CSS in Action: Invisible Content Just for Screen Reader Users
http://www.webaim.org/techniques/css/invisiblecontent/

WebAIM
2007

Abstract:

Most of the techniques for making web content accessible to screen readers are invisible to visual users. Alternative (alt) text, table header tags, table summaries, and form tags are examples of techniques that make a big difference for screen reader users, but which have little or no impact on the visual appearance of the web content. Every once in a while, though, web designers confront situations in which the addition of accessible markup does have an impact on the visual layout. In some cases, this visual impact can decrease the usability of the content for visual users. In other cases, designers simply want to provide a more pleasing layout or appearance that would be compromised by including all of the text in a semantically correct format.