Web Accessibility Should Not Restrict Web Usability

"[Usability] is the practice of designing Websites so visitors can do what they wish without undue impediment. (Some impediments are necessary, like typing in personal identification numbers to look at your bank statement. Other impediments, like indulgent Flash intros, are rather quite unnecessary.) You can see a clear kinship between usability and accessibility, given that we are trying to lessen the effects of the impediment of disability…But we should not expect a one-to-one relationship. Usable sites can be inaccessible. Conversely, accessible sites can be unusable.

Further, we are forced to expand our conception of usability here. Is your site fully and conveniently usable by sighted people but an exercise in tedium for the blind? If so, is it still usable?"

Joe Clark, Building Accessible Websites (Indianapolis: New Riders Publishing, 2003)