It do not make sense.
Not when you have 8 cores + HT == 16 “workable” cores.
We have reached the end of what single core can boost, now gains in that regard comes out of better manufacturing (smaller parts == more power efficient).
Adding more and more cores is the way to go forward.
So if you can split CSS onto 3 cores that otherwise would be idle? (IF you can do it in performance-wise manner!) Nobrainer answer: Yes!
What this whole project is about is to explore ways to do just that (by finding algorithms and by creating programming tools!).
And its no by anny means “official” mozilla project.