renderFrame(n) is a pure function of the frame number.
A virtual clock coerces Date, requestAnimationFrame, and Math.random to frame-time, so there is no wall-clock and no un-seeded randomness. The same scene renders byte-identical frames regardless of order.
That is what lets frames shard across parallel browser tabs, and what makes every video reproducible and diff-able. It is guarded by make probe, which renders sampled frames in scrambled order and diffs the DOM.
renderFrame(n) // pure in n → same bytes, any order → make probe // verifies it