Just one example: http://twgljs.org
JavaScript, for all its flaws, is far less painful to use than C for this.
If there is a need to break compatibility, I think it's an opportunity to provide access to a standardized browser vm or a new language. If google or mozilla do and attract developers, it would be a nice thing: make it so it can attract users to use firefox, while making it open so that apple and google can implement it.
It could happen if it could be included inside mozilla nightly, but unless there is wide acceptance developers might not choose it, so it's an eternal chicken egg problem.
To me, there are sane alternative to js that are compatible with all platforms, only that it's very cumbersome. Js is just easy to deploy, but that's its only advantage. I don't think I should use a shitty language just because OS developers refuse to deliver and support better tools. I don't think I should give up all the existing C and C++ libs because they don't work anymore on android/iOS.
There are some differences, yes. Writing code in the subset that works in both non-strict and strict modes is easy, however. And old browsers aren't a big problem.
Also, I've come into many situations where webgl doesn't work in some particular situations, would it be a a combination of OS, webgl driver support or web browser.
I guess it should work and will as time passes, but as long as there is nothing else than javascript, I won't bother.