A 404 is the one page on a site nobody meant to open. It is also the one page with nothing to do on it, which is why this one has a runner on it instead of an apology.
It is a canvas and a loop, not a framework. The scenery is three parallax strips, each drawn once into an offscreen canvas and then scrolled, so a frame costs one blit per layer rather than a redraw of everything on it. The score is written straight into a text node by the animation loop, which means a whole run happens without React rendering once.
The name goes to the leaderboard when the run ends. Start it with an @ and it borrows your x.com photo, because a scoreboard of strangers is more fun with faces on it.