Cascading Style Sheets (CSS) Snapshot 2007

W3C Working Draft 27 July 2010

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Elika J. Etemad


This document collects together into one definition all the specs that together form the current state of Cascading Style Sheets (CSS) as of 2007. The primary audience is CSS implementors, not CSS authors, as this definition includes modules by specification stability, not Web browser adoption rate.

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 Draft 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 specification. When sending e-mail, please put the text “css-beijing” in the subject, preferably like this: “[css-beijing] …summary of comment…

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This specification is a Last Call Working Draft. All persons are encouraged to review this document and send comments to the www-style mailing list as described above. The deadline for comments is 15 August 2010.

Table of contents

1. Introduction

When the first CSS specification was published, all of CSS was contained in one document that defined CSS Level 1. CSS Level 2 was defined also by a single, multi-chapter document. However for CSS beyond Level 2, the CSS Working Group chose to adopt a modular approach, where each module defines a part of CSS, rather than to define a single monolithic specification. This breaks the specification into more manageable chunks and allows more immediate, incremental improvement to CSS.

Since different CSS modules are at different levels of stability, the CSS Working Group has chosen to publish this profile to define the current scope and state of Cascading Style Sheets as of late 2007. This profile includes only specifications that we consider stable and for which we have enough implementation experience that we are sure of that stability.

Note that this is not intended to be a CSS Desktop Browser Profile: inclusion in this profile is based on feature stability only and not on expected use or Web browser adoption. This profile defines CSS in its most complete form.

Note also that although we don't anticipate significant changes to the specifications that form this snapshot, their inclusion does are not mean they are frozen. The Working Group will continue to address problems as they are found in these specs. Implementers should monitor www-style and/or the CSS Working Group Blog for any resulting changes, corrections, or clarifications.

1.1. The W3C Process and CSS

This section is non-normative.

In the W3C Process, a Recommendation-track document passes through five levels of stability, summarized below:

Working Draft (WD)
Published during the process of drafting the specification, the purpose of a public Working Draft is to create a snapshot of the specification's current state and to solicit input from the W3C and the public. The document is known to be unstable, and is often incomplete.
Last Call Working Draft (LC or LCWD)
By publishing a Last Call Working Draft, a working group is expressing that they consider the spec to be complete and all issues to be resolved. Publishing a Last Call Working Draft announces that this specification will move toward Candidate Recommendation unless significant issues are brought up. The Last Call period is a last chance for others to submit issues before the transition to CR.
Candidate Recommendation (CR)
By publishing a Candidate Recommendation, a working group is expressing that have resolved all known issues and they believe the spec is ready for implementation.
Proposed Recommendation (PR)
To exit CR and enter this stage, the spec needs a comprehensive test suite and implementation reports proving that every feature in the spec is interoperably implemented in at least two shipping implementations. Entering the Proposed Recommendation stage signals to the W3C that these requirements have been met. Once the W3C officially approves the specification, it becomes a Recommendation.
Recommendation (REC)
This is the final stage. At this point there should need to be no more changes.

In the CSSWG's experience, the recommendation track is not linear. The wider review triggered by an LCWD often results in at least another working draft, possibly several. More significantly, our experience is that many specs enter CR twice, because implementation testing often uncovers significant problems in the spec and thus pushes it back to working draft. Additionally, fixing even minor problems forces a CR to re-enter the Working Draft stage. As a result, although the CSSWG has a clear idea of the stability of the CSS specs, it is very difficult for someone outside the working group to come to that same understanding based on a specification's official status. The CSS Working Group's motivation for creating this document is thus to communicate to others our understanding of the state of CSS.

2. CSS Levels

Cascading Style Sheets does not have versions in the traditional sense; instead it has levels. Each level of CSS builds on the previous, refining definitions and adding features. The feature set of each higher level is a superset of any lower level, and the behavior allowed for a given feature in a higher level is a subset of that allowed in the lower levels. A user agent conforming to a higher level of CSS is thus also conformant to all lower levels.

2.1. CSS Level 1

The CSS Working Group considers the CSS1 specification to be obsolete. CSS Level 1 is defined as all the features defined in the CSS1 specification (properties, values, at-rules, etc), but using the syntax and definitions in the CSS2.1 specification. CSS Styling Attributes Level 1 defines its inclusion in element-specific style attributes.

2.2. CSS Level 2

Although the CSS2 specification is technically a W3C Recommendation, it passed into the Recommendation stage before the W3C had defined the Candidate Recommendation stage. Over time implementation experience and further review has brought to light many problems in the CSS2 specification, so instead of expanding an already unwieldy errata list, the CSS Working Group chose to define CSS Level 2 Revision 1 (CSS2.1).

CSS2.1 is now a Candidate Recommendation—effectively though not officially the same level of stability as CSS2—and should be considered to obsolete the CSS2 Recommendation. In case of any conflict between the two specs CSS2.1 contains the definitive definition. Features in CSS2 that were dropped from CSS2.1 should be considered to be at the Candidate Recommendation stage, but note that many of these have been or will be pulled into a CSS Level 3 working draft, in which case that specification will, once it reaches CR, obsolete the definitions in CSS2.

The CSS2.1 specification defines CSS Level 2 and the CSS Styling Attributes Level 1 specification defines its inclusion in element-specific style attributes.

2.3. CSS Level 3

This section is non-normative.

CSS Level 3 builds on CSS Level 2 module by module, using the CSS2.1 specification as its core. Each module adds functionality and/or replaces part of the CSS2.1 specification. The CSS Working Group intends that the new CSS modules will not contradict the CSS2.1 specification: only that they will add functionality and refine definitions. As each module is completed, it will be plugged in to the existing system of CSS2.1 plus previously-completed modules.

From this level on modules are levelled independently: for example Selectors Level 4 may well be defined before CSS Line Module Level 3.

3. Cascading Style Sheets Definition

As of 2007, Cascading Style Sheets (CSS) is defined by the following specifications. Each specification in this list builds on and possibly modifies the definitions in the previous specifications, with the base formed by CSS Level 2 Revision 1. (In other words, CSS is defined as CSS Level 2 Revision 1, modified by CSS Namespaces, modified by Selectors Level 3, etc.) A valid CSS document is one that conforms to this definition.

  1. CSS Level 2 Revision 1 (including errata)
  2. CSS Styling Attributes Level 1
  3. CSS Namespaces
  4. Selectors Level 3
  5. CSS Color Level 3

3.1. Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS layout implementations 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.

3.2. CSS Profiles

Not all implementations will implement all functionality defined in CSS. For example, an implementation may choose to implement only the functionality required by a CSS Profile. Profiles define a subset of CSS considered fundamental for a specific class of CSS implementations. The W3C CSS Working Group defines the following CSS profiles:

3.3. Experimental Implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary property and value extensions to CSS. The CSS Working Group recommends that experimental implementations of features in CSS Working Drafts also use vendor-prefixed property or value names. This avoids any incompatibilities with future changes in the draft. Once a specification reaches the Candidate Recommendation stage, implementors should implement the non-prefixed syntax for any feature they consider to be correctly implemented according to spec.

4. Changes

The primary substantive change from the previous draft is the addition of the CSS Styling Attributes specification, which more rigorously defines a feature that previously had been only loosely defined by the HTML4 specification and had been implemented well before 2007.