backside-visibility not working in IE10 - works fine in webkit
===

I'm building a simple pure-css 'card flip' animation, it *has* to work in IE10, but sadly what I've written doesn't.

[jsFiddle demo here][1] or [sample html zip here][2]


I can see that backside-visibility works in IE10 from [their demo here][3] so maybe I've just overlooked something stupid, maybe a fresh pair of eyes might help!

Thanks in advance!
  [1]: http://jsfiddle.net/felixthehat/7FeEz/1/
  [2]: https://www.dropbox.com/s/kbp8rh0sbh49ixe/backside%20vis%20test%20case.zip
  [3]: http://ie.microsoft.com/testdrive/Graphics/hands-on-css3/hands-on_3d-transforms.htm