Sophisticated services that offer SSE will have many different endpoint domains to get around browser limits on simultaneous connections to the same service.
That's what I'm saying: it's not that good for non-sophisticated services, you need workarounds to make this "great technology" work. Might as well switch to websockets and not worry about this.
Right, but with websockets you wouldn't have to think about this at all.
You'd also need to ensure http2 for local development, proxies in between, etc. You _can_ do all this, but you can also avoid putting any thought into this matter.
endpoint-00.example.com endpoint-01.example.com endpoint-02.example.com
etc. It isn't specific to SSE. It's any HTTP request. SSE is just the most obvious because the connections stay open for long periods.