HTTP 103 – Early Hints
Used to return some response headers before the final HTTP message, e.g. for preloading resources.
Common use case
A server can send Link: rel=preload headers in a 103 while it is still assembling the real page, letting the browser start fetching CSS, fonts, and scripts before the final HTML arrives.
Example
HTTP/1.1 103 Early Hints
Link: </style.css>; rel=preload; as=style
History
Standardised in RFC 8297 (2017) after Google and Cloudflare experimented with it to shave time off page loads.
Did you know?
Chrome, Firefox, and major CDNs (Cloudflare, Fastly) all shipped support quickly because the performance win - effectively free preloading - was so easy to measure.