[malandrew]

So earlier I said that making another rendering engine would be a 5-7+ project. How about taking WebKit, forking it and making it highly parallelizable, piece by piece? Is that even possible, or is a big rewrite from scratch with parallelism in mind the only way to go?

[bzbarsky]

We certainly considered that path, with both Gecko and WebKit.

In theory, that's possible, but in practice it would likely take just as long, and at the end you would have a rendering engine that was years behind the competition. Not least because intermediate stages wouldn't be upliftable back to the main codebase because partially parallelizing things leaves you with worse performance than either parallelizing nothing or everything, in many cases.