> If it becomes less attractive for web developers to write code that works across browsers, consumers could ultimately be persuaded to leave Firefox.

Developers already hate supporting multiple browsers. I can understand adopting new ECMAScript and CSS features at different paces, but having fundamentally different default DOM styling is just annoying. CSS rules interacting different between browsers is maddening. I would be perfectly fine with an open source core that all browsers used. Python provides a reference implementation for its spec, and still has a healthy (I think) ecosystem of alternate implementations.