FYI; this would affect the interpretation of <link> in Atom as well...
Begin forwarded message:
Resent-From: [email protected]
From: Mark Nottingham <[email protected]>
Date: 8 April 2009 9:10:58 AM
To: HTTP Working Group <[email protected]>
Cc: Ian Hickson <[email protected]>
Subject: [link draft] Changing the model for links
Archived-At: <http://www.w3.org/mid/[email protected]
>
I've been discussing the link draft with Ian Hickson, who points out
that in HTML4, there's a difference between
<link rel="stylesheet" href="a"/>
<link rel="stylesheet alternate" href="a"/>
and
<link rel="stylesheet alternate" href="a/>
(see <http://www.w3.org/TR/html401/present/styles.html#specifying-external
> for the background of why these are different)
In the current link draft, there isn't any way to express the
difference between these; the underlying model is
[ context ] ---[ relation type ]---> [ target ]
where 'relation type' is singular.
To accommodate this use, the model would need to be something like
[ context ] ---[ list of relation types ]---> [ target ]
noting that there may be more than one list of relation types
between any context and target.
Personally, I think that it would be only in pathological cases that
it would be necessary to know the difference between the two (i.e.,
real world Web pages will not point to a URI as both a stylesheet
and as an alternate for themselves, so it's safe to say that even
the first example above means that "a" is an alternate stylesheet).
However, it is important for Link to interoperate well with HTML4.
Also, the HTML5 folks plan to use this model for other purposes
(e.g., "up up" to indicate a parent of a parent).
The practical impact of making this change is that serialisations of
links won't be able to collapse multiple relation types between two
URIs into one link; they'll have to be separate to allow this
interpretation.
So, for example, if you have link types ['w', 'x', 'y z'] between A
and B, it will have to be serialised as
Link: <B>; rel="w"
Link: <B>; rel="x"
Link: <B>; rel="y z"
in HTTP headers, NOT
Link: <B>; rel="w x y z"
because that's ambiguous.
The alternative is to say that the 'stylesheet alternate'
combination isn't specific to how it's serialised, but is tied to
the occurrence of the links. I.e., when both relations are present
in links between the same resources, these special semantics take
affect. However, this does seem to directly conflict with the HTML4
language (see link above), so I don't think doing so is viable.
Comments?
--
Mark Nottingham http://www.mnot.net/
--
Mark Nottingham http://www.mnot.net/
