This section defines the syntax and semantics of all standard HTTP/1.1 header fields.
If an Accept header field is present, and if the server cannot send a response which is acceptable according to the combined Accept field value, then the server SHOULD send a 406 (not acceptable) response.
A more elaborate example is Verbally, this would be interpreted as "text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi entity, and if that does not exist, send the text/plain entity." Media ranges can be overridden by more specific media ranges or specific media types.
If more than one media range applies to a given type, the most specific reference has precedence.
media-range = ( "*/*" | ( type "/" "*" ) | ( type "/" subtype ) ) *( ";" parameter ) accept-params = ";" "q" "=" qvalue *( accept-extension ) accept-extension = ";" token [ "=" ( token | quoted-string ) ] The asterisk "*" character is used to group media types into ranges, with "*/*" indicating all media types and "type/*" indicating all subtypes of that type.
The media-range MAY include media type parameters that are applicable to that range.
Each media-range MAY be followed by one or more accept-params, beginning with the "q" parameter for indicating a relative quality factor.
The first "q" parameter (if any) separates the media-range parameter(s) from the accept-params.
Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the qvalue scale from 0 to 1 (section 3.9). Note: Use of the "q" parameter name to separate media type parameters from Accept extension parameters is due to historical practice.
Although this prevents any media type parameter named "q" from being used with a media range, such an event is believed to be unlikely given the lack of any "q" parameters in the IANA media type registry and the rare usage of any media type parameters in Accept.
Future media types are discouraged from registering any parameter named "q".
SHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality." If no Accept header field is present, then it is assumed that the client accepts all media types.