I assume you meant "race conditions" when you say "racey multithreaded DOM". The multithreaded part here is still isolated in its own context (iFrame), you should never have a race condition with your main DOM thread.
I don’t know if I agree with that last bit. “Don’t do work on the gui thread” is basically a mantra. It’s also not very effective to show a progress bar in a gui unless you don’t mind the rest of the gui to be unresponsive.