http://lists.w3.org/Archives/Public/www-style/2012Aug/0125.html
Looks like this works correctly other than when there's a minus sign in the keyframe selector, which causes a very early rejection of the keyframe.
A key of "+10%" doesn't work either, and should.
<rdar://problem/18326801>
At least the negative keyframes (-20%) were fixed in r149106
And the positive forms work as well