In IE 9 (but same concept should apply to any browser without animationend or prefixed equivalent), the hover cards will show fine. However, when you mouse out to leave, it just stays there.
It can't show the animation without CSS3 animation support (http://caniuse.com/#search=animation). However, it should still close.
The solution is probably to use feature detection to determine if animationend exists, and either hide it immediately (or with a simple jQuery fade), or let animationend do its job in supported browsers.
It looks like http://davidwalsh.name/css-animation-callback (from Modernizr) should work (using animation instead of transition), if all browsers with CSS animations support animationend).
Version: master
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=64242