The book title, Adding Ajax, should be synonymous with the concept of progressive enhancement in Ajax development, and I’ve gone through the earlier chapters and adjusted accordingly.
Progressive enhancement (or should that be Progressive Enhancement?) is the philosophy that you create web pages that don’t require any scripting at all, first; then add scripting effects in such a way that each degrades gracefully and doesn’t impact on the accessibility of the page.
Most web development should use progressive enhancement, because most sites aren’t creating Word in a web page, or yet another spreadsheet. Most web sites are providing information, stores, how-tos, whatever that aren’t dependent on scripting. The scripting is an enhancement.
Starting with the script and then adding accessibility comes off as clumsy. Starting with the basic application and adding scripting and Ajax effects is a much more elegant approach.
The one last step in all of this is how to make dynamic web page changes apparent to screen readers. Juicy Studio has documented approaches with JAWS, first in Making Ajax Work with Screen Readers and recently with Improving Ajax Applications for JAWS Users.