CSS Text Decoration Module Level 4

1. Introduction

This is the initial draft of CSS Text Decoration Level 4; it is being maintained as a diff spec against CSS Text Decoration Level 3.

2. Additional Controls for Line Decorations

2.1. 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.

2.1.1. Automatic Thickness of Text Decoration Lines

Insert L3 text here.

2.2. 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.

2.3. 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

The line is aligned to the outside of the specified position. (Below for under/auto positions, above for over.)
Positive lengths represent inward distances; negative lengths outward.
Automatic adjustments made to accommodate descendant content are maintained; the text-underline-offset is in addition to those. Should this be removed?

When the value of the text-decoration-line property is either spelling-error or grammar-error, the UA may ignore the value of text-underline-position.

2.4. Text Decoration Line Continuity

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

Name:	text-decoration-skip
Value:	none \| [ objects \|\| [ spaces \| [ leading-spaces \|\| trailing-spaces ] ] \|\| 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

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.

This property will be split into individual properties along the lines of text-decoration-skip-ink, to improve its cascading behavior. See discussion and resolution.

This property specifies what parts of the element’s content any text decoration affecting the element must skip over. It controls all text decoration lines drawn by the element and also any text decoration lines drawn by its ancestors. Values have the following meanings:

none: Skip nothing: text-decoration is drawn for all text content and across atomic inline-level boxes.
objects: Skip this element (its entire margin box) if it is an atomic inline (such as an image or inline-block).
spaces: Skip all spacing, i.e. all typographic character units with the Unicode White_Space property [UAX44], plus any adjacent letter-spacing or word-spacing.
edges: The UA should place the start and end of the line inwards slightly (e.g. by half a line thickness) from the content edge of the decorating box so that, e.g. two underlined elements side-by-side do not appear to have a single underline. (This is important in Chinese, where underlining is a form of punctuation.)

text-decoration-skip: edges for 石井艾俐俐
box-decoration: Skip over the box’s margin, border, and padding areas. Note that this only has an effect on decorations imposed by an ancestor; a decorating box never draws over its own box decoration.
leading-spaces: Skip all spacing, i.e. all typographic character units 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 typographic character units 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.

Note that this property inherits and that descendant elements can have a different setting.

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

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

When the value of the text-decoration-line property is either spelling-error or grammar-error, the UA may ignore the value of text-decoration-skip.

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

Name:	text-decoration-skip-ink
Value:	auto \| none
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 controls how overlines and underlines are drawn when they cross over a glyph.

When enabled, decoration lines skip over where glyphs are drawn: interrupt the decoration line to let the shape of the text show through where the text decoration would otherwise cross over a glyph. The UA must also skip a small distance to either side of the glyph outline.

An alphabetic underline through Myanmar text skips around descenders and the vertical strokes of combining characters that drop below the alphabetic baseline.

text-decoration-skip: ink

Ideographic scripts do not want to skip when auto. How can we define this behavior? Are there more scripts wanting not to skip? Need some normative text describe how auto works. See telcon minutes, alreq#86, csswg#1288

This property only applies to overlines and underlines; line-throughs are unaffected.

auto: UA should skip over where glyphs are drawn.
none: UA must draw contiguous lines without interruptions, even when they cross over a glyph.

3. Additional Controls for Text Shadows

Level 4 adds a spread radius argument to text-shadow, using the same syntax and interpretation as for box-shadow, except that corners are always rounded (since the geometry of a glyph is not so simple as a box).

4. Additional Controls for Emphasis Marks

4.1. 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?

5. 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:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with , like this: UAs MUST provide an accessible alternative.

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.

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

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.

CSS Text Decoration Module Level 4

W3C First Public Working Draft, 13 March 2018

Abstract

Status of this document