[lostmsu]

As I mentioned in the sibling, there's multicast/broadcast (classic multiplayer games used it) and you could do IP scanning.

[jzm2k]

I still don't understand what this has to do with WebRTC/WebSocket. You can combine, e.g., mDNS with WebRTC and build a pure peer-to-peer network.

[lostmsu]

You can't use mDNS in the browser.

The standards are carefully defined in a way that creates server dependency. WebSocket outright does not allow connections to other browsers directly. WebRTC does, but you are extremely limited in how you could do discovery. I think both are intentional.