Reframe is a free browser tool that turns a horizontal video into 9:16, 4:5 or 1:1: it follows faces (BlazeFace, on-device) or motion with a smooth virtual camera, resets on scene cuts instead of whip-panning, supports two-subject framing, lets you drag keyframes or letterbox a segment instead of cropping, and exports MP4/WebM plus the crop path as JSON. It runs entirely in your browser and needs JavaScript.