Section 508 of the U.S. Code is the law that requires federal agencies and their contractors to adhere to a minimal level of accessibility. The law can be loosely interpreted and basically boils down to:

the use of text labels or descriptors for graphics and certain format elements. (HTML code already provides an "Alt Text" tag for graphics which can serve as a verbal descriptor for graphics). This section also addresses the usability of multimedia presentations, image maps, style sheets, scripting languages, applets and plug-ins, and electronic forms.