#MediaWiki does not comply with section 508 of VPAT. This answer is properly right for now. It is certainly the answer the Wikimedia Foundation had to give for now.
This answer raises a problem not only for the Unites States government but also for many of the readers of Wikipedia. When you have to conform to specifications, you only know if you do and, to what extend when you check these specifications out and test compliance.
MediaWiki has as far as I am aware never been tested. I do however know that a "visually impaired" man with his dog visited a Dutch chapter meeting and an "audible impaired" man visited with his Dutch sign language translator in an other chapter meeting. So it cannot be all bad. When you add that the SignWriting Foundation is working on an extension to enable SignWriting it is obvious that it cannot be that bad and, that compliance with VPAT for 508 should be reachable.
MediaWiki is open source and, we are quite happy that the United States government uses our software. We would welcome it when they test MediaWiki for compliance and help us where we do not meet the mark. This will not only help those branches of government that use MediaWiki, it helps impaired people all over the world to learn about everything that can be found in our many projects.