I added If-Modified-Since: and 304 Use Local Copy to HTTP spec.
I also added 204 No Response that seemed to be missing from there.
If-Modified-Since: date
This request header is used with GET method to make it conditional: if
the requested document has not changed since the time specified in
this field the document will not be sent, but instead a Use Local Copy
304 reply.
Format of this field is the same as for Date:.
USE LOCAL COPY 304
If the client has done a conditional GET and access is allowed, but
the document has not been modified since the date and time specified
in If-Modified-Since field, server responds with 304 status code, and
does not send the document back to the client.
This is to allow fast update of caches.
NO RESPONSE 204
Server has received the request but there is no information to send
back, and the client should stay in the same document view. This is
mainly to allow input for scripts without changing the document at the
same time.
-- Cheers, Ari --