[AndriyKunitsyn]

Which begs the question - if all of these projects got it "wrong", what's the chance that the "right" thing isn't right at all?

All animation is inherently discrete. No matter how many threads you have, there always has to be the last rendering thread, the thing that actually prepares the calls to the rendering backend. It always has to have frames, and in every frame, in the timestamp T, it will be interested in getting the world state in the timestamp T. So, the things that work on the world state - they have to prepare it as it was in T, not earlier, not later. You cannot completely decouple it.

In one of game projects that I worked on, a physics thread and a game thread actually were pretty decoupled, and what the game thread did was extrapolating the world state from the information provided by physics, because it knew not only the positions of physics objects, but also their velocities. Can we make every web developer to set velocities to the HTML elements explicitly? Probably not.