network readiness / captive portal detection?

On macOS at least, browsers will sometimes pause an HTTP request in progress and block until captive portal authentication has taken place. I have not been able to find much documentation, but https://developer.apple.com/news/?id=q78sq5rv seems like an interesting jumping-off point. I'm pretty sure Windows has something similar. I think that these services are mostly implemented by using newer / weirder ways to open a socket, but I'm honestly not entirely sure how it works!