[fiatjaf]

Ok, let me jump in with https://fiatjaf.alhur.es/pouch-replicate-webrtc/, you can replicate a totally in-client database between browsers.

The example doesn't use a signaling server, you must copy and paste the ICE candidates between browsers (do not use the same browser for the example, that's cheating) and then the connection is established and the database is replicated every time you add a document.

Also, read the source, it's very very simple (since you must provide your own WebRTC DataChannels).