The 1xx (Informational) class of status code indicates an interim response for communicating connection status or request progress prior to completing the requested action and sending a final response. Since HTTP/1.0 did not define any 1xx status codes, a server MUST NOT send a 1xx response to an HTTP/1.0 client.

A 1xx response is terminated by the end of the header section; it cannot contain content or trailers.

A client MUST be able to parse one or more 1xx responses received prior to a final response, even if the client does not expect one. A user agent MAY ignore unexpected 1xx responses.

A proxy MUST forward 1xx responses unless the proxy itself requested the generation of the 1xx response. For example, if a proxy adds an “Expect: 100-continue” header field when it forwards a request, then it need not forward the corresponding 100 (Continue) response(s).

https://www.rfc-editor.org/rfc/rfc9110.html#name-informational-1xx