Cascading Style Sheets (CSS)
Cascading Style Sheets (CSS) is a mechanism for changing the appearance of HTML or XML elements, by assigning styles to element types, self-defined classes of elements or individual instances. Style sheets can be used to consistently define the appearance of an entire site. Following the introduction of CSS, the W3C (World Wide Web Consortium) recommended that layout-specific features in HTML be phased out and replaced by style sheets, creating a simpler and more structural World Wide Web.
Benefits
Macromedia, the developer of Flash and Dreamweaver tools, has identified a number of benefits of CSS.
Site wide look and feel consistency
Separating structure (or content) from presentation increases web design consistency. With CSS, presentation is defined using a layout-oriented CSS language, which can be applied to a set of HTML documents, thus permitting a complete change of presentation for all the documents at once.
Lower bandwidth usage
HTML code is often much more compact (therefore easier to read and maintain) when used in conjunction with CSS. CSS makes it possible to transform table-based layouts into CSS-based layouts, which, according to various case studies and articles, can reduce page size by 25 to 50%.
Better User Experience: faster site loading
Reduced page size leads to faster site loading, which translates into better user experience.
Better search engine ranking
Being well-placed in search engines is one of the hardest challenges faced by webmasters. Search engine agents crawl the web to index it. Dynamic effects such as those created by JavaScript are not taken into account, and text rendered with graphics cannot be read and parsed either. Separating presentation from content increases the information/markup ratio, making CSS-based documents more pertinent with regard to the search terms, which makes them rank higher in search results. Using CSS to render titles instead of graphics also helps in this regard.
Cross-Browser Compatibility
One of the major headaches for web developers comes from web pages being displayed differently in various browsers. Such rendering differences do exist in older browsers, but most of them are well known and documented. In many cases, rendering differences are caused by ambiguous coding. Well-formed code significantly reduces rendering differences in various browsers.
Future Compatibility
A very significant portion of electronically-stored information is produced for the web and written in HTML format. Most of this information uses invalid HTML which happens to be displayed correctly in permissive older browsers. As the web evolves, web browsers may eventually become either less permissive, or behave differently, when given invalid markup (e.g., parse invalid data and render it slightly differently). Using valid, standards-compliant markup (which CSS provides) ensures that data will be re-usable for a long time, as specifications on how to parse the standard formats are well documented and here to stay.
Accessibility
Providing content that is accessible to people with disabilities is mandatory in many countries. Coding with standards (particularly CSS for positioning, and strict HTML) makes accessibility an easier goal to achieve, as standards have been created with accessibility in mind. Taking care of accessibility means being able to serve web content to a larger audience, increasing web site usability, even for non-disabled people.
Reducing dependency on any one developer
A common challenge for web developers when assuming management of existing sites is the code written by other developers. In many cases, they are faced with poor HTML code quality that may also rely on undocumented features or hacks and specific coding practices. Lack of documentation is also often problematic. Because standards, such as CSS, are well documented, another person taking over standards-compliant code can hit the ground running - and will not need to become familiar with the previous developer's coding practices. This also significantly reduces vendor lock-in.
