It is beyond the scope of a protocol specification to mandate
compliance with previous versions. HTTP/1.1 was deliberately
designed, however, to make supporting previous versions easy. It is
worth noting that at the time of composing this specification, we
would expect commercial HTTP/1.1 servers to:
recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1
requests;
understand any valid request in the format of HTTP/0.9, 1.0, or
1.1;
respond appropriately with a message in the same major version used
by the client.
And we would expect HTTP/1.1 clients to:
recognize the format of the Status-Line for HTTP/1.0 and 1.1
responses;
understand any valid response in the format of HTTP/0.9, 1.0, or
1.1.
For most implementations of HTTP/1.0, each connection is established
by the client prior to the request and closed by the server after
sending the response. A few implementations implement the Keep-Alive
version of persistent connections described in section 19.7.1.1.