As per https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/animation/keyframe_effect.cc?l=169&rcl=ff8ac07365b339290a799c776f877db74dcc1c47, we do not currently support setKeyframes for CSS Transitions due to the complexity involved. The setKeyframes API can basically mutate a CSS Transition into a full Animation.

We should, of course, support this API for CSS Transitions.