>Except this is an engine that can be fixed by anyone when it's broken
If that were true then there wouldn't be any odd quirks of behavior specific to WebKit. And there are plenty.
It's not so much about going by the core specifications - browsers have become better at that - it's about all the tiny pieces of behavior around the edge that aren't in specifications. And with a monoculture of browser engines it's far more likely that a piece of unspecified behavior *becomes* a de-facto standard because "most browsers" "do it like that".
And that's where you start entering dangerous territory, standards-wise.