Kevin asks, “What do you think is the single most important thing to do to a website to make it accessible to the most people possible?”
This is a tough question that I want to answer in several different ways.
Methodologically, you should integrate accessibility into the development process—thinking about it from the very first stages of developing a site’s architecture, technological framework, and design.
From a code perspective, use semantic markup built on valid HTML and CSS. This simplifies HTML, making it more palatable for screen readers.
For usability, you should add accessibility affordances like tab indexing, access keys, and skip navigation links. These hooks are not hard to implement, they make a site easier to use, and they also make a committment to accessibilty apparent.
Finally, if you can’t or won’t do one of the things above, add a link with accessibility contact information—someone to call or email to request information on your web site in an alternative format.