Practical benefits of accessible websites
Easier to find
The better code and structure typical of accessible websites helps search engines analyse the website and assess the content, leading to better rankings.
Wider range of visitors
Accessible websites help some visitors browse them who otherwise couldn't, which increases the size of the audience. Many websites exclude visitors through poor website construction (BBC report of survey findings).
Easier to use
Accessible website designs are usually clearer, and more focused on the needs of the visitor - accessible Web designers focus on ease-of-use, and relevance.
Better compliance with legislation
Accessible Web designers have the same goals as the legislation - to help those with impairments or disabilities get more out of the Web.
