In general, enforcing the separation of content and structure from presentation and layout results in Web documents that load faster and are easier to maintain and update. Source documents that contain only content and structural markup are easier for people (like Web developers) to read and understand than documents that include lots of complex formatting instructions. Therefore, those documents are easier and less expensive to update when necessary even if the person responsible for updating the site is not the person who designed and built it.
Content can be changed independently of formatting because presentation and layout are handled in CSS. This means you can leverage the skills of graphic designers and typographers on your team by allowing them to focus on the visual design while writers and communication specialists work on textual content. And you can change presentation and layout on hundreds or even thousands of pages at once simply by changing the external style sheet to which those pages are linked.
Style sheets have similar benefits even for single documents. For example, suppose you repeatedly use the same element such as a <blockquote> or a horizontal rule (<hr>) at different times on a page. If you want to change the way these elements look but don't use style sheets, you'll have to specify the appearance of the element each time you use it; with a style sheet, you only have to do it once. Obviously, this can be a huge time-saver.
Working with style sheets also has important benefits for accessibility. First, you can offer users a choice of styles without having to modify the source document. And you can easily verify that the source document works effectively with assistive technologies before applying any styles at all. Finally, using markup and style sheets together, and doing so correctly (WCAG 1.0 Guideline 3), makes it possible to avoid design and coding practices that may create unintended barriers to accessibility. (We'll show you some examples later in this chapter.)