The World Wide Web Consortium (W3C) is a member-driven organization that develops open web standards. Their mission is "to lead the World Wide Web to its full potential by developing protocols and guidelines that ensure the long-term growth of the Web".
With the merger of the IDPF, W3C is now the home for development of EPUB as well as future web-based publishing initiatives.
Refer to the W3C mission statement for more information.
The Web Content Accessibility Guidelines (WCAG, pronouned wuh-cag) are a set of guidelines developed by the W3C that establish the criteria web content need to meet and exceed to be accessible to a broad audience of users.
WCAG is not a checklist of markup practices, but a set of four principles for accessible documents: that they be perceivable, operable, understandable and robust. Each of these four principles is divided into specific success criteria to evaluate a document against (e.g., that all non-text content have text equivalents and that content can be zoomed without having to be scrolled).
>Each of the success criteria is assigned a conformance level of A, AA or AAA. Level A is the baseline that documents have to achieve, and does not provide a high level of accessibility (i.e., some users will still have difficulty reading the document).
Level AA is the level most often cited in accessibility legislation, as content that meets this level will be usable by users with a wide variety of reading needs.
Level AAA is the highest level a document can achieve, but is considered more of an aspirational goal than a practical one (i.e., some success criteria simply cannot be met by some types of content). Authors should try to achieve any level AAA success criteria they can, but meeting them all is not expected.
Informative guidance on how to meet each of the success criteria is provided in a separate techniques document and there is also a guide to understanding WCAG.