Even the visualization is drawn in the parallel worker. Which CANNOT be put in a worker. If you wait long enough on the demo page, you will see how much time just redrawing the DOM is consuming.
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.
Heavy data visualizations, interactive infographics etc. Also, if your app has the capability to run third party plugins you generally want to run it in a separate context for security reasons. With PDom, you also isolate yourself from the perf implication the 3rd pary code may have.
There are no good webGL charting libraries. SVG is the gold standard as the fidelity is way higher. No one wants to look at charts which look like science experiments.
Except it’s literally the opposite. Chrome gets a lot of hate for being associated with google, but it’s always innovating and implementing the latest standards. Internet explorer, at least in it’s last years was the complete opposite.