Today you can have two animations running by having two values in the -webkit-animation-name. If you remove one of those names (by changing -webkit-animation-name to have only one of the names) the removed animation will stop, but the remaining one will reset and start over. It should not affect the running animation.
Created attachment 23203 [details] Patch, including LayoutTest file
Comment on attachment 23203 [details] Patch, including LayoutTest file r=me
M LayoutTests/ChangeLog A LayoutTests/animations/change-one-anim-expected.txt A LayoutTests/animations/change-one-anim.html M WebCore/ChangeLog M WebCore/page/animation/AnimationBase.cpp M WebCore/page/animation/AnimationController.cpp M WebCore/page/animation/CompositeAnimation.cpp M WebCore/page/animation/ImplicitAnimation.cpp M WebCore/page/animation/KeyframeAnimation.h Committed r36356 Required a little bit of merging thanks to a clashing patch by mitz, but i think it is ok.