I've had this question on Sitecore's ATAG compliance when working on an opportunity in my previous agency. Unfortunately i don't have access to the materials and the response was from Sitecore sales and for a much older version of Sitecore than the current (8.2 Update-2).
The question is specifically for compliance of Content Management environment (Content Editor, Experience Editor, Analytics Dashboards etc).
I thought it was worth asking here on our Sitecore StackExchange to help others if the question crops up on another opportunity.