Semantic Structure
http://www.webaim.org/techniques/structure/

WebAIM
2003

Abstract:

Sighted people are able to quickly scan over an entire Web page. They can visually skip past the advertisements, past the main navigation and go directly to the main content. They can ignore the extraneous information all around the main content, without ever having to pay attention to it. Those who use screen readers experience Web sites a little differently. They are not able to immediately zero in on the main content of a Web page. They must find it by listening to the advertisements and the main navigation system. They can speed up the process by tabbing from link to link, but they risk missing an important piece of information this way. The experience is definitely different.