In "Accessibility is not Enough," Jakob Nielsen explains the problem with thinking about accessibility as a checklist of things that need to be done to make it accessible for those with handicaps. Each person who designs a website that might be used by a disabled person should make sure that the site works for that person. It is easy to do a check with disabled people that you know use it. Have them do a few tasks to make sure that they can do them well with their accessibility tools. I am summarizing this article because I think it is an important topic for me and all who design to understand.
It is incorrect to assume that because there are no accessibility errors when you validate the document that is truly is accessible. For instance, Nielsen points out the problem with thinking that the document is accessible if there is an alt tag. That is not true. You need to also know how to write the alt tag so that a person who can't see the picture can still understand what it is.
Think about the purpose of making it accessible. If you have a website that sells products, it is so that you can sell more. If you have disabled employees and are using an intranet, it is so that you can have more productivity. The goals are the same for any usability demand.
Keep in mind that is not just a person has a disability or she does not. It is really a continuum of people with different disabilities. A person over 45 may be seen as a non-disabled person, but in reality, they may have bad eyesight, and they need a page with resizable text so that they can read it.
Does this sound familiar? If something is difficult to use, people don't want to try and figure it out. For example, if your site works on a handheld computer, that is good for you. But, if the people who need to use your site can't figure out how to use it on the handheld, they won't. If they can use it easily, and it is optimized for their screen, they will be all over it.
If it is too cumbersome to navigate the site or too difficult to learn how it works, people will leave.
Accessibility is not the objective. Usability is the objective. Make sure that you are designing sites that are accessible and usable.