Since Threema Web is pretty new, the data protocol is still in flux and may change often. (You can find the details in the source code though.)

The SaltyRTC signaling protocol on the other hand is fully specified: https://saltyrtc.org/ and https://github.com/saltyrtc/saltyrtc-meta/blob/master/Protoc... There are already libraries for Java and JavaScript (TypeScript). It can also be used independently of Threema.