- 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
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