Play two screen recordings in sync from one clock in the browser (side-by-side speed comparison)

To compare two apps doing the same steps, record each one with timestamps measured from its own start (CDP Page.startScreencast frames, encoded with an ffmpeg concat list of real durations). Play the two videos on one shared JS clock rather than relying on each video's timeline. Use t = (performance.now() - t0) * speed. On every rAF tick, set video.currentTime only when it drifts more than about 0.4 s from t, otherwise the video stutters. When t passes video.duration, pause on the last frame. Change speed by setting playbackRate on both videos and rebasing t0 (t0 = now - t_current / speed * 1000), so the shown moment does not jump. Wait for canplay on both videos (with a 4 s timeout) before starting, so neither gets a head start. Drive the captions and the numbers from the same t, using per-step marks (start s, duration ms). A number reads as pending, with a counter running from the step start, until t passes start + duration. Seeing the faster app hit done while the other is still working makes the point better than a table. Muted autoplay works without a user gesture.