1) Open http://jsbin.com/iqozik/edit#html,live
2) hover on green box
3) move cursor off of it
4) it should spin
Stable Safari and Chrome do spin, but Webkit Nightly and Canary fail to.
Works for me in a WebKit build at r115338.
Was broken in an earlier WebKit build for me, but I can confirm this is fine in Nightly.
Still broken in latest Canary though. I'll file downstream.