CSS Containment Module Level 3

1. Privacy Considerations

Not applicable.

2. Security Considerations

Not applicable.

Appendix A. Changes

This appendix is informative.

Changes since the 18 August 2022 Working Draft

Significant changes since the 18 August 2022 Working Draft include:

Move the definition of inline-size containement from this Level 3 specification to Level 2. (Issue 10433)
Move Container Queries to [CSS-CONDITIONAL-5] (Issue 10433)

Prior to the move, the following significant changes had been made, and are reflected in [CSS-CONDITIONAL-5]:

List all values of contain in order to clarify serialization order (Issue 8600)
Add the containerName and containerQuery attributes. (Issue 7033)
Correct typo in container-type syntax, to clarify that normal cannot be combined with other values. (Issue 7669)
Allow var() references in container size queries. (Issue 8088)
Allow comma-separated <container-condition>s to match different containers. (Issue 7551)

Changes since the 21 December 2021 First Public Working Draft

Significant changes since the 21 December 2021 First Public Working Draft include:

Allow the computed value of container-name to include duplicate identifiers. (Issue 7181)
Make the <'container-name'> in the container shorthand required. (Issue 7142)
Clarify handling of shorthand properties in container style queries. (Issue 7095)
Cascade-dependent keywords are not allowed as values in a style feature, and cause the container style query to be false. (Issue 7080)
Change the initial value of container-type to be normal, which establishes elements as containers for style features. (Issue 6393, Issue 7066, Issue 7402)
Remove the block-size value from container-type, since single-axis block-size containment is not currently possible. (Issue 1031)
Remove the <string> option from the container-name syntax. Container names must be <custom-ident>s. (Issue 6405)
Reverse the order of <'container-name'> and <'container-type'> in the container shorthand property, with both being optional. (Issue 6393)
Allow <general-enclosed> syntax in <container-condition>s, for the sake of forward compatibility. (Issue 6396)
Remove the size function syntax from <size-feature> queries. (Issue 6870)
Update the query container selection process to account for necessary container-types, and removed the explicit type-selection syntax. (Issue 6644)
Remove state query features, which have been deferred. (Issue 6402)
Clarify container selection around pseudo-elements and the shadow-DOM. (Issue 5984 and Issue 6711)

Changes from CSS Containment Level 2

Introduces inline-size containment.
Defines the terms, properties, units, and at-rule needed for Container Queries

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:

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

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

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

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.

CSS Containment Module Level 3

Abstract

Status of this document