How Style Sheets Benefit Accessibility
Written by jon on 11:26 AMCSS benefits accessibility primarily by separating document structure from presentation. Style sheets were designed to allow precise control - outside of markup - of character spacing, text alignment, object position on the page, audio and speech output, font characteristics, etc. By separating style from markup, authors can simplify and clean up the HTML in their documents, making the documents more accessible at the same time.
CSS allows precise control over spacing, alignment and positioning. Authors can thereby avoid "tag misuse" - the practice of misusing a structural element for its expected stylistic effects. For example, while the BLOCKQUOTE and TABLE elements in HTML are meant to mark up quotations and tabular data, they are frequently used to create visual effects instead such as indentation and alignment. When specialized browsing software such as a speech synthesizer encounters elements that are misused in this way, the results can be unintelligible to the user.
In addition to preventing element misuse, style sheets can help reduce image misuse. For instance, authors sometimes use 1-pixel invisible images to position content. This not only bloats documents, making them slow to download, but can also confuse software agents looking for alternative text (the "alt" attribute) for these images. CSS positioning properties mean that invisible images are no longer required to control positioning.
CSS provides precise control over font size, color, and style. Some authors have used images to represent text in a particular font when they are uncertain of the availability of the font on the client's machine. Text in images is not accessible to specialized software such as screen readers, nor can it be cataloged by search robots. To remedy this situation, the powerful WebFonts of CSS allows users much greater control of client-side font information. With WebFonts, authors can rely on fallback mechanisms on the client when the author's preferred fonts are not available. Fonts can be substituted with more accuracy, synthesized by client software, and even downloaded from the Web, all according to author specification.
CSS allows users to override author styles. This is very important to users who cannot perceive a page with the author's chosen fonts and color. CSS allows users to view documents with their own preferred fonts, colors, etc. by specifying them in a user style sheet.
CSS provides support for automatically generated numbers, markers, and other content that can help users stay oriented within a document. Long lists, tables, or documents are easier to navigate when numbers or other contextual clues are provided in an accessible manner.
CSS supports aural style sheets, which specify how a document will sound when rendered as speech. Aural style sheets (or "ACSS" for short) allow authors and users to specify the volume of spoken content, background sounds, spatial properties for sound, and a host of other properties that can add effects to synthesized speech analogous to those achieved with styled fonts for visual output.
CSS provides more precise control over the display of alternative content than HTML alone. CSS2 selectors give access to attribute values, often used to provide alternative content. In CSS2, attribute values may be rendered in a document along with an element's primary content.
CSS Implementations
At the publication of this NOTE, widely deployed browsers do not implement CSS consistently. However, the latest generation of browsers from a number of vendors demonstrates solid implementations of most of CSS1 and much of CSS2, and implementations continue to improve.
Obviously, the benefits of the features described in this document will not be realized without proper implementation of CSS1 and CSS2. Part of designing an accessible document with CSS involves ensuring that the document remains accessible when style sheets are turned off or not supported.
Until most browsers support CSS consistently, content developers may still create accessible documents that mix supported features of CSS with some presentation features of HTML. Documents that use HTML presentation features instead of CSS must transform gracefully. For example, tables used for layout must make sense when rendered serially.
| Posted in »
0 comments: Responses to “ How Style Sheets Benefit Accessibility ”