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[link draft] Changing the model for links

From: Mark Nottingham <mnot@mnot.net>
Date: Wed, 8 Apr 2009 09:10:58 +1000
Message-Id: <C1CD0848-13F0-4FCF-91FF-52126622C3B8@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>
Cc: Ian Hickson <ian@hixie.ch>
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/
Received on Tuesday, 7 April 2009 23:11:44 GMT

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