103 Early Hints

An interim response: the server sends it before the final one, and the client keeps waiting.

When a server should send it. While the final response is still being prepared, to give the browser Link headers for files it will certainly need: stylesheets, fonts or a preconnect to another origin. Only hint what every response for that URL uses, because the hints go out before the server knows the outcome.

Common causes. A server or CDN telling the browser to start fetching CSS, fonts or scripts while the page is still being generated.

What to do. Nothing. It is a speed feature; the final status code follows.

Browsers and HTTP clients. Chromium-based browsers act on preload and preconnect hints in a 103 sent over HTTP/2 or HTTP/3; support in other browsers is partial. Clients that do not understand it simply wait for the final response.

Google Search. An interim response has no effect on indexing; Google acts on the final status that follows it. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Never cached: an interim response is not a final answer to the request.

HTTP/1.1 103 Early Hints
Link: </style.css>; rel=preload; as=style
Link: <https://fonts.example>; rel=preconnect

(the final response follows on the same connection)