Note: A protocol-independent system for feature tag registration
      is currently being developed in the IETF.  This specification does
      not define any feature tags.  In experimental situations, the use
      of tags which start with "x." is encouraged.

   Feature tags are used in feature sets (section 6.2) and in feature
   predicates (section 6.3).  Feature predicates are in turn used in
   features attributes (section 6.4), which are used in variant
   descriptions (section 5).  Variant descriptions can be transmitted in
   Alternates headers (section 8.3).

   The US-ASCII charset is used for feature tags.  Feature tag
   comparison is case-insensitive.  A token tag XYZ is equal to a
   quoted-string tag "XYZ". Examples are

      tables, fonts, blebber, wolx, screenwidth, colordepth




 
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   An example of the use of feature tags in a variant description is:

      {"index.html" 1.0 {type text/html} {features tables frames}}

   This specification follows general computing practice in that it
   places no restrictions on what may be called a feature.  At the
   protocol level, this specification does not distinguish between
   different uses of feature tags: a tag will be processed in the same
   way, no matter whether it identifies a property, capability, or
   preference.  For some tags, it may be fluid whether the tag
   represents a property, preference, or capability.  For example, in
   content negotiation on web pages, a "textonly" tag would identify a
   capability of a text-only user agent, but the user of a graphical
   user agent may use this tag to specify that text-only content is
   preferred over graphical content.

6.1.1 Feature tag values

   The definition of a feature tag may state that a feature tag can have
   zero, one, or more values associated with it.  These values
   specialize the meaning of the tag.  For example, a feature tag
   `paper' could be associated with the values `A4' and `A5'.

      tag-value  = token | quoted-string

   The US-ASCII charset is used for feature tag values.  Equality
   comparison for tag values MUST be done with a case-sensitive, octet-
   by-octet comparison, where any ""%" HEX HEX" encodings MUST be
   processed as in [1].  A token value XYZ is equal to a quoted-string
   value "XYZ".

6.2 Feature sets

   The feature set of a user agent is a data structure which records the
   capabilities of the user agent and the preferences of the user.

   Feature sets are used by local variant selection algorithms (see
   appendix 19 for an example).  A user agent can use the Accept-
   Features header (section 8.2) to make some of the contents of its
   feature set known to remote variant selection algorithms.

   Structurally, a feature set is a possibly empty set, containing
   records of the form

      ( feature tag , set of feature tag values )







 
RFC 2295            Transparent Content Negotiation           March 1998


   If a record with a feature tag is present in the set, this means that
   the user agent implements the corresponding capability, or that the
   user has expressed the corresponding preference.

   Each record in a feature set has a, possibly empty, set of tag
   values.  For feature tags which cannot have values associated with
   it, this set is always empty.  For feature tags which can have zero,
   one, or more values associated with it, this set contains those
   values currently associated with the tag.  If the set of a feature
   tag T has the value V in it, it is said that `the tag T is present
   with the value V'.

   This specification does not define a standard notation for feature
   sets.  An example of a very small feature set, in a mathematical
   notation, is

      { ( "frames" , { } ) ,
        ( "paper"  , { "A4" , "A5" } )
      }