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Re: Link relations in RDFa (Was: Re: Guidance on publishing in multiple formats)

From: Ivan Herman <ivan@w3.org>
Date: Thu, 10 Nov 2011 10:19:05 +0100
Cc: HTML Data Task Force WG <public-html-data-tf@w3.org>
Message-Id: <F6669D2E-9E35-4C1F-91ED-4CC018F682C5@w3.org>
To: Jeni Tennison <jeni@jenitennison.com>

On Nov 9, 2011, at 22:59 , Jeni Tennison wrote:

> On 9 Nov 2011, at 09:36, Ivan Herman wrote:
>> 2. As for the default context of HTML: indeed, there is the issue of the default prefixes. But there is another default set, namely the set of 'link relations' that are recognized as valid in a @rel or @property[1]. The decision of the RDFa WG is to rely on the IANA set of link relations in this respect[2]. It is worth referring to that, too (note that those terms are used only if there is no @vocab, meaning that in probably 99%, if not 100%, of the schema.org cases they would have no effect).
> 
> 
> Thanks, Ivan, for pointing out the discrepancies in the use of link relations. We need to work through the implications of this...
> 
> If someone uses an unprefixed term within a @rel attribute and there is no in-scope @vocab then an RDFa processor will interpret the relationships based on the IANA link relations whereas an HTML processor will interpret them based on the HTML link relations. Has the RDFWAWG done an analysis on whether the semantics are comparable? (It seems to me that rel="alternate stylesheet" has particular issues.) If it hasn't, it would be a useful thing for this group to do.

Yes, there was a discussion about this. Some in the group felt that the list, ie, the ones 'accepted' by RDFa, should be drastically reduced because the semantics may not really be clear, but others felt that the RDFa Group is not in position to override the consensus represented by the IANA registry. The latters' opinion carried...

> 
> If there *is* a @vocab in scope then the semantics of the relationship could be radically different between an HTML/microformats and RDFa processor. For example (ok, a bit of a reach), someone could define a vocabulary about graffiti artists in which the 'tag' property pointed to an imagine of their tag. We should warn publishers against using @vocab to expand properties that have the same short name as those in HTML, and warn vocabulary authors not to use properties with the same name as the HTML link relationships if they aim to have their vocabulary used by RDFa.

Correct. Note that there is some sort of a danger in general with the usage of @vocab (after all, the link relations could be considered to be a default @vocab, although the RDFa processor would accept a limited set of terms only, whereas there is no such check for @vocab in general). Indeed, authors should really be careful and possibly avoid nested @vocab. Although, if used with care, that might work well (and may be necessary with a cut-and-paste) it is also error prone a bit in a way you describe above.

> 
> In fact, using properties that are named the same as HTML link relationship terms is problematic generally, as the *subject* of the relationship will almost certainly be different. The subject of the HTML link relationships is usually (but not always) the document, whereas the subject of a property in RDFa is determined through the @about, @typeof etc attributes. So again publishers are best advised not to use @vocab for such properties, and vocabulary authors to try to avoid those terms.

Absolutely.

> 
> If publishers use normal HTML link relations within the scope of @vocab then they will be interpreted as being part of that vocabulary (which they almost certainly aren't). We should warn publishers to avoid using @vocab where there are normal HTML relations in scope (in particular not to use it on the html or head element where they are common place), or to use vocab="http://www.w3.org/1999/xhtml/vocab#" on links that have normal HTML relationships within that scope.

I agree. 

Note, however, that there is one linked relation, namely describedby, which is explicitly referring to the POWDER Spec, meaning that its full URI is not in the xhtml/vocab namespace. This could happen for other links in the future, so the usage of the full URI should be done with care.


> 
> The microformats list of rel values [3] need to be considered as well. That's a much bigger list to beware of...

The (anecdotical) information we got was that the HTML5 group is considering the IANA registry as the authoritative source. That is why the RDFa group has chosen it...

Ivan


> 
> Have I missed anything?
> 
> Jeni
> 
>> