[roca]

Indeed you can incrementally improve existing engines, and certainly that's what you would try to do if you wanted one of those specific features. But I didn't write the post because I want those features, I wrote it in the hope that it might be helpful to people who are already planning to write a competitive browser engine from scratch.

Yes, for almost every conceivable features you'd be much better off adding it to an existing engine. Maybe you won't get buy-in from the core maintainers, so you'll have to maintain an out-of-tree branch, but that's still going to be much less work than doing your own engine.