It is vital that all designs are 'usable' and that usability considerations are central to the development of a site. There is, of course, a natural relationship between W3C/WAI compliance and a usable website - usability being a broader, but less quantifiable metric. We believe that both standards compliance and general usability best practice are vital to a successful website.
Usability is a quality attribute that assesses how easy user interfaces are to use. The word “usability” also refers to methods for improving ease-of-use during the design process.
Usability is defined by five quality components:
"On the Web, usability is a necessary condition for survival. If a website is difficult to use, people leave. If the homepage fails to clearly state what a company offers or what users can do on the site, people leave. If users get lost on a website, they leave. If a website's information is hard to read or doesn't answer users' key questions, they leave."