While only certain websites are required by law to be accessible for persons with disabilities, it is generally acknowledged to be a best practice to build accessibility into all websites. There is a great, free tool that allows web developers to check accessibility: WAVE.
WAVE is developed and made available as a free community service by WebAIM. Originally launched in 2001, WAVE has been used to evaluate the accessibility of millions of web documents. WAVE shows the original web page with embedded icons and indicators that reveal the accessibility of that page.
To start, simply enter a URL, upload a web document or paste in HTML code. Results are quick and easy to understand.
Give it a try.