This specification defines a subset of Cascading Style Sheets Level 2
and CSS3 module: Color specifications tailored to the needs and
constraints of TV devices.
This profile is obsolete. Please see the latest CSS Snapshot for the specifications that make up CSS.
Status of this document
This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.
Publication as a Working Group Note does not imply endorsement by the
W3C Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than work in progress.
The (archived) public
mailing list www-style@w3.org
(see instructions) is
preferred for discussion of this document. When sending e-mail, please put
the text “css-tv” in the subject, preferably like this:
“[css-tv] …summary of comment…”
At this time, the CSS Working Group does not envisage
further work on this specification and does not plan to propose it as a
W3C Recommendation.
1. Overview
This document specifies a profile of the Cascading Style Sheets level 2
(CSS2) and CSS3 module: Color specifications appropriate for TV devices
such as set top boxes or integrated interactive television sets that
display their output on a television screen. Conformance to this profile
means that a user agent supports, at minimum, the features defined in this
specification. This subject is addressed in Section 2, Conformance, below.
CSS2 is a style sheet language that allows authors and users to attach
style (e.g., fonts, spacing, and aural cues) to structured documents
(e.g., HTML documents and XML applications). By separating the
presentation style of documents from the content of documents, CSS2
simplifies Web authoring and site maintenance. CSS2 builds on CSS1 (see
[CSS1]) and,
with very few exceptions, all valid CSS1 style sheets are valid CSS2
style sheets. CSS2 supports media-specific style sheets so that authors
may tailor the presentation of their documents to visual browsers, aural
devices, printers, braille devices, handheld devices, etc.
In summary, CSS2 specifies how developers can author style sheets for
presenting documents across multiple devices and media types. While this
is very important, it is also important that authors have an understanding
of what features are supported on these different devices. Likewise, it is
important that similar devices operate in a similar manner. Otherwise,
authors will need to develop style sheets for each version of each device
-- raising the cost of content development and decreasing
interoperability.
The CSS TV Profile specifies a conformance profile for TV devices,
identifying a minimum set of properties, values, selectors, and cascading
rules. The resulting CSS TV Profile includes the vast majority of CSS1,
portions of CSS2 and CSS3 module: Color. The CSS TV Profile is a proper
superset of the CSS Mobile Profile, with the use of the ‘tv’ media type instead of the ‘handheld’ media type.
2. Conformance
The primary role of a profile is to define a subset of features that
provides a minimal guarantee of interoperability. In the case of the CSS
TV Profile, this guarantee is that a conforming user agent will support
the features defined in this specification following the CSS2 conformance
clause ([CSS2]
Section 3.2), recast and summarized below:
A CSS TV Profile conforming user agent (TV-UA) MUST support the
all and tv CSS2 media types. A TV-UA MAY support other
CSS2 media types, as well.
For each source document, a TV-UA MUST attempt to retrieve all
associated style sheets that are appropriate for the supported media
types.
A TV-UA MUST parse the style sheets according to this specification.
In particular, the TV-UA MUST recognize all CSS TV Profile at-rules,
blocks, declarations, and selectors. If a TV-UA encounters a property
that applies for a supported media type, the TV-UA MUST parse the value
according to the property definition. This means that the TV-UA MUST
accept all valid values and MUST ignore declarations with invalid values.
TV-UA MUST ignore rules that apply to unsupported media types.
For each element in a document tree, the TV-UA MUST assign a value for
every applicable property according to the property's definition and the
rules of cascading and inheritance.
If the source document comes with alternate style sheets (such as with
the "alternate" keyword in HTML 4.0 [HTML40]), the TV-UA SHOULD allow the
user to select one from among these style sheets and apply the selected
one.
As with CSS2, there are qualifications to this conformance clause:
Values MAY be approximated when required by the TV-UA.
The inability of a TV-UA to implement part of this specification due
to the limitations of a particular device (e.g., a TV-UA cannot render
colors on a monochrome monitor or page) SHALL NOT imply non-conformance.
