I'd put the count at 2¾: Blink counts for ½ since it's an ex-fork of WebKit. While a lot of the new feature development of the two are independent of each other, they still largely share a lot of the same architecture, which means there's going to be correlation in things like ease of implementation. The extra ¼ comes from Servo, which ends up having the opposite edge of the dichotomy from Gecko; it's effectively a ground-up re-implementation of the layout engine stack, so its internal architecture is quite different from everybody else, but it's goal isn't to drive new feature development in web standards.
