The official reference: Section 508 of the Rehabilitation Act of 1974 (as amended, 1998). Includes the “Buy Accessible” reference section for procurement officials, product vendors, and service providers.

The U.S. Access Board is an independent federal agency devoted to accessibility for people with disabilities. It operates with about 30 staff members and a governing board of representatives from Federal departments and public members appointed by the President. In particular, take a look at their “Electronic and Information Technology Accessibility Standards” page.

WebAIM (Web Accessibility In Mind) has a concise checklist of accessibility features required for websites under Section 508 and Pass/Fail criteria.