RFC 2295            Transparent Content Negotiation           March 1998


8.2 Accept-Features

   The Accept-Features request header can be used by a user agent to
   give information about the presence or absence of certain features in
   the feature set of the current request.  Servers can use this
   information when running a remote variant selection algorithm.

      Note: the name `Accept-Features' for this header was chosen
      because of symmetry considerations with other Accept- headers,
      even though the Accept-Features header will generally not contain
      an exhaustive list of features which are somehow `accepted'.  A
      more accurate name of this header would have been `Feature-Set-
      Info'.

       Accept-Features = "Accept-Features" ":"
                   #( feature-expr *( ";" feature-extension ) )

       feature-expr = [ "!" ] ftag
                    | ftag ( "=" | "!=" ) tag-value
                    | ftag "=" "{" tag-value "}"
                    | "*"

       feature-extension = token [ "=" ( token | quoted-string ) ]

   No feature extensions are defined in this specification.  An example
   is:

       Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
                  paper = A4, paper!="A2", x-version=104, *

   The different feature expressions have the following meaning:

      ftag       ftag is present

      !ftag      ftag is absent

      ftag=V     ftag is present with the value V

      ftag!=V    ftag is present, but not with the value V

      ftag={V}   ftag is present with the value V, and not with any
                 other values

      *          the expressions in this header do not fully describe
                 the feature set: feature tags not mentioned in this
                 header may also be present, and, except for the case
                 ftag={V}, tags may be present with more values than
                 mentioned.




 
RFC 2295            Transparent Content Negotiation           March 1998


   Absence of the Accept-Features header in a request is equivalent to
   the inclusion of

      Accept-Features: *

   By using the Accept-Features header, a remote variant selection
   algorithm can sometimes determine the truth value of a feature
   predicate on behalf of the user agent.  For example, with the header

       Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
                  paper = A4, paper!="A2", x-version=104, *

   the algorithm can determine that the following predicates are true:

       blex, colordepth=[4-], colordepth!=6, colordepth, !screenwidth,
       paper=A4, colordepth=[4-6]

   and that the following predicates are false:

       !blex, blebber, colordepth=6, colordepth=foo, !colordepth,
       screenwidth, screenwidth=640, screenwidth!=640,

   but the truth value of the following predicates cannot be
   determined:

       UA-media=stationary, UA-media!=screen, paper!=a0,
       x-version=[100-300], x-version=[200-300], x-version=99,
       UA-media=screen, paper=A0, paper=a4, x-version=[100-199], wuxta

8.3 Alternates

   The Alternates response header is used to convey the list of variants
   bound to a negotiable resource.  This list can also include
   directives for any content negotiation process.  If a response from a
   transparently negotiable resource includes an Alternates header, this
   header MUST contain the complete variant list bound to the negotiable
   resource.  Responses from resources which do not support transparent
   content negotiation MAY also use Alternates headers.