It is recommended that authors use this conformance profile to take
advantage of forward compatibility. Authors should be able to use style
properties with an understanding that the cascading rules are processed
correctly and that unknown properties and values are ignored. For example:
h3 {
display: inline;
display: run-in;
}
A TV-UA that can process the ‘run-in’
value for the ‘display’ property will
accept the first display declaration and then "write over" that value with
the second display declaration. A TV-UA that cannot process the ‘run-in’ value will process the first display
specification and ignore the second display specification.
3. Selectors and
at-rules
3.1. Selectors
In CSS2, pattern matching rules determine which style rules apply to
elements in the document tree [CSS2].
The following table summarizes CSS TV Profile selector syntax. In
addition to these selectors, the CSS TV Profile includes the CSS2 grouping
mechanism (See [CSS2] Section 5.2.1).
Note. Unsupported selectors are parsed as invalid
(which would affect valid selectors in the same group), the same as if a
purely CSS1 UA were to encounter selectors new in CSS2. Of course a TV-UA
is free to implement more selectors than required by CSS TV.
Pattern
Meaning
Selector type
CSS TV
*
Matches any element
Universal selector
Yes
E
Matches any E element (i.e., and element of type E)
Type selectors
Yes
E F
Matches any F element that is a descendent of an E element
Descendent selectors
Yes
E > F
Matches any F element that is a child of an element E
Child selectors
Yes
E:first-child
Matches element E when it is the first child of its parent
The :first-child pseudo-class
Yes
E:link
E:visited
Matches element E if E is the source anchor of a hyperlink of which
the target is not yet visited (:link) or already visited (:visited).
The link pseudo-classes
Yes
E:active
Matches E during certain user actions.
The dynamic pseudo-classes
Yes
E:hover
Matches E during certain user actions.
The dynamic pseudo-classes
No
E:focus
Matches E during certain user actions.
The dynamic pseudo-classes
Yes
E:lang(c)
Matches element of type E if it is in (human) language c (the
document language specifies how language is determined).
The :lang() pseudo-class
No
E + F
Matches any F element immediately preceded by an element E.
Adjacent selectors
No
E[foo]
Matches any E element with the "foo" attribute set (whatever the
value).
Attribute selectors
No
E[foo="bar"]
Matches any E element whose "foo" attribute value is exactly equal
to "bar".
Attribute selectors
No
E[foo~="bar"]
Matches any E element whose "foo" attribute value is a list of
space-separated values, one of which is exactly equal to "bar".
Attribute selectors
No
E[foo|="bar"]
Matches any E element whose "foo" attribute value has a
hyphen-separated list of values beginning (from the left) with "bar".
Attribute selectors
No
E:first-line
Matches the first formatted line of an E element.
The :first-line pseudo-element
Yes
E:first-letter
Matches the first formatted letter of an E element.
The :first-letter pseudo-element
Yes
E:before
Matches/creates generated content before an E element.
The :before pseudo-element
No
E:after
Matches/creates generated content after an E element.
The :after pseudo-element
No
E.bar
Matches any E element whose class attribute (as defined by the
document language) value is a list of space-separated values, one of
which is exactly equal to "bar".
Class selectors
Yes
E#bar
Matches any E element whose ID attribute (as defined by the document
language) is equal to "bar".
ID selectors
Yes
3.2. at-rules
The following table summarizes CSS TV Profile at-rule syntax.
at-rule
Function
CSS TV
@import
Imports an external style sheet.
Yes
@charset
Defines character set for the style sheet.
Yes
@media
Groups a set of style rules to apply only to one or more particular
media.
Yes
@font-face
Defines a named font-family, including for downloading.
No
@page
Defines a (optionally named) page formatting context.
No
@color-profile
Defines a named color-profile.
No
4. Properties
The following table summarizes CSS TV Profile properties and property
values from CSS2. Refer to [CSS2] for the definition of these
properties and values.
Name
CSS TV
CSS Values
Initial value
‘azimuth’
No
<angle> | [ left-side | far-left | left | center-left | center
| center-right | right | far-right | right-side ] || behind ] |
leftwards | rightwards | inherit
Yes. Note that a TV UA may implement a UA style sheet rule of
*{max-width: 100%} to avoid horizontal scrolling.
