What is web accessibility? This is a difficult question to answer, but my short answer is:
An accessible web page makes all pertinent content available to any human user who wants it regardless of browsing technology.
Web accessibility benefits more than people with a disability; keep in mind that a user may be temporarily disabled or using older technology.
The W3C’s description says:
Web accessibility means that people with disabilities can use the Web. More specifically, Web accessibility means that people with disabilities can perceive, understand, navigate, and interact with the Web, and that they can contribute to the Web. Web accessibility also benefits others, including older people with changing abilities due to aging.