Posted way back in 2000 [!] by John Allsopp of A List Apart fame. It's still an inspirational primer to the philosophy of accessible site design.

http://www.alistapart.com/articles/dao/

Firstly, think about what your pages do, not what they look like. Let your design flow from the services which they will provide to your users, rather than from some overarching idea of what you want pages to look like. Let form follow function, rather than trying to take a particular design and make it “work”.

This is a must-read for web designers and clients - anyone interested in maximizing the benefits of the medium.