CSS Mobile Profile 1.0

Abstract

This specification defines a subset of the Cascading Style Sheets Level 2 specification tailored to the needs and constraints of mobile devices.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. The latest status of this document series is maintained at the W3C.

This is a working draft of the CSS Mobile Profile specification. It is guaranteed to change; anyone implementing it should realize that we will not allow ourselves to be restricted by experimental implementations when deciding whether to change the specifications.

This specification is a Working Draft of the CSS Working Group (part of the Style activity) for review by W3C members and other interested parties. Publication as a Working Draft does not imply endorsement by the W3C membership, nor of members of the CSS working group. As a W3C Working Draft, this document may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress".

The mailing list www-style@w3.org (see instructions) is preferred for discussion of this and other drafts in the Style area.

A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR.

Contents

1. Overview

This document specifies a profile of the Cascading Style Sheets, level 2 (CSS2) specification appropriate for mobile devices such as wireless phones. Conformance to this profile means that a user agent supports, at minimum, the features defined in this specification per the CSS2 conformance. This subject is address in Section 2, Conformance, below.

As defined in [CSS2]:

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 Mobile Profile specifies a conformance profile for mobile devices, identifying a minimum set of properties, values, selectors, and cascading rules. The resulting CSS Mobile Profile is very similar to CSS1

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 Mobile 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:

1. A CSS Mobile Profile conforming user agent (MP-UA) MUST support the handheld CSS2 media type.
2. For each source document, a MP-UA MUST attempt to retrieve all associated style sheets that are appropriate for the supported media types.
3. A MP-UA MUST parse the style sheets according to this specification. In particular, it must recognize all CSS Mobile Profile at-rules, blocks, declarations, and selectors. If a MP-UA encounters a property that applies for a supported media type, the MP-UA must parse the value according to the property definition. This means that the MP-UA must accept all valid values and must ignore declarations with invalid values. MP-UA must ignore rules that apply to unsupported media types.
4. For each element in a document tree, the MP-UA MUST assign a value for every applicable property according to the property's definition and the rules of cascading and inheritance.
5. If the source document comes with alternate style sheets (such as with the "alternate" keyword in HTML 4.0 [HTML40]), the MP-UA MUST 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:

1. Values MAY be approximated when required by the MP-UA.
2. The inability of a MP-UA to implement part of this specification due to the limitations of a particular device (e.g., a MP-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:

  body {
    background-position: center center;
    background-position: 45% 55%;
  }
  

A MP-UA that can process percentage values for the background-position property will process the first background-position specification and then "write over" that value with the second background-position specification. A MP-UA that cannot process percentage values will process the first background-position specification and ignore the second background-position specification.

3. Selectors

In CSS2, pattern matching rules determine which style rules apply to elements in the document tree [CSS2].

The following table summarizes CSS Mobile Profile selector syntax. In addition to these selectors, the CSS Mobile Profile shall support the CSS2 grouping mechanism (See [CSS2] Section 5.2.1).

4. Properties

The following table summarizes CSS Mobile Profile properties and property values. Refer to [CSS2] for the definition of these properties and values.

5. CSS Syntax

The CSS Mobile Profile uses the same syntax as specified in [CSS2]. The CSS Mobile Profile uses a subset of the values used in CSS2. Specifically:

1. The MP-UA SHALL support integer and real numbers ([CSS2] Section 4.3.1).
2. The MP-UA SHALL support the following lengths ([CSS2] Section 4.3.2):
   • px
   • em
   • ex
   The MP-UA MAY support other lengths.
3. The MP-UA SHALL support percentage values ([CSS2] Section 4.3.3).
4. The MP-UA SHALL support URI values ([CSS2] Section 4.3.4).
5. The MP-UA MAY support counter values ([CSS2] Section 4.3.5).
6. The MP-UA SHALL support the following color values ([CSS2] Section 4.3.6):
   • The 16 colors defined in HTML 4.0 [HTML4]
   • A numerical RGB specification ([CSS2] Section 4.3.6)
   The MP-UA MAY support other color values.

Similarly, the CSS Mobile Profile requires that conforming user agents support the character encoding mechanisms specified in [CSS2]. Specifically:

1. The MP-UA SHALL support priorities specified in [CSS2] to determine a document's character encoding.
2. The MP-UA SHALL support the CSS2 @charset rules.

6. Assigning Property Values, Cascading, and Inheritance

In general, the CSS Mobile Profile uses the same cascading rules as in CSS2. Specifically:

1. The MP-UA SHALL assign values as described in CSS2 ([CSS2] Section 6.1).
2. The MP-UA SHALL support inheritence as described in CSS2 ([CSS2] Section 6.2).
3. The MP-UA SHALL support the CSS2 @import rules as specified in CSS2 ([CSS2] Section 6.3).
4. The MP-UA SHALL support style sheets author originating style sheets. The MP-UA MAY support user or user-agent originating style sheets ([CSS2] Section 6.4).
5. The MP-UA SHALL support all CSS2 cascading rules ([CSS2] Sections 6.4.1-6.4.4).

7. Media Types

A CSS Mobile Profile conforming user agent must be able to process media-dependent stylesheets as specified in CSS2 ([CSS2] Section 7). Specifically:

1. The MP-UA SHALL support the CSS2 @media rules as specified in CSS2 ([CSS2] Section 7).
2. The MP-UA SHALL accept and process style sheets that target the handheld media type.
3. The MP-UA SHALL accept style sheets that contain other (non-handheld) media-dependent style sheets.
4. The MP-UA MAY process other media types (such as screen or print), but it is not required to do so.

The MP-UA is not required to satisfy the CSS2 conformance statement pertaining to the handheld media type (see [CSS2] Section 7.3.1); the MP-UA need only satisfy the conformance statements in this specification.

Appendix A. References

[CSS2]

"Cascading Style Sheets, level 2", Bos B., et.al., 12 May 1998. Available at http://www.w3.org/TR/REC-CSS2/.

[CSS1]

"Cascading Style Sheets, level 1", Lie H.W. and Bos B., 17 December 1996. Available at http://www.w3.org/TR/REC-CSS1-961217.html.

[HTML4]

"HTML 4.0 Specification", D. Raggett, A. Le Hors, I. Jacobs, 8 July 1997. Available at http://www.w3.org/TR/REC-html40/.