I am about halfway through Adaptive Web Design by Aaron Gustafson, a book about progressive enhancement. Even being familiar with the concepts, a couple of points caught my attention.

The Web’s experience layers are usually thought of as content, presentation, and behavior which are mapped to HTML, CSS, and JavaScript. Aaron expands this to five layers with text and ARIA. This means you can think of HTML as an enhancement to the text providing additional semantics, and ARIA adds additional semantics in more interactive experiences. Basic HTML can then be enhanced even further with commonly accepted conventions such as microformats.

Nicholas Lindley