Progressive enhancement

"Progressive Enhancement consists of the following core principles:

• basic content should be accessible to all web browsers
• basic functionality should be accessible to all web browsers
• sparse, semantic markup contains all content
• enhanced layout is provided by externally linked CSS
• enhanced behavior is provided by unobtrusive, externally linked JavaScript
• end-user web browser preferences are respected

Progressive enhancement is a strategy for web design that emphasizes accessibility, semantic HTML markup, and external stylesheet and scripting technologies. Progressive enhancement uses web technologies in a layered fashion that allows everyone to access the basic content and functionality of a web page, using any browser or Internet connection, while also providing an enhanced version of the page to those with more advanced browser software or greater bandwidth.

The strategy is an evolution of a previous web design strategy known as "graceful degradation", wherein designers would create Web pages for the latest browsers that would also work well in older versions of browser software. "

http://en.wikipedia.org/wiki/Progressive_enhancement