Web content accessibility is defined by the W3C as

making web content accessible for people with disabilities

( Web Content Accessibility Guidelines 1.0 - ).

The philosophy of a growing initiative to make all web content accessible to all people.

Accessibility is still largely obscure in regards to common knowledge and application, however I believe that it is an issue that SEOs and web designers alike will soon find to be a primary consideration in their work.

Of interest to the SEO industry is the empirical evidence that suggests positive effects on rankings for websites that conform to official accessibility guidelines. Conformance to accessibility standards does not of course improve the relevance of a hyper-document for any given search phrase, it may however improve the general opinion which a search engine holds for a given web document (much the same way that the use of valid HTML does.). Astute SEO firms can take advantage of these trends much to the benefit of their clients.

And while accessibility (and valid HTML for that matter) may seem insignificant issues at present, the tide is turning to favor all things which are standards compliant. This is the natural back-lash of the current WWW, participants of which have taken advantage of the inherent freedom (and forgiveness) of the Internet to the point where usability and quality are being compromised.