Is there any P2P network that fits that criteria? The first time you connect, you need to know at least one publicly reachable node but I fail to see how that relates to WebSocket/WebRTC. I think central server is somewhat ambiguous because the bootnode could be any node on the network you have prior knowledge of.
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.