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The API exposed by this specification is designed to provide basic font metrics
for both in-document and out-of-document content.
Note: In a future version of this spec support may be added for exposing
information about individual runs of text, including information about
directionality, script, and character properties.
width The advance width of the line box, in CSS pixels.
advances List of advances for each codepoint in the given text relative to the preceding
codepoint, in CSS pixels. Where a glyph is composed of a sequence of codepoints
the advance for the all but the first codepoint in the sequence will be zero.
boundingBoxLeft The distance parallel to the dominantBaseline from the alignment
point given by the text-align property to the left side of the bounding
rectangle of the given text, in CSS pixels; positive numbers indicating a
distance going left from the given alignment point.
Note: The sum of this value and boundingBoxRight can be wider
than the width, in particular with slanted fonts where
characters overhang their advance width.
boundingBoxRight The distance parallel to the dominantBaseline from the alignment
point given by the text-align property to the right side of the bounding
rectangle of the given text, in CSS pixels. Positive numbers indicating a
distance going right from the given alignment point.
height The distance between the highest top and the lowest bottom of the em squares in
the line box, in CSS pixels.
emHeightDescent The distance from the dominantBaseline to the lowest bottom of
the em squares in the line box, in CSS pixels.
Positive numbers indicating that the dominantBaseline is below
the bottom of that em square (so this value will usually be negative).
Zero if the dominantBaseline is the bottom of that em square.
Note: This value and fontBoundingBoxDescent are useful when
metrics independent of the actual text being measured are desired as the values
will be consistent regardless of the text as long as the same fonts are being
The boundingBoxAscent attribute (and its corresponding attribute
for the descent) are useful when metrics specific to the given text are desired.
glyphsRendered Number of glyphs used from the specific font. If multiple fonts are required to
render the specified text this attribute will indicate how many glyphs where
used from each font.
Note: Indicates the number of glyphs which may be lower than the number of
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