Accessibility testing is a subset of usability testing where in the users under consideration are people with all abilities and disabilities. The significance of this testing is to verify both usability and accessibility.
Accessibility aims to cater people of different abilities such as:
A good web application should cater to all sets of people and NOT just limited to disabled people. These include:
Users with poor communications infrastructure
Older people and new users, who are often computer illiterate
Users using old system (NOT capable of running the latest software)