Ideally, 60fps. The game has an update() function that simulates 1/60th of a second of gameplay, and it tries to call that function every 1/60th of a second, then renders the game to the screen. Unfortunately, this can't always happen, due to technical reasons. If a computer is too slow to update/render the game every 1/60th of a second, it calls update() multiple times, then renders the result to the screen. Since rendering takes up way more computing power than updating (for this game, at least), this ensures the game runs at a correct speed. If a browser chooses to render content faster than 60fps, the game skips calling update on some frames to keep the game running at the correct speed. And I haven't even started talking about interpolating between frames, or browsers that render content at framerates that aren't multiples/factors of 60fps. (You can read up on some of the technical details here; there's a lot of stuff about framerates in browsers that even I don't know.).
Even considering all that, I'd estimate that you could consistently perform/time runs within at least 50ms. But I guess it depends on a lot of factors (as I mentioned).