DOOM running in WebAssembly — a NOMMU Linux on Adrian Cable's single-instruction SUBLEQ VM, with sound effects and OPL3 music. This is a standalone demo of the client engine; the Vaadin Directory add-on wraps this exact engine as a server-side Flow component. Click the frame and play. First paint takes ~15 seconds (it boots Linux, fetches the WAD and launches DOOM).
The game data is a WAD fetched over the network — here id Software's freely
redistributable shareware episode, from the Internet Archive. The Flow component takes
that URL as its constructor argument, so an IWAD you own and host yourself
(doom.wad, doomu.wad, doom2.wad) plays the full game.