Oh interesting. TBH, I have zero knowledge on how to do any of this on the web side. All my experience is from working with traditional game engines on the native side.
Of course, you're still dealing with the event loop in most cases, which is probably a stumbling block when it comes to really low-level stuff. That said, there are even facilities for shared memory and atomics operations [3] these days, which helps. I've messed around with it a little bit on a side project- as a JS developer, it's really weird and fun to say "screw the event loop!" and just enter an endless synchronous loop. :D
[1] https://developer.mozilla.org/en-US/docs/Web/API/Worker [2] https://developer.mozilla.org/en-US/docs/Web/API/MessageChan... [3] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...