So I (lets be honest, Claude) compiled Quake 3 to WebAssembly. One link, no installs, no accounts. Click Host, share the 4-letter code, and you're fragging. Game traffic goes peer-to-peer over WebRTC, signaling is a single Lambda function. The whole thing costs about $1/day under heavy load.
Notes: There's probably a ton here that could be better, faster, cooler. Please fork and DIY if you're into this sort of thing :-) https://github.com/JWally/web-quaker