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.
You forgot to mention that the images should be converted to ascii art
before sending to the browser, for greater compatibility.