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We've had a few people ask about website accessibility, so I asked a friend over at NSW Education who works with their Accessibility about the standards, and what resources she recommended for accessibility. Her recommendations are below:
The guidelines are the Web Content Accessibility Guidelines (WCAG) 2.0: https://www.w3.org/TR/WCAG20/
Minimum requirements for government agencies is AA, some federal agencies require an AAA
They're a bit hard to digest, but there is a quick reference guide which is a lot more manageable: https://www.w3.org/WAI/WCAG20/quickref/
For additional recourses, webAIM has some great resources, tips and techniques: http://webaim.org/resources/
And she said to remember the guidelines are not just about HTML content, all website documents (word, pdf, ppt etc.) should pass the guidelines too, and recommend this resource: http://a11yproject.com/