Incompatibility is certainly a headache, but having worked on a 10k-line, Firefox-only Javascript project, I can say with confidence that there are a number of

Incompatibility is certainly a headache, but having worked on a 10k-line, Firefox-only Javascript project, I can say with confidence that there are a number of pain-points in the language itself. Scoping weirdness, the erratic semantics of the 'this' keyword, etc. If you had multiple languages atop a VM, it would be simpler, in fact. The browser only needs to worry about bytecode, except in the case of Javascript, where the inclusion of Rhino (or its equivalent) would be needed for pragmatic reasons. Any hooks into the DOM would be exposed through the VM, and a sensible way to talk to the VM would be the language implementor's problem.