The 4xx class of status code is intended for cases in which the client
seems to have erred. Except when responding to a HEAD request, the server
SHOULD include an entity containing an explanation of the error situation,
and whether it is a temporary or permanent condition. These status codes
are applicable to any request method. User agents SHOULD display any included
entity to the user.
If the client is sending data, a server implementation using TCP SHOULD
be careful to ensure that the client acknowledges receipt of the packet(s)
containing the response, before the server closes the input connection.
If the client continues sending data to the server after the close, the
server's TCP stack will send a reset packet to the client, which may erase
the client's unacknowledged input buffers before they can be read and
interpreted by the HTTP application.
The resource identified by the request is only capable of generating response
entities which have content characteristics not acceptable according to
the accept headers sent in the request.
Unless it was a HEAD request, the response SHOULD include an entity containing
a list of available entity characteristics and location(s) from which
the user or user agent can choose the one most appropriate. The entity
format is specified by the media type given in the Content-Type header
field. Depending upon the format and the capabilities of the user agent,
selection of the most appropriate choice MAY be performed automatically.
However, this specification does not define any standard for such automatic
selection.
If the response could be unacceptable, a user agent SHOULD temporarily
stop receipt of more data and query the user for a decision on further
actions.