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Layout as described by CSS produces boxes that control how content is displayed and positioned. This specification describes an API for accessing information about these boxes.
The layout stage of CSS is responsible for generating the position and size of
a document’s content. During this process, each DOM element produces
potentially many boxes, each of which in turn produce potentially many
This specification describes an API that gives developers access to geometry,
text and other information about boxes and fragments.
2. Boxes and Fragments
The [css-display-3] specification describes the relationship between
elements, boxes, and fragments.
Elements and pseudo-elements generate zero or more fragments, each of which
generates a fragment tree.
Fragments do not in general form a tree that maps cleanly back to the DOM. When
an element generates multiple fragment trees, the element that generates a least common ancestor can be arbitrarily far up the DOM tree.
Assuming that layout places "foo bar" on the first line, and "baz" on the
second, the following HTML produces six fragments in a single tree.
the first letter fragment, parented by the first line box and styled by the first-letter rule.
a fragment containing "oo", parented by the first line box and styled by the first-line rule.
a fragment containing "bar", parented by the first line box and italic.
a fragment containing "baz", parented by the second line box and italic.
The italic element produces two fragments ("bar" and "baz"), each in its own
tree. In this example, the paragraph element generates the common root for
these fragments; however if (for example) the paragraph element were itself a
descendant of a multi-column div then the common root may be further up the
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.
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
Implementations of Unstable and Proprietary Features
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