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
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.
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).
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",
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 to this specification
is defined for three 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.
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.
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
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