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Re: Linking to alternate representation in HTML responses using the Link element

From: Jo Rabin <jrabin@mtld.mobi>
Date: Thu, 22 May 2008 21:51:29 +0100
Message-ID: <4835DCD1.7050400@mtld.mobi>
To: Francois Daoust <fd@w3.org>
CC: public-bpwg-ct <public-bpwg-ct@w3.org>

OK. Then this now assumes the status of a recommendation that content 
that is mobile SHOULD contain such a link?

(Though if I were to be picky I'd wonder how a resource can be an 
alternative for itself ... )


On 21/05/2008 16:33, Francois Daoust wrote:
> Replying to myself after further investigations...
> Dom reminded me that instead of "rev", which is probably correct but not 
> really ever used in practice, it is actually quite simple and 
> semantically valid to do the "linking to self" stuff with a <link> 
> element. The [uri] to set is simply... an empty string!
> Leading to:
> <link rel="alternate" media="handheld" type="[content-type]" href="" />
> Indeed, the href attribute may be an absolute or a relative link, and 
> thus the empty string here means "current page", which is exactly what 
> we need for B) below.
> Francois.
> Francois Daoust wrote:
>> Hi Aaron,
>> Hi all!
>> [Aaron, I'm pinging you more specifically on this because it seems 
>> that Google's transcoding proxy uses this mechanism, so you might be 
>> able to help with concrete practice here]
>> We talked a bit about using the "link" element in HTML responses in a 
>> previous call [1] and a bit on the mailing-list [2] to advertise the 
>> fact that:
>>  A) a page _has_ a mobile representation that may be requested
>>  B) a page _is_ a mobile representation of a resource
>> The linking mechanism seems simple in theory:
>>  <link rel="alternate" media="handheld" type="[content-type]" 
>> href="[uri]" />
>> as defined in the HTML4.01 spec [3]
>> In practice, this addresses A, but not B, IMHO, at least not directly. 
>> The definition of rel="alternate" [4] makes it clear that [uri] points 
>> to an alternative representation of the _document_, from which I 
>> understand that "linking to self" should not be permitted, at least 
>> not in theory. Am I wrong?
>> If I am, then [uri] can be the document itself, but then the questions 
>> Jo mentioned in [2] need to be answered to determine what constitutes 
>> a link to self:
>> - when the server uses redirection, does [uri] target the originally 
>> requested URI, the final one, any of them?
>> - what about query strings? It has to be part of [uri] if one is using 
>> "?experience=handheld", but aren't resources usually identified 
>> without the query string?
>> It occurs to me that we haven't discussed the "reverse linking" 
>> mechanism, that may help address B. If you have a main page 
>> "index.html" that contains a link such as:
>>  <link rel="section" href="section1.html">
>> then "section1.html" may identify itself as a section of "index.html" 
>> by defining:
>>  <link rev="section" href="index.html" />
>> Given "desktop.html" and "handheld.html", can we define:
>>  <link rel="alternate" media="handheld" type="application/xhtml+xml" 
>> href="handheld.html" />
>> in "desktop.html" and:
>>  <link rev="alternate" media="handheld" type="application/xhtml+xml" 
>> href="desktop.html" />
>> in "handheld.html" to state that "handheld.html" is the "handheld" 
>> representation of "desktop.html"?
>> I'd say yes, but I'm not quite sure this is a valid use of the "rev" 
>> mechanism.
>> (and I don't quite think that anyone really uses the "rev" mechanism 
>> at all actually, but that should not be such a problem).
>> Going back to the guidelines, I'd say:
>> A) if a _forward_ (rel) link with a "handheld" media attribute is 
>> encountered, the proxy should redirect the user to the alternate 
>> representation
>> B) if a _reverse_ (rev) link with a "handheld" media attribute is 
>> encountered, then that's it, we've found the handheld version!
>> [1] http://www.w3.org/2008/05/06-bpwg-minutes.html#item03
>> [2] http://lists.w3.org/Archives/Public/public-bpwg-ct/2008May/0011.html
>> [3] 
>> http://www.w3.org/TR/1999/REC-html401-19991224/struct/links.html#h-12.3
>> [4] http://www.w3.org/TR/1999/REC-html401-19991224/types.html#type-links
Received on Thursday, 22 May 2008 20:52:50 UTC

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