent’s native UI?
No.
Does this specification expose temporary identifiers to the web?
No.
Does this specification distinguish between behavior in first-party and third-party contexts?
No.
How should this specification work in the context of a user agent’s "incognito" mode?
No difference in behavior is needed.
Does this specification persist data to a user’s local device?
No.
Does this specification have a "Security Considerations" and "Privacy Considerations" section?
Yes, this is the section you are currently reading.
Does this specification allow downgrading default security characteristics?
No.

Appendix A. Changes

This appendix is informative.

Changes from the Recommendation of 21 November 2019

Changes from the Candidate Recommendation of 30 April 2019

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.

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.

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.

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.

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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 17 October 2017. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-BREAK-3]
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 4 December 2018. CR. URL: https://www.w3.org/TR/css-break-3/
[CSS-CONTAIN-2]
Tab Atkins Jr.; Florian Rivoal; Vladimir Levin. CSS Containment Module Level 2. 3 June 2020. WD. URL: https://www.w3.org/TR/css-contain-2/
[CSS-DISPLAY-3]
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 19 May 2020. CR. URL: https://www.w3.org/TR/css-display-3/
[CSS-IMAGES-3]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 3. 10 October 2019. CR. URL: https://www.w3.org/TR/css-images-3/
[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 3 June 2020. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-SIZING-3]
Tab Atkins Jr.; Elika Etemad. CSS Box Sizing Module Level 3. 23 October 2020. WD. URL: https://www.w3.org/TR/css-sizing-3/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 31 January 2019. WD. URL: https://www.w3.org/TR/css-values-4/
[CSS2]
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/
[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
[SVG2]
Amelia Bellamy-Royds; et al. Scalable Vector Graphics (SVG) 2. 4 October 2018. CR. URL: https://www.w3.org/TR/SVG2/

Informative References

[CSS-CONTENT-3]
Elika Etemad; Dave Cramer. CSS Generated Content Module Level 3. 2 August 2019. WD. URL: https://www.w3.org/TR/css-content-3/
[CSS-GRID-1]
Tab Atkins Jr.; et al. CSS Grid Layout Module Level 1. 21 October 2020. CR. URL: https://www.w3.org/TR/css-grid-1/
[CSS-MULTICOL-1]
Håkon Wium Lie; Florian Rivoal; Rachel Andrew. CSS Multi-column Layout Module Level 1. 15 October 2019. WD. URL: https://www.w3.org/TR/css-multicol-1/
[CSS-OVERFLOW-4]
David Baron; Florian Rivoal. CSS Overflow Module Level 4. 13 June 2017. WD. URL: https://www.w3.org/TR/css-overflow-4/
[CSS-PAGE-3]
Elika Etemad; Simon Sapin. CSS Paged Media Module Level 3. 18 October 2018. WD. URL: https://www.w3.org/TR/css-page-3/
[CSS-REGIONS-1]
Rossen Atanassov; Alan Stearns. CSS Regions Module Level 1. 9 October 2014. WD. URL: https://www.w3.org/TR/css-regions-1/
[CSS-SIZING-4]
Tab Atkins Jr.; Elika Etemad. CSS Box Sizing Module Level 4. 20 October 2020. WD. URL: https://www.w3.org/TR/css-sizing-4/
[CSS-UI-3]
Tantek Çelik; Florian Rivoal. CSS Basic User Interface Module Level 3 (CSS3 UI). 21 June 2018. REC. URL: https://www.w3.org/TR/css-ui-3/
[CSS-UI-4]
Florian Rivoal. CSS Basic User Interface Module Level 4. 24 January 2020. WD. URL: https://www.w3.org/TR/css-ui-4/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
contain none | strict | content | [ size || layout || paint ] none See below no n/a not animatable per grammar specified keyword(s), except strict and content (see prose)