It wouldn't matter if the DOM layout engine ran on a separate thread, but it runs on the main thread, so it blocks your games update loop for a few frames every time you shoot, or whatever.

Now, that's not to say it has to be that way; real engines have a separate render thread that would sort of solve that problem. And they run the game update code in parallel, and all kinds of other stuff. But, by the time you have all that stuff, you can probably rasterize a font and draw quads ;)

But don't take my word for it. Maybe it's better now. Make a thing that moves some divs around in a requestAnimationFrame callback and take a look at the profile. I bet 2% of the time the layout engine takes 30ms+ to do basically nothing