If you work in the states, and do a project for the government, then your projects should be accessible according section 508 guideline. So if you are doing a web site, what does that mean?
- If you have an image on the page, the image tag should have an alt attribute describing what the image is about ( .net puts the alt attribute in the code but it is blank )
- if you have complex table,you should define scope and table header (gridview supports this)
- if you put a video, you should have subtitles
- if you have a form and labels regarding to textboxes, you should use label for to associate labels to textboxes, in asp.net you have a label, use associtedcontrolid property
- dont imitate the header using css
- make sure ur site is visible when you disable the css
these are the common problems i see in the web sites but not all :)
good luck making ur site accessible