In the old days (2011), the WMF had design guidelines that discussed accessibility issues such as appropriate font sizes, use of colors, and text contrast. These guidelines were later replaced with the Agora guidelines (https://www.mediawiki.org/wiki/Wikimedia_Foundation_Design) which specify only that "We must enable access for users with impairments."

Accessibility is central to our mission as an organization and very important to our community. In fact the en.wiki community has enacted their own comprehensive accessibility guidelines for content:
https://en.wikipedia.org/wiki/Wikipedia:Accessibility
https://en.wikipedia.org/wiki/Wikipedia:Accessibility_dos_and_don'ts

Mediawiki developers also have a set of published accessibility guidelines:
https://www.mediawiki.org/wiki/Accessibility_guide_for_developers

The issue of accessibility in MediaWiki UX design has been raised numerous times in the recent past, most commonly in regard to font sizes and colors. I'm personally aware of it coming up at least 5 times in the past year (Typography Refresh, Flow, Echo, Mobile, NavPopups). Rather than rehashing the same discussions each time, I would encourage the design team to come up with a new set of accessibility guidelines that everyone can refer to and agree on. I would encourage stealing ideas from the en.wiki guidelines and the WCAG guidelines (http://www.w3.org/TR/UNDERSTANDING-WCAG20/). I would also suggest that the design team invest in a pair of scratched-up coke-bottle glasses that each design mock-up can be tested with :)

Ryan Kaldari