The current experience clicking on a Wikimedia video in IE is very poor; you just get a direct link to a .webm file which by default won't play.
There's a vaguely worded link about support which, if you carefully read the linked page, can direct you to the Google WebM for IE plugin download page -- which makes video "just work" once installed -- but it's really not easy to find.
Ideally, clicking on a WebM-mode video playback should take you directly to https://tools.google.com/dlpage/webmmf/
[Deploying ogv.js (bug 61823) should also help by making default-size playback dependent only on standard JS and Flash audio, but we'll still need the IE WebM plugin to support HD playback.]
Might implement this in TimedMediaHandler something like the iOS VLC App mode -- enable only when needed, claim WebM support, but link out to the plugin download page. Then when coming back, reload and the page should pick up the plugin 'natively'.