This module contains the features of CSS relating to text decoration, such as underlines, text shadows, and emphasis marks.
CSSis a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.
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Table of ContentsThis 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 propertyThis 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:
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):
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.
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 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 propertyThis 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:
The initial position of the underline depends on the value of text-underline-position as detailed below.
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 propertyThe 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:
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 propertyThis 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.
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
autoworks. See
telcon minutes,
alreq#86,
csswg#1288This property only applies to overlines and underlines; line-throughs are unaffected.
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 MarksSee also issue about continuity in size/position.
4.1. Emphasis Mark Skip: the text-emphasis-skip propertyThis 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 specifiedThis property describes for which characters marks are drawn. The values have following meanings:
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?
See also discussion of the initial value.
5. Privacy and Security ConsiderationsThis specification introduces no new privacy or security considerations.
Conformance Document conventionsConformance 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 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.
Requirements for Responsible Implementation of CSSThe following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.
Partial ImplementationsSo 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 FeaturesTo 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 FeaturesOnce 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 https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.
Index Terms defined by this specificationFor 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.
↵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
autoworks. See
telcon minutes,
alreq#86,
csswg#1288 ↵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?
↵ #propdef-text-decoration-widthReferenced in:RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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