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The CSS2 Recommendation is based upon CSS1, a W3C Recommendation issued in December 1996, and is a prerequisite for the Document Object Model (DOM), W3C's platform- and language-neutral interface, which allows programs and scripts to dynamically access and update the content, structure, and style of documents.
CSS2 offers precise control over the presentation of Web pages. The ability to position elements explicitly greatly enhances control of document layout, both on screen and in print. Relatively positioned elements are shifted, by an amount specified by the designer, from the position they would have occupied in normal flow. Absolutely positioned elements are taken out of the normal flow of text entirely, and can be placed elsewhere to create navigation bars, indexes, and similar features. Floated elements can be used to place text or graphics in the margins, with text flowing around the floated element.
CSS2 has a rich WebFonts capability, offering dynamic download of fonts from a Web site (just as images are downloaded today). The fonts can be locked to a particular Web site and are not installed on the client machines. WebFonts also includes the information needed to synthesize fonts or to select similar looking fonts on the client, if the fonts specified by the designer are not available.
CSS selectors, which determine what style rules are applied to which parts of the document, have been made more powerful in CSS2. This gives designers greater flexibility and expressive power, particularly when styling XML documents.
Prior to style sheets, the markup needed to simulate common typographic effects such as exdented headings, wide margins, and drop capitals caused documents to bloat and tied them to a single style of presentation. For example, without style sheets, to make all headings appear in a particular font, extra markup must be placed around every single heading in the document. It is easy to miss one heading, giving an inconsistent look. Redesigning such a document implies changing all of the tags.
"Consolidating all the presentation information into one part of the document, and not having to repeat it, makes the document shorter and simpler to edit," explained Chris Lilley, chair of the CSS&FP working group that produced CSS2. "Moving it into a separate style sheet is even better, allowing re-use and easing maintenance."
The separation of style and content allows a single style sheet to define the style for a group of related Web pages, or even an entire Web site. The result is shorter documents, which in turn, load faster. Once the first document has loaded, the rest are even faster because the browser need only fetch the style sheet once. A recent W3C study showed that using CSS with W3C's Portable Network Graphics (PNG) and HTTP/1.1 can dramatically reduce page download times and ease the load on the global Internet.
CSS2 introduces the concept of named media. Portions of a style sheet can be marked as only applying to certain media. For example, one part of a style sheet can set colors just for the screen, one part can set margins for when the page is printed, and the rest can specify what is common to both screen and print media.
Audio presentation of Web content -- using speech synthesis -- is an attractive alternative for accessing information, particularly suitable for home entertainment, industrial and medical information systems, and in-car browsers. CSS2 allows designers to control how HTML and XML documents are spoken, including the volume, speed, stress, and richness of the computer generated voices. The stereo position of voices, audio clips, and background music can also be controlled with CSS2.
Besides the significant increase in accessibility of Web pages that use style sheets, CSS2 includes a number of specific features that improve accessibility. Users, as well as document authors, can specify style sheets; these are cascaded together to produce the end result. User style sheets can range from simple (e.g., increasing the overall size of the text) to complex (e.g., specifying full aural rendering). The ability to select elements with particular attributes and to generate content allows users to specify that, for example, alternate text ("alt" text) or titles on images should be displayed. Auto numbering of headings can also be a useful navigation aid.
Continuing the W3C goal of ensuring a truly World Wide Web, the members of the W3C CSS&FP Working Group drew on the experience of leading experts on internationalization and fonts. To accommodate internationalization, CSS2 fully supports the international ISO 10646 character set, allowing authors to manage differences in language, text direction, and character encoding schemes. CSS2 can display left-to-right, right-to-left, or mixed text such as a Hebrew document, containing a French quote, which itself contains a phrase in Arabic.
CSS2 enables document authors to apply specialized formatting to portions of documents depending on the language in which they are written. Font sets can be constructed to display multilingual documents. CSS2 extends list numbering to allow additional international styles. In addition, CSS2, when coupled with internationalization features, makes it easier to seach through content.
