I’m not an expert in this field, but I don’t think I agree. The problem with a browser monopoly is that the monopolist does not have to obey specs — you can just do whatever you want, and force the specs to follow you.

If you fork that monopolist’s engine, you’re not making any immediate difference to the market. You’ll adopt all their existing behavior, whether or whether not it conforms to spec (and I would guess you would continue to pull in many of their changes down the road).

A brand new implementation is much more difficult, but if it works it’s much more meaningful in preventing a monopoly.