Preliminary headers so the browser can start preloading.
When to use it
Send Link headers for CSS and fonts while the page is still being built.
Good to know
103 lets the server send Link headers with rel=preload or rel=preconnect while it is still building the real response, so the browser fetches CSS, fonts and scripts during server think time. Chromium browsers act on it only over HTTP/2 and HTTP/3, and only for the main navigation; support in Firefox and Safari is more limited. Old clients and some proxies mishandle unexpected 1xx responses, so test before rolling out.
Hints must be safe for every possible final response: if the page ends as a redirect or an error, the preloaded files were wasted bandwidth. Cloudflare can generate 103 responses for you from Link headers it saw on earlier responses from your origin. Check the result with curl -v, which prints the interim 103 before the final status, or in DevTools, where early-hinted requests appear before the document finishes.
Informational (1xx)
1xx codes are informational: the server has received the request and the exchange continues. Browsers and HTTP libraries handle them for you.
What it looks like
HTTP/1.1 103 Early HintsHow to check it
curl -s -o /dev/null -w "%{http_code}\n" https://example.com/