Actually, it's the other way around: the single thread in a browser is an UI thread, and what's seriously broken is using it for anything else!
If you ever want to do non-UI work, you should use Web Workers, which were invented specifically because doing non-UI complex tasks in the UI thread got really common when HTML unexpectedly became an application platform.
If you ever want to do non-UI work, you should use Web Workers, which were invented specifically because doing non-UI complex tasks in the UI thread got really common when HTML unexpectedly became an application platform.