HTTP 103 – An HTTP Status Code for Indicating Hints
Summary: a new status code that lets the server send headers early, before the main headers. This helps with optimisations like preloading. Example from the document:     HTTP/1.1 103 Early Hints
     Link: ; rel=preload; as=style

     HTTP/1.1 103 Early Hints
     Link: ; rel=preload; as=style
     Link: ; 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: ; rel=preload; as=style
     Link: ; rel=preload; as=style
     Link: ; rel=preload; as=script

The client can start preloading the CSS and JavaScript before the main headers arrive. This is a nice optimisation.There are various security risks with sending multiple headers to non-conforming clients hence: "Therefore, a server might refrain from sending Early Hints over HTTP/1.1 unless the client is known to handle informational responses correctly."