CSS Text Decoration Module Level 4

Unofficial Proposal Draft,

This version:
https://drafts.csswg.org/css-text-decor-4/
Latest published version:
https://www.w3.org/TR/css-text-decor-4/
Issue Tracking:
Inline In Spec
http://www.w3.org/Style/CSS/Tracker/products/10
Editors:
Elika J. Etemad / fantasai (Invited Expert)
(Invited Expert)

Abstract

This module contains the features of CSS relating to text decoration, such as underlines, text shadows, and emphasis marks.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This is just a place to keep ideas at the moment. It has no status at W3C and has not yet been discussed by the CSSWG.

1. Things To Include Here

In addition to all of the features in Level 3, a completed initial draft of this module will include:

1.1. Text Decoration Line Thickness: the text-decoration-width property

Name: text-decoration-width
Value: auto | <length>
Initial: auto
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: as specified
Canonical order: per grammar
Animatable: no

This property, which is also a sub-property of the text-decoration shorthand, sets the stroke thickness of underlines, overlines, and line-throughs.

Values have the following meanings:

auto
The UA chooses an appropriate thickness for text decoration lines; see below.
<length>
Specifies the thickness of text decoration lines as a length. The UA must floor the actual value at one device pixel. Authors are strongly encouraged to use em units so that the line thickness scales with the font.

1.1.1. Automatic Thickness of Text Decoration Lines

Insert L3 text here.

1.2. Determining the Position and Thickness of Line Decorations

This section is copied over from early drafts of Text Decoration Level 3. It is still under review, and needs integration with text-underline-offset and text-decoration-width.

Since line decorations can span elements with varying font sizes and vertical alignments, the best position for a line decoration is not necessarily the ideal position dictated by the decorating box. Instead, it’s calculated, per line, from all text decorated by the decorating box on that line, the considered text. However, descendants of the decorating box that are skipped due to text-decoration-skip, descendant inlines with text-decoration-skip: ink, and any descendants that do not participate in the decorating box’s inline formatting context are excluded from the set of considered text.

The line decoration positions are then calculated per line as follows (treating over-positioned underlines as over lines and under-positioned overlines as under lines):

over lines
Align the line decoration with respect to the highest over EM-box edge of the considered text.
alphabetic underlines

The alphabetic underline position is calculated by taking the ideal offset (from the alphabetic baseline) of each run of considered text, averaging those, and then using the lowest alphabetic baseline to actually position the line. (Alphabetic baselines can differ between baseline-aligned boxes if the dominant baseline is non-alphabetic.) To prevent superscripts and subscripts from throwing this position off-kilter, an inline with a non-initial computed vertical-align is treated as having the ideal underline position of its parent.

non-alphabetic under lines
Position the line decoration with respect to the lowest under EM-box edge of the considered text.
line-throughs
Line-throughs essentially use the same sort of averaging as for alphabetic underlines, but recompute the position when drawing across a descendant with a different computed font-size. (This ensures that the text remains effectively “crossed out” despite any font size changes.) For each run of considered text with the same font-size, compute an ideal position averaged from its font metrics. To prevent superscripts and subscripts from throwing this position off-kilter, an inline with a non-initial computed vertical-align is treated as having the ideal underline position of its parent. Position the portion of the line across each decorated fragment at that position.

For simplicity, line-throughs should draw over each element at that element’s preferred/averaged position. This can produce some undesirable jumpiness, but there doesn’t appear to be any way to avoid that which is correct in all instances, and all attempts are worryingly complex. What position should line-throughts adopt over elements that have a different font-size, but no considered text?

CSS does not define the thickness of line decorations. In determining the thickness of text decoration lines, user agents may consider the font sizes, faces, and weights of descendants to provide an appropriately averaged thickness.

The following figure shows the averaging for underline:

In the first rendering of the underlined text '1st a'
								 with 'st' as a superscript, both the '1st' and the 'a'
								 are rendered in a small font. In the second rendering,
								 the 'a' is rendered in a larger font. In the third, both
								 '1st' and 'a' are large.

In the three fragments of underlined text, the underline is drawn consecutively lower and thicker as the ratio of large text to small text increases.

Using the same example, a line-through would in the second fragment, instead of averaging the two font sizes, split the line-through into two segments:

In both cases, however, the superscript, due to the vertical-alignment shift, has no effect on the position of the line.

1.3. Text Underline Offset: the text-underline-offset property

Name: text-underline-offset
Value: auto | <length>
Initial: auto
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
Canonical order: per grammar
Animatable: no

This property, which is not a sub-property of the text-decoration shorthand, sets the offset of underlines from their initial position.

Values have the following meanings:

auto
The UA chooses an appropriate offset for underlines.
<length>
Specifies the offset of underlines as a length. This replaces any information in the font or derived from glyph shapes / character ranges. Authors are strongly encouraged to use em units so that the offset scales with the font.

The initial position of the underline depends on the value of text-underline-position as detailed below.

Interaction of text-underline-position and text-underline-offset
text-underline-position Initial Position Positive Direction
auto alphabetic baseline over
under text-under edge over
over text-over edge under

1.4. Text Decoration Line Continuity: the text-decoration-skip property

This specification extends the text-decoration-skip property present in level 3 by adding the trailing-spaces value.

Name: text-decoration-skip
Value: none | [ objects || [ spaces | [ leading-spaces || trailing-spaces ] ] || ink || edges || box-decoration ]
Initial: objects leading-spaces trailing-spaces
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified
Canonical order: per grammar
Animatable: no

Note: The initial value is different from the one specified at the previous level, which was only text-decoration-skip: objects.

