The fetch API is asynchronous. The initial page load would deliver the payload that then initiates the streaming connection in the background.

That sounds less like a problem with HTTP streaming (initiated from JavaScript) and more like a page with some hanging resource.

I'm guessing you would use JS to fetch() the stream resource separately.

Not sure what you mean. How else are you going to make the request if not using fetch()?

You could use an iframe as well, with certain caveats.