One of the better legislative rules that have forced web developers to pay attention is the introduction of the ADA and the resultant guidelines that they force developers to use.
The primary function is to provide alternative means and to facilitate assistance technology like screen readers to access content and functionality on your site.
Generally, the guidelines state that you must
a. provide text equivalents for non text elements (i.e alt attributes and/or LONGDESC)
b. avoid dependence on colours (i.e. red lettering for error messages)
c. avoid actions that are not triggered by user input.
HTML should be used for structure and CSS for style.
