I'm having a problem with the back face-visibilty property. It works how I want it to in Safari/Chrome, but for some reason it's not working correctly in Firefox.
When the box flips, I get a mirror image of the front face.
This bug has been acknowledged by Mozilla.
The fix is to add transform: rotateX(0deg)
to the front and back objects.
Adding backface-visiblity:hidden
on the container, as suggested by @yalia, makes the back unresponsive in Chrome, hence that fix should be avoided.
Here's a fiddle with the fix.