Currently, the fast path handles keywords, hex colours and rgb/rgba, but doesn't handle rgb/rgba when the colour is terminated early and doesn't handle hsl/hsla. It would be helpful to handle those other cases, mainly so that OffscreenCanvas doesn't need to use the full parser to parse colours. Though this would be a small perf win in some (what I expect are) uncommon situations, this is mainly to help with OffscreenCanvas.
I no longer think this is a good idea, given better understanding of the colour spec. I don't think hsl(a) is used often enough to warrant maintaining a fast-path for, and I don't think the various odd but accepted ways of specifying colours make sense to complicate this code over. Regards to OffscreenCanvas, I'm taking the path of just making the full parser path verified thread-safe.