This CSS module defines properties for text manipulation and specifies their processing model. It covers line breaking, justification and alignment, white space handling, and text transformation.
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
This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents by W3C.
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This module describes the typesetting controls of CSS;
that is, the features of CSS that control the translation of
source text to formatted, line-wrapped text.
This specification describes features introduced in Level 5;
it is a diff against [css-text-4].
Specifies that all lines in the container are scaled with a single
scaling factor.
This keyword has no effect if none is specified.
If none of consistent, per-line, or
per-line-all are specified, consistent is
assumed.
per-line
Specifies that each line is scaled with its own scaling factor.
However, the last line of the block and lines that end in a forced
break are not scaled.
This keyword has no effect if none is specified.
per-line-all
Specifies that each line is scaled with its own scaling factor,
including the last line of the block and lines that end in a forced
break.
This keyword has no effect if none is specified.
<percentage>
Specifies the limit of the scaling factor.
If grow is specified and the value is 100% or greater,
it is the maximum scaling factor.
If shrink is specified and the value is between 0% and
100% inclusive, it is the minimum scaling factor.
Otherwise, or if this component is omitted, there is no limit on
the scaling factor.
This property provides a functionality to make inline-level contents
exactly fill the inline size of the line box.
Unlike justify of the text-align property, which achieves
this by adjusting spacing between characters, this property scales the
font size.
When grow or shrink is specified,
a line scaling factor is computed for each line box.
If consistent applies, all line boxes are scaled by the
smallest computed line scaling factor.
Otherwise, each line box is scaled by its own line scaling factor.
This property does not affect the font-sizecomputed value,
and thus does not affect font-size-relative <length> values of other properties.
For example, "line-height: 1.5em" and "letter-spacing: 0.1em" are not
affected by this scaling.
Scaling can change the block size of a line box,
which can in turn change its position along the block-axis.
If any feature is active that would cause the inline size of the
line box to change based on its block-axis position (such as
float or initial-letter), scaling is disabled for the block.
If the inline size of a block container depends on the size of
the viewport, its apparent inline size may not change even if
the user changes the page zoom level.
In that case, if text is fitted with this feature, the text size may
not change at all even though the zoom level has changed.
There is no agreement yet on how to deal with this issue. See
csswg#12886.
1.1.1. Computing line scaling factor
The parts of the line box’s contents that can be scaled by this
property are called scalable parts.
These include:
Text, including text in inline boxes, but excluding trailing
white space.
A simple calculation for the line scaling factor would be
(A + B) / A, where A is the total inline size of
scalable parts and B is the remaining space.
However, due to optical sizing, the
inline size of text might not be perfectly proportional to its
font-size.
The exact method for determining a reasonable line scaling factor
is up to the implementation.
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.
Tests
Tests relating to the content of this specification
may be documented in “Tests” blocks like this one.
Any such block is non-normative.
Conformance classes
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.
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
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.
If the inline size of a block container depends on the size of
the viewport, its apparent inline size may not change even if
the user changes the page zoom level.
In that case, if text is fitted with this feature, the text size may
not change at all even though the zoom level has changed.
There is no agreement yet on how to deal with this issue. See
csswg#12886.
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