What it means
An interim response that sends Link headers early so the browser can preload CSS, fonts or preconnect to origins while the server is still building the final page.
Common causes
- A CDN such as Cloudflare or Fastly sends Early Hints for cached Link headers
- The application server emits 103 before slow rendering
How to fix it
As a client or visitor
Nothing to do; supported browsers start preloads and then use the final response. Clients that do not understand 103 simply ignore it.
As the site or API owner
Send only resources the final page will really use, and keep the hints identical to the Link headers of the 200 response to avoid wasted downloads.
Example response
HTTP/1.1
HTTP/1.1 103 Early Hints
Link: </styles/main.css>; rel=preload; as=style
Link: <https://cdn.example.com>; rel=preconnectSpecification
FAQ
› Which browsers support 103 Early Hints?
Chrome, Edge, Firefox and Safari support preload and preconnect hints from 103 over HTTP/2 and HTTP/3.
› Does 103 improve Core Web Vitals?
It can improve LCP and FCP when server think-time is long, because critical assets download during that wait.