From [1]: "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 M

From [1]: "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." They don't need to be requested. Supporting servers will just send them, unsupporting clients will just ignore them (barring the potential security issue with broken HTTP/1.1 implementations which is noted in the 103 spec). [1] https://tools.ietf.org/html/rfc7231#section-6.2 https://tools.ietf.org/html/rfc7231#section-6.2