Media feature considerations in HTTP

> >Also, a reason to be careful of feature tags is that they circumvent
> >reification in HTTP header assertions.  For example, if I wanted to say
> >that the negotiated content varied by some attribute that was expressed
> >as a feature tag rather than as an HTTP header, I cannot use the HTTP
> >Vary header.
> 
> ?  I'd suggest just use "Vary: Content-features".  This header was designed 
> exactly *for* use in content negotiation.

Yes, but "Vary: Content-features" can only, I think, be interpreted to
mean that the content varies with *all* media features, not any
particular one.  Information very useful to HTTP intermediaries is being
lost because it's been encapsulated such that it cannot be referred to by
other headers.

With "Man" from RFC 2774, "Man: Content-features" means that the client
is requiring that the Content-features header be understood.  If that
client wanted to ask that a particular media feature, say "xmlns", be
understood, it would not be able to use 2774.

MB
-- 
Mark Baker, Chief Science Officer, Planetfred, Inc.
Ottawa, Ontario, CANADA.      mbaker@planetfred.com
http://www.markbaker.ca   http://www.planetfred.com