Well, this is pretty cool: HTTP status code 103 approved!
An HTTP Status Code for Indicating Hints
A new status code that lets the server send headers early, before the main headers
Example from the docs:
HTTP/1.1 103 Early Hints Link: </main.css>; rel=preload; as=style HTTP/1.1 103 Early Hints Link: </style.css>; rel=preload; as=style Link: </script.js>; rel=preload; as=script HTTP/1.1 200 OK Date: Fri, 26 May 2017 10:02:11 GMT Content-Length: 1234 Content-Type: text/html; charset=utf-8 Link: </main.css>; rel=preload; as=style Link: </newstyle.css>; rel=preload; as=style Link: </script.js>; rel=preload; as=script