The WCAG 2.0 is a stable, referenceable technical standard that helps developers of any kind of online content (from websites to text and PDF files), create or check their materials for accessibility. Many grant givers or governments (like the European Union) even require institutions to follow those guidelines when publishing public sector information or education resources. https://www.w3.org/TR/WCAG20/
The ultimate resource on accessibility directly from the World Wide Web Consortium (W3C). WCAG 2.0 documentation is the definitive guide for accessibility on the web and is aimed at web developers, designers, and content creators.