http://www.mediawiki.org/wiki/Compatibility#CSS
MediaWiki is compatible with user agents which do not process CSS3 markup. Some additional features are available to browsers which can process these styles.
I.e., tough luck for browsers who can't. https://bugzilla.wikimedia.org/show_bug.cgi?id=26266 etc. etc.
JavaScript MediaWiki is accessible to user agents which do not process JavaScript. MediaWiki degrades gracefully and all essential functionality works without JavaScript, although some things might be less convenient.
Like no Site Notices. Oh, that's Wikipedia, not Mediawiki.
Text-only browsers MediaWiki is accessible to text-only user agents such as Lynx, and to interpreters such as JAWS. Some functionality is reduced.
...to the point that you can take your business elsewhere. < Like what? Dig through my closed invalid etc. bugs.
Why don't you add:
In general the attitude here at MediaWiki is you are crazy for not using what we here at MediaWiki expect a browser to be. Even if making our stuff work fully via pure HTML would allow for machine processing and more devices etc... who know what the future might offer.