- It's good business
19.3% of the United States population in the year 2000 had some kind of disability. 17% of adult Americans report having some hearing loss. While not everyone who falls in the first 19.3% necessarily has a condition that affects how they use the Internet that is still a large number of people to ignore as potential customers. Would you have your sales people refuse to describe a shirt to a blind customer in your store? Then why do that with your web site?
"The ADA prevents that anyway" you say. I say that's a next segue for my next point: not making your site accessible could get you sued. Target ended up settling a lawsuit brought forth by the National Federation of the Blind for 6 million dollars because they were unable to use their site. Target's settlement with the NFB also required that they make the site accessible.
If you don't care about increasing your customer base you should at least care about covering your butt. - One day it could be you
Think about the amount of time you spend online and all the sites you like to visit. Now think about how you'd feel if you suddenly couldn't use most of those web sites anymore or if the number of sites you could use got smaller over time.
Not everyone who has a disability was born with it. Some people developed impairments with age. Some were caused by injury or illness. You might not care about sites being accessible now but you will when your short term memory won't hold onto which page you just visited, that artsy site becomes harder to read, that episode of your favorite show becomes harder to hear, and moving the mouse gets that much more painful.
Unless you're planning on becoming completely disinterested in the Internet at some point.
It's worth noting that while I keep using the word 'disability' there are things that affect your vision, hearing, cognitive abilities, and/or mobility that aren't considered disabilities, at least not by the general population.
It's also worth noting that you're probably taking advantage of things that done for accessibility already and not just on the web. (Sliding doors anyone?) - Professionalism
Just like coding to standards, making your sites accessible is just the right way to do it. If you want to be perceived as a professional in the industry then your sites will work for 100% (or as near to that as possible) of the population, not just 80%.
Some of this should happen anyway without the client asking for it, like alternate text. A lot of it will require input and/or permission from the client like the color scheme.
Useful Links
W3C Web Accessibility Initiative - "[Develops] strategies, guidelines, and resources to help make the Web accessible to people with disabilities."
United States Section 508 Guidelines - Web sites for federal agencies must follow these guidelines as well as state and local agencies that receive federal money.
WebAIM - Web Accessibility in Mind has a lot of information on the subject like this article on cognitive disabilities and the web but they've also created a tool to help you check that your sites are accessible called WAVE and it's free.
Think Vitamin's Videos on Accessibility - Think Vitamin recently made all of their videos on accessibility free forever.
Vischeck - This site has two useful tools. The first, Vischeck, is a color-blindness simulator that allows you to see what an image or web page looks like for someone with one of three different types of color blindness. The second, Daltonize, alters the colors of an image so that someone with colorblindness can still see the same contrast as in the original image.
Color Scheme Designer - 4th link in the utility navigation at the upper right is Vision Simulation and it has 8 different conditions that affect how someone views color. It's a good way to make sure that your palette has sufficient contrast for those that don't have normal vision (which according to this site is over 10% of the population).
