If that’s your metaphore, then the browser engine is the kernel. Have you ever seen a Linux distro forking the kernel and taking it into a different direction? It’s always just (relatively) minor changes, if any, so that keeping up with the upstream kernel is possible. Mostly a matter of built-in drivers, and a security feature here and there.
How is it any different from developing a new Linux distro?
If that’s your metaphore, then the browser engine is the kernel. Have you ever seen a Linux distro forking the kernel and taking it into a different direction? It’s always just (relatively) minor changes, if any, so that keeping up with the upstream kernel is possible. Mostly a matter of built-in drivers, and a security feature here and there.