CSS - Cascading Style Sheets
Cascading Style Sheets (CSS) is a mechanism for adding style (e.g. graphics, fonts, colors, layout) to Web documents. Style sheets determine how Web documents are presented on PC Screens, on PDAs, on Mobile Phones, and in Print. CSS simplifies and therefore reduces the cost of Web authoring and Web site maintenance.
CSS Helps To Maximise Returns
By attaching style sheets to structured documents on the Web (e.g. XHTML/HTML), developers and site visitors can determine the presentation of Web documents without sacrificing device-independence or diminishing accessibility. This means that more people, irrespective of accessing platform or ability can access your Web documents reliably and effectively, maximising your potential returns.
CSS Improves Download Speeds.
CSS 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 positioning, audio and speech output, font characteristics, etc. By separating style from markup, authors can simplify and clean up the XHTML/HTML in their documents, making the documents more structured, more compact and therefore quicker to download, and also more accessible at the same time.
CSS Benefits Web Accessibility.
CSS allows precise control over spacing, alignment and positioning. Authors can thereby avoid "tag misuse" - the practice of misusing a structural element for purely stylistic effects. For example, while the BLOCKQUOTE and TABLE elements in XHTML/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 specialised browsing software such as a speech synthesizer encounters elements that are misused in this way, the results can be unintelligible to the user. Different browsers will also display your documents differently.
CSS Helps to Prevent Image Misuse.
In addition to preventing element misuse, style sheets can help reduce image misuse. For instance, designers 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 Helps With Search Engines.
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 specialised software such as screen readers, nor can it be cataloged by search engine robots.
For a complete explanation of CSS and its associated technologies, visit the W3C's Cascading Style Sheet section.