<length> | <percentage> | auto | inherit
auto
‘word-spacing’
No
normal | <length> | inherit
normal
‘z-index’
Yes
auto | <integer> | inherit
auto
The following table summarizes CSS TV Profile properties and property
values from the CSS3 module: Color. Refer to [CSS3COLOR] for the definition
of these properties and values.
Name
CSS TV
CSS Values
Initial value
‘color’
HTML4 keywords, RGB and RGBA values, transparent, orange, inherit
HTML4 keywords, RGB, RGBA, HSL, and HSLA values, transparent, X11
keywords, System Colors, flavor, attr(X,color), inherit
The CSS TV Profile uses the same syntax as specified in [CSS2]. The CSS TV
Profile uses a subset of the values used in CSS2. Specifically:
The TV-UA SHALL support integer and real numbers ([CSS2] Section 4.3.1).
The TV-UA SHALL support the following lengths ([CSS2] Section 4.3.2):
px
em
ex
in
cm
mm
pt
pc
The TV-UA MAY support other lengths.
The TV-UA SHALL support percentage values ([CSS2] Section 4.3.3).
The TV-UA SHALL support URI values ([CSS2] Section 4.3.4).
The TV-UA MAY support counter values ([CSS2] Section 4.3.5).
The TV-UA SHALL support the following color values ([CSS2] Section
4.3.6):
The 16 named colors defined in HTML 4.0 [HTML401], orange and
transparent as defined in CSS3 module: Color [CSS3COLOR]
A numerical RGB specification ([CSS2] Section 4.3.6)
A numerical RGBA specification as defined in [CSS3COLOR] Section 4.2.2
The TV-UA MAY support other color values.
Similarly, the CSS TV Profile requires that conforming user agents
support the character encoding mechanisms specified in [CSS2]. Specifically:
The TV-UA SHALL support priorities specified in [CSS2] to determine a document's
character encoding.
The TV-UA SHALL support the CSS2 @charset rules.
6. Assigning Property
Values, Cascading, and Inheritance
In general, the CSS TV Profile uses the same cascading rules as in CSS2.
Specifically:
The TV-UA SHALL assign values as described in CSS2 ([CSS2] Section
6.1).
The TV-UA SHALL support inheritance as described in CSS2 ([CSS2] Section
6.2).
The TV-UA SHALL support the CSS2 @import rules as specified in CSS2
([CSS2] Section
6.3).
The TV-UA SHALL support author originating style sheets. The TV-UA MAY
support user or user-agent originating style sheets ([CSS2] Section 6.4).
The TV-UA SHALL support all CSS2 cascading mechanisms ([CSS2] Sections
6.4.1-6.4.4).
7. Media Types
A CSS TV Profile conforming user agent MUST be able to process
media-dependent stylesheets as specified in CSS2 ([CSS2] Section 7). Specifically:
The TV-UA SHALL support the CSS2 @media rules as specified in CSS2 ([CSS2] Section 7).
The TV-UA SHALL accept and process style sheets that target the
tv media type.
The TV-UA SHALL accept and process style sheets that target the
all media type.
The TV-UA SHALL accept style sheets that contain other (non-tv)
media-dependent style sheets.
The TV-UA MAY process other media types (such as screen
or print).
The TV-UA is not required to satisfy the CSS2 conformance statement
pertaining to the tv media type (see [CSS2] Section 7.3.1); the TV-UA
need only satisfy the conformance statements in this specification.
8. Acknowledgements
This document derives from the CSS Level 1 and CSS level 2
Recommendations, the CSS3 module: Color and the CSS Mobile Profile 1.0. We
thank all CSS1, CSS2, CSS3 module: Color and CSS Mobile Profile authors,
editors and contributors.
We would like to thank the members of the CSS working group, especially
Bert Bos, Chris Lilley, and Tapas Kanti Roy for their direct feedback on
the draft.
We would like to thank the contributors on www-style@w3.org. We would
like to especially thank ARIB, Akihiko Handa, Joel Zdepski, Etan Wexler,
Kynn Bartlett, Björn Höhrmann, and fantasai for their valuable comments
on the draft.