Marble Run, a 3D physics sandbox in the browser that exports real MP4

I built this one myself. No signup, no payment, it all runs in the browser. You place ramps, walls, pegs, spinners, boxes and balls on a lit 3D wall, press play, and a real rigid body simulation runs. Export a 9:16 MP4 with sound when a run feels right. Two details that mattered more than I expected. The export is a real MP4 encode, not a screen recording, because a screen recording of 3D looks flat and the audio never lines up with the impacts. And the whole thing runs client side, so there is