It's every established. Go build even a basic list with a slide transition, wrap it in Phonegap, and deploy it to an HTC device from the last year running Android 2.3. It won't work well on 90% of the smartphones out there. Simply put, even the latest devices struggle with basic CSS animations/transitions.
Hell, scrolling isn't even smooth on most devices for even a fairly simple DOM.
You misunderstood my point -- I was not talking about the current situation, but what today's devices will be capable of with software from a year in the future.
Even now, I believe Mobile Firefox is much faster than the stock webkit browser for javascript processing.
I don't really think that's established yet -- Android's default browser doesn't have the same engine that chrome does. It's obviously going to be slower than desktop, but it might still be improved to the point of being fast enough.
Also, see this post about code generation on ARM: http://news.ycombinator.com/item?id=2805825