The minimum would be a stateless server handling <input type=file>, storing it in the file system and responding with the path exposed by nginx or something. The user agent will take care of the playback, be it Firefox, VLC or mpv.
Now if your average users upload high bitrate videos but have shitty bandwidth, you'll need to transcode them down to lower resolution or higher compression to save them from rebuffering. Still, no client-side scripting needed.
In case their connection quality is unstable, HLS is finally necessary for on-the-fly adjustment of playback bitrate. This is the secondary purpose of JS viewers around the web these day (the primary one being DRM). The other possibility with custom viewer is to lighten the server load by enabling P2P transfering, e.g. in case of PeerTube. Realistically, you'd either deploy the barebone one I mentioned in the beginning or set up a PeerTube instance; anything in between is probably a waste of engineering effort.