Bottom up approach.
Design for the lowest denominator and then enhance it for better ones.
More emphasis on content and functionality NOT presentation.
For example, design for IE6 with all it’s limitations and then enhance it for newer browsers like Chrome/Firefox.
1. Basic content access to all
2. basic functionality access to all
3. content encapsulated inside semantic markup
4. layout and presentation through CSS
6. end user browser preferences are respected
Top down approach.
Focuses on presentation rather than content or functionality.
Try to make the system work for the latest version of software and mainstream users, then tries to add handlers for the less capable softwares.
Using the alt attribute in the img tag, so that people with non graphical browsers can at least understand what the image is about.
Using the noscript tag to support for non-js browsers. Using tables to allow older browsers to render page layout without CSS.