Sometimes it’s doable if you can call the API and check that the result is what you’d expect

Yeah, you can even test visual and network stuff at a cost of latency, but it’s hard and lots of developers are too lazy to do this, I’ve often seen sites that don’t even check if function exists before calling it, crashing the entire site because adblock cut out google tags or they call API that isn’t even implemented in firefox.

I’ve never worked with WebRTC but I imagine it might be difficult to do that with some of its APIs given they require camera or microphone access

I did. It’s a complete mess. First and foremost exactly because it’s a soup of completely unrelated tech - P2P, webcams, audio in&out, stream processing and compression, SIP(!?). There’s no good debug tooling available and lots of stuff is buried inside browser’s implementation. And, on top of that, any useful info on the topic is usually buried under lots of “make a skype killer in 5 minutes” kind of libraries with hardcoded TURN servers - the developer’s overpriced TURN servers, that is.