Besides those guidelines listed in Research-Based Web Design & Usability Guidelines by U.S. Department of Health and Human Services, someone else are working on other set of guidelines too. Recently, I found a Report from Nielsen Norman Group titled:
Beyond ALT Text:
Making the Web Easy to Use for Users with Disabilities
This is actually a research report which contains the usability test results, test data and 75 additional design guidelines. The following are part of the good guidelines:
The above guidelines extracted are also the important guideline to normal website. It helps making the web easy to use for all users. For the details, the report explains why such guideline is important and there are some illustrations too.
You may find the full version by downloading the report. The report seems FREE to get it now. It should be a good reference to the web designer.