CSS content-size Property

Unofficial Proposal Draft,

This version:
Tab Atkins-Bittner (Google)
Suggest an Edit for this Spec:
GitHub Editor
Issue Tracking:
GitHub Issues


This specification defines the content-size property, which, in conjunction with size containment, defines a 'default size' for the contents of an element.

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

1. Introduction

When size containment is applied to an element, it lays out as if it were completely empty, ignoring any child content it might have. This directive lets the layout engine "scope" sizing-related layout changes; the UA knows, for 100% certain, that no changes in the contents of the element will have any effect on the element’s own size, and so the UA can immediately skip redoing layout for the element or its ancestors without having to rely on heuristics.

However, making the element completely empty isn’t always desirable; it can allow the element to shrink down to zero size. Setting an explicit width/height (or min-width/min-height) on the element can prevent this, but that can have its own possibly unwanted implications on layout, making it act differently than an ordinary element with children would in the same situation.

The content-size property, when set on an element with size containment, causes the element to pretend to have a single, fixed-size child of the specified size, rather than pretending to be completely empty. This accomplishes the same "hiding" of layout dirtying that size containment normally does, but allows the author to provide a more useful "default size" for the element, and avoid accidentally letting the element shrink down to a useless size.

2. The content-size Property

Name: content-size
Value: none | <length>{1,2}
Initial: none
Applies to: elements with size containment
Inherited: no
Percentages: n/a
Computed value: the specified keyword, or a pair of absolutized <length>s
Canonical order: per grammar
Animation type: discrete

The content-size property specifies whether a container with size containment should pretend to be empty for layout purposes, or pretend to have a single child of the specified size. Its values are:


If the element has size containment and is a non-replaced element, it lays out as if it were empty.

If the element is a replaced element, its intrinsic width and height are both zero.


If the element has size containment and is a non-replaced element, it lays out as if it had a single child element, with the child‘s width property set to the first <length>, and its height property set to the second <length> (defaulting to the first if only one is specified).

If the element is a replaced element, its intrinsic width is the first <length>, and its intrinsic height is the second <length> (defaulting to the first if only one is specified).

If the element does not have size containment, this property has no effect.


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.
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.


Terms defined by this specification

Terms defined by reference


Normative References

Tab Atkins Jr.; Florian Rivoal. CSS Containment Module Level 1. 30 April 2019. CR. URL: https://www.w3.org/TR/css-contain-1/
Tab Atkins Jr.; Elika Etemad. CSS Intrinsic & Extrinsic Sizing Module Level 3. 22 May 2019. WD. URL: https://www.w3.org/TR/css-sizing-3/
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 31 January 2019. CR. URL: https://www.w3.org/TR/css-values-3/
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/
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/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
content-size none | <length>{1,2} none elements with size containment no n/a discrete per grammar the specified keyword, or a pair of absolutized <length>s