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This module defines a pattern for tagging alternate style sheets using a microformat. It can be used to define
standardized classes that can be hooked into the browser UI. Sample use cases include horizontal-vertical layout
switches in Japanese ebook readers; automatically selecting high-contrast styles; and other accessibility
concerns.
HTML 4.0 defines the linking of external style sheets using the
link element. Although typically a style sheet is linked in unconditionally, HTML 4 also defines
alternate style sets, which are named, mutually-exclusive sets of style sheets that the reader
can use to alter the display.
Until now, few [HTML] user agents have exposed alternate styles in a meaningful way. However,
alternate styles are a very simple way to allow the user to alter a document's presentation to his or her preferences
and accessibility needs.
This specification takes advantage of the class attribute of the HTML link
element (introduced in [HTML], but supported in many user agents for much longer) to define
style tags: a markup pattern for tagging style sheets with standardized class names. These class
names can be used by a UA to offer special UI access to such tagged style sets.
The original impetus for this proposal was to offer standard controls for switching between horizontal and vertical
layouts in Japanese ebook readers; this use case is addressed in Appendix A, Vertical Text Style Set Tags.
2 Conformance
None of this specification applies if the UA has unilaterally turned author style sheets off. (This ability is
mandated by HTML and CSS.) In this case, of course, all style sheets are disabled.
The [CSSOM] has more low-level controls for controlling whether a given style sheet is enabled
or disabled; scripting the CSSOM may alter the behavior defined here.
note
Future versions of this specification will define error handling rules and interaction with [CSSOM].
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL
NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be interpreted as described in [RFC2119].
All sections and appendixes of this specification are normative except where identified by the informative status
label "This section is informative". The application of informative status to sections and appendices applies to all
child content and subsections they contain.
All examples in this specification are informative.
3 Style Sets
This section is informative
This is an informative summary of HTML's declarative alternate style set mechanism, with a few slight
changes to terminology.
HTML 4 defines two types of style sheets in its linking mechanism: persistent style sheets and alternate style
sheets.
A persistent style sheet is one that is always enabled, regardless of which alternate style
set is enabled. A persistent style sheet is indicated by omitting the title attribute from the
link or style element that imports it.
An alternate style sheet is one that is part of an alternate set, and can be enabled or
disabled as part of that set. An alternate style sheet is indicated by setting a non-empty value for the
title attribute of the link or style that imports it. Except for
style sheets belonging to a preferred style set (see below), alternate style sheets have to have
an alternate value in the rel attribue value addition to the
stylesheet value.
HTML allows the grouping of multiple alternate style sheets into a single, named alternate style
set. All alternate style sheets with the same title value belong to the same style
set.
The author can indicate whether an alternate style set is preferred by omitting the alternate value from the rel attribute of each of its component alternate
style sheets. If no preferred style sets are defined, the default preferred style set is an unnamed set containing no
alternate style sheets.
By default, the first preferred style set is loaded. The author can override this behavior with the Default-Style meta or HTTP header as defined in [HTML401].
4 Tagging Style Sets
A style sheet can be tagged by adding the tag's name to the class attribute of the
link or style element that imports it. Tagging a style sheet with a
standard style tag (one defined by a specification referencing this specification) indicates
that it implements the functionality described by the style tag's definition. Both alternate style
sheets and persistent style sheets may be tagged in this way.
A style set can be tagged by tagging any one of its component style sheets. Tagging a style set indicates that it
offers the functionality described by the style tag's definition.
Authors using standard tags should tag all alternate style sheets within a style
set with the same set of standard tags.
note
Future versions of this specification might allow UAs to selectively disable tagged style sheets within a style
set. Therefore all alternate style sheets that need to be enabled or disabled together have to have the same set of
standard tags.
If two style sheets within a style set are tagged with mutually exclusive tags, both tags are ignored.
A specification that defines style tags may indicate that a particular tag is
assumed to be the default for untagged style sets.
Appendix A. Vertical Text Style Set Tags
The following style tags are defined by this appendix:
horizontal
This style set provides a primarily horizontal presentation of text. This tag is mutually exclusive with
vertical.
vertical
This style set provides a primarily vertical presentation of text. This tag is mutually exclusive with
horizontal.
The UA may guess whether an untagged style set is horizontal or vertical, or may instead
assign such a style set an indeterminate state.
Examples
The following examples use XHTML syntax; to use HTML syntax, simply drop the slash before the
> in the tags.
In the following example, the author provides a style sheet for a primarily horizontal-text presentation of the
document:
Since only the second style set is marked with the alternate value of the
rel attribute, the author has indicated a preference for the horizontal style set
which, in the absence of a user override, should be used by default.
In the following example, the author provides style sheets for both horizontal-text and vertical-text
presentations of the document:
Since only the first style set is marked with the alternate value of the
rel attribute, the author has indicated a preference for the vertical style set which, in the
absence of a user override, should be used by default.
In the following example, the author provides style sheets for both horizontal-text and vertical-text
presentations of the document:
Since neither is marked with the alternate value of the rel
attribute, both are preferred presentations by the author. Since the horizontal presentation is provided first, in
the absence of a user preference to the contrary, it will be used by default.
Appendix B. Night Vision Style Set Tags
The following style tags are defined by this appendix:
day
This style set provides a color scheme suited for daytime use (light background, dark text). This tag is
mutually exclusive with night.
night
This style set provides a color scheme suited for nighttime use (dark background, light text). This tag is
mutually exclusive with day.
Examples
In the following example, the author provides style sheets for both both day vision and night vision
presentations of the document. Both horizontal-text and vertical-text presentations are available, but the horizontal
presentation is preferred:
