It is somewhat limited, but using message passing to the main thread, you can render a UI or even use WebGL or Web Audio or anything else. See the link for an example with WebGL.

Oh, I know. But it's rare that a webapp does things computational enough that it's worth the extra overhead - most lagginess is actually at the UI level, in my experience (and why virtual DOMs are so popular now).

I've seen Web Workers used in things like graphing libraries, though.