Title: CSS Text Decoration Module Level 4
Shortname: css-text-decor
Level: 4
Status: FPWD
Date: 2018-03-13
Prepare for TR: true
Work Status: Exploring
Group: csswg
ED: https://drafts.csswg.org/css-text-decor-4/
TR: https://www.w3.org/TR/css-text-decor-4/
Issue Tracking: Tracker http://www.w3.org/Style/CSS/Tracker/products/10
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Koji Ishii, Google, kojiishi@gmail.com, w3cid 45369
Abstract: This module contains the features of CSS relating to text decoration, such as underlines, text shadows, and emphasis marks.
Link Defaults: css-color-3 (property) color, css-break-3 (dfn) fragment
Ignored Terms: svg shape, svg shapes, invalid, repeatable list, simple list, valid image

spec:css-text-3; type:dfn; text:character

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.

Additional Controls for Line Decorations

Text Decoration Line Thickness: the 'text-decoration-width' property

			Name: text-decoration-width
			Value: auto | from-font | <>
			Initial: auto
			Applies to: all elements
			Inherited: no
			Percentages: N/A
			Media: visual
			Computed value: as specified
			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.
from-font
If the first available font has metrics indicating a preferred underline width, use that width, otherwise behaves as ''text-decoration-width/auto''.
<>
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.

Automatic Thickness of Text Decoration Lines

Issue: Insert L3 text here.

Determining the Position and Thickness of Line Decorations

Issue: 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 ''vertical-align/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.

Text Underline Offset: the 'text-underline-offset' property

			Name: text-underline-offset
			Value: auto | from-font | <>
			Initial: auto
			Applies to: all elements
			Inherited: yes
			Percentages: N/A
			Media: visual
			Computed value: as specified
			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.
from-font
If the first available font has metrics indicating a preferred underline offset, use that offset, otherwise behaves as ''text-underline-offset/auto''.
<>
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
''text-underline-position/auto'' alphabetic baseline over
''text-underline-position/under'' text-under edge over
''text-underline-position/over'' text-over edge under
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'.

Text Decoration Line Continuity

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
			Animatable: no
Issue: 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. Issue: 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.)

An underline below a series of Chinese characters has a gap between two adjacent underlining elements.

''text-decoration-skip: edges'' for <u>石井</u><u>艾俐俐</u>

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'.

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
			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 ''text-decoration-skip-ink/auto''. How can we define this behavior? Are there more scripts wanting not to skip? Need some normative text describe how ''text-decoration-skip-ink/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.

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).

Additional Controls for Emphasis Marks

Issue: See also issue about continuity in size/position.

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 punctuation
			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 other 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 P* category that are not defined as ''symbols'' (see below).
symbols
Skip symbols. Symbols in this definition includes all typographic character units belonging to the Unicode S* general category as well as any which are NFKD-equivalent [[!UAX15]] to the following characters from the Unicode Po category:
# U+0023 NUMBER SIGN
% U+0025 PERCENT SIGN
‰ U+2030 PER MILLE SIGN
‱ U+2031 PER TEN THOUSAND SIGN
٪ U+066A ARABIC PERCENT SIGN
؉ U+0609 ARABIC-INDIC PER MILLE SIGN
؊ U+060A ARABIC-INDIC PER TEN THOUSAND SIGN
& U+0026 AMPERSAND
⁊ U+204A TIRONIAN SIGN ET
@ U+0040 COMMERCIAL AT
§ U+00A7 SECTION SIGN
U+00B6 PILCROW SIGN
⁋ U+204B REVERSED PILCROW SIGN
〽️ U+303D PART ALTERNATION MARK
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? Issue: See also discussion of the initial value. Privacy and Security Considerations {#priv-sec} =============================================== This specification introduces no new privacy or security considerations.