In web-animation an animation with negative playbackrate does not start playing from "end" to "begin". The current time simply progresses backwards. So for animation to play from "end" to start one has to explicitly set current time to "end". Example in [2] shows this behavior .

However, in cc when an animation has a negative playback rate, the animation simply behaves as if time goes from "end" to "start". [1] 

AFIACT, web animations with negative playback rate are not currently composited so this does not actually break anything but this difference in behavior prevents us from ever doing so and creates confusion. We should try to fix this as a matter of code health.

[1] https://cs.chromium.org/chromium/src/cc/animation/keyframe_model.cc?type=cs&sq=package:chromium&g=0&l=229
[2] https://jsbin.com/curuyobuco/1/edit?html,css,output