The CSS2 Recommendation is supported by the W3C CSS2 Package, consisting of the CSS2 Validation Service, a set of W3C Core Style Sheets, and the CSS Test Suite. The CSS2 Package will help document authors use CSS2 and also help developers create CSS2-compliant software.
Today, W3C expanded its HTML Validation Service to include full CSS validation (both levels 1 and 2) at http://jigsaw.w3.org/css-validator. Content providers can be sure their style sheets are valid, either by giving the Web address or by pasting the style sheet into a form.
The service outputs error and warning messages, suggests style sheet improvements, and formats the corrected style sheets so they are easy to read.
"Browsers silently ignore style sheet errors, doing their best to work around errors," said Bert Bos, co-architect of CSS and one of the editors of the CSS2 Recommendation. "That's good for the user but doesn't help authors correct bugs. The best way to know whether your style sheets are correct is to run them by the CSS Validation Service."
The Core Style Project proposes a modular architecture for Web style sheets. It builds upon the CSS1 Base Style Sheet incorporated into the CSS2 Recommendation. The project aims to promote cascadability among Web style sheets of diverse origins by providing style authors with generic, yet attractive models and bases for their own elaboration.
Shared CSS style sheets offer benefits to Web site developers, content providers, and users alike. Style sheet reuse means less site management and more consistent appearance. Consistency sends a strong message about corporate identity. It also improves accessibility by making it easier to navigate the site. Shared styles free content providers from the burden of rewriting style rules for each document. The cascade gives them the best of both worlds: reuse and extensibility.
"With CSS2 and the Core Styles," said Todd Fahrner, the Studio Verso designer behind the Core Style Sheets, "CSS moves beyond the 'good idea' phase and becomes a critical element of a new, more attractive and manageable Web, where substance and style complement one another as peers."
The W3C Core Style Sheets page (http://www.w3.org/StyleSheets/Core/) explains how to link to the style sheets and provides samples that illustrate the effects of the Core Styles. W3C will continue to add to these Core Styles and intends to make css.w3.org a gallery of style sheets contributed by Web designers for communal consumption.
To ensure that pages designed with CSS work best on any browser, W3C is releasing a reference suite of CSS test documents (http://www.w3.org/Style/CSS/Test).
"The W3C CSS Test Suite is a tool for implementors to ensure interoperability with other CSS-based software," said Eric Meyer of Case Western Reserve University, who is coordinating the test suite efforts. "Key contributions from Tim Boland of NIST, W3C and the Web community have produced a test suite that covers all of CSS1". The Test Suite will be completed with CSS2 test documents in the near future.
Implementors can conduct tests on their CSS clients (authoring tools, browsers, format converters, etc.) by reading test suite documents and verifying the results against the expected rendering described in the test suite.
Specifications developed within W3C working groups must be formally approved by the Membership. Consensus is reached after a specification has proceeded through the following review stages: Working Draft, Proposed Recommendation, and Recommendation.
Working Drafts are written by a Working Group, who typically meet by teleconference every week and also meet in person four to six times a year. Once stable, working drafts are submitted by working groups to the W3C Director for consideration as a Proposed Recommendation. Upon the Director's approval, the document becomes a Proposed Recommendation and is forwarded to the W3C Membership, who votes whether it should become an official W3C Recommendation. The W3C Advisory Committee -- comprised of one official representative from each Member organization -- submits one of the following votes on the Proposed Recommendation: yes; yes, with comments; no, unless specified deficiencies are corrected; no, this Proposed Recommendation should be abandoned.
During the Member review and voting period (approximately 6 weeks), the Working Group resolves minor technical issues (if any) and communicates its results to the W3C Director. After this time, the Director announces the disposition of the document; it may become a W3C Recommendation (possibly with minor changes), revert to Working Draft status, or may be dropped as a W3C work item.