I built CinePeer , a desktop streaming app: a Prime Video-style interface where you browse with TMDB metadata, click play, and the video starts while the torrent is still downloading . The UI took a weekend. The part that took actual thinking was this: How do you make <video> play a file that doesn't exist yet? The problem An HTML5 video element needs an HTTP URL. It seeks by sending Range headers — "give me bytes 1048576–2097151". A normal file server reads...