Yes. "100 Continue". [1] If client intends to send a POST request with request body large enough to try to avoid wasting bandwidth (in case server might reply

Yes. "100 Continue". [1] If client intends to send a POST request with request body large enough to try to avoid wasting bandwidth (in case server might reply with 3xx,4xx or 5xx), the client will send only headers including "Expect: 100-continue" header, then server indenting to reply with "200 OK", will instead first reply with "100 Continue" than client will send POST body, and finally server sends "200 OK". [1] https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.2.3 https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8....