The initial value is quite verbose, which makes it inconvenient to set the property to the initial value plus something else. We should make this easier to specify.

Add final level 3 content

none
objects
spaces
ink
edges
box-decoration

Add final level 3 content

leading-spaces
Skip all spacing, i.e. all characters with the Unicode White_Space property [UAX44] and all word separators plus any adjacent letter-spacing or word-spacing, when located at the start of the line.
trailing-spaces
Skip all spacing, i.e. all characters with the Unicode White_Space property [UAX44] and all word separators plus any adjacent letter-spacing or word-spacing, when located at the end of the line.

The following addition are made to the default UA stylesheet for HTML:

ins, del { text-decoration-skip: none; }

2. Rescued L3 Brainstorming: Ignore For Now

2.1. Emphasis Mark Skip: the text-emphasis-skip property

This section is under brainstorming. It’s also not yet clear if this property is needed quite yet, despite differences in desired behavior among publications.

Name: text-emphasis-skip
Value: spaces || punctuation || symbols || narrow
Initial: spaces
Applies to: all elements
Inherited: yes
Percentages: N/A
Media: visual
Computed value: as specified

This property describes for which characters marks are drawn. The values have following meanings:

spaces
Skip Word separators or characters belonging to the Unicode separator category (Z*). (But note that emphasis marks are drawn for a space that combines with any combining characters.)
punctuation
Skip punctuation. Punctuation in this definition includes characters belonging to the Unicode Pc, Pd, Ps, Pe, Pi, or Pf categories. It also includes characters where the Unicode category is Po and the Sentence_Break property [UAX29] of the Unicode database [UAX44] is ATerm, Close, SContinue, or STerm.
symbols
Skip symbols. Symbols in this definition includes characters belonging to the Unicode S* category. It also includes the Unicode Po category that are not defined as punctuation above.
narrow
Skip characters where the East_Asian_Width property [UAX11] of the Unicode database [UAX44] is not F (Fullwidth) or W (Wide).

Characters belonging to the Unicode classes for control codes and unassigned characters (Cc, Cf, Cn) are skipped regardless of the value of this property.

This syntax requires UA to implement drawing marks for spaces. Is there any use case for doing so? If not, should we modify the syntax not to allow drawing marks for spaces?

3. Privacy and Security Considerations

This specification introduces no new privacy or security considerations.

Conformance

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.
renderer
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.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-CASCADE-4]
Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. URL: http://dev.w3.org/csswg/css-cascade/
[CSS-FONTS-3]
John Daggett. CSS Fonts Module Level 3. URL: http://dev.w3.org/csswg/css-fonts/
[CSS-LINE-GRID-1]
Elika Etemad; Koji Ishii; Alan Stearns. CSS Line Grid Module Level 1. URL: http://dev.w3.org/csswg/css-line-grid/
[CSS-RUBY-1]
Elika Etemad; Koji Ishii. CSS Ruby Layout Module Level 1. URL: http://dev.w3.org/csswg/css-ruby-1/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. URL: https://drafts.csswg.org/css-values/
[CSS-WRITING-MODES-4]
CSS Writing Modes Module Level 4 URL: https://drafts.csswg.org/css-writing-modes-4/
[CSS22]
Bert Bos. Cascading Style Sheets Level 2 Revision 2 (CSS 2.2) Specification. URL: http://dev.w3.org/csswg/css2/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[UAX11]
Asmus Freytag. East Asian Width. 23 March 2001. Unicode Standard Annex #11. URL: http://www.unicode.org/unicode/reports/tr11/tr11-8.html
[UAX29]
Mark Davis. Text Boundaries. 25 March 2005. Unicode Standard Annex #29. URL: http://www.unicode.org/unicode/reports/tr29/tr29-9.html
[UAX44]
Mark Davis; Ken Whistler. Unicode Character Database. 25 September 2013. URL: http://www.unicode.org/reports/tr44/

Property Index

Name Value Initial Applies to Inh. %ages Media Ani­mat­able Canonical order Com­puted value Computed value
text-decoration-width auto | <length> auto all elements no N/A visual no per grammar as specified
text-underline-offset auto | <length> auto all elements yes N/A visual no per grammar as specified
text-decoration-skip none | [ objects || [ spaces | [ leading-spaces || trailing-spaces ] ] || ink || edges || box-decoration ] objects leading-spaces trailing-spaces all elements yes N/A visual no per grammar as specified
text-emphasis-skip spaces || punctuation || symbols || narrow spaces all elements yes N/A visual as specified

Issues Index

This is just a place to keep ideas at the moment. It has no status at W3C and has not yet been discussed by the CSSWG.
Insert L3 text here.
This section is copied over from early drafts of Text Decoration Level 3. It is still under review, and needs integration with text-underline-offset and text-decoration-width.
For simplicity, line-throughs should draw over each element at that element’s preferred/averaged position. This can produce some undesirable jumpiness, but there doesn’t appear to be any way to avoid that which is correct in all instances, and all attempts are worryingly complex. What position should line-throughts adopt over elements that have a different font-size, but no considered text?
Should this be removed?
The initial value is quite verbose, which makes it inconvenient to set the property to the initial value plus something else. We should make this easier to specify.
Add final level 3 content
Add final level 3 content
This section is under brainstorming. It’s also not yet clear if this property is needed quite yet, despite differences in desired behavior among publications.
This syntax requires UA to implement drawing marks for spaces. Is there any use case for doing so? If not, should we modify the syntax not to allow drawing marks for spaces?