You still ultimately need some channel to set up that initial P2P connection. This allows the peers to share the IP addresses at which they’re accessible, as well as the formats and codecs they each support.

There’s nothing to stop you doing this through broadcast messages on a local network, though I’m not aware of any way that could be done from within a browser. You could extract the SDP and ICE messages from the browser environment and handle that in Node though, if you were using Electron or something.