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

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 Wednesday, 21 May 2008 15:34:24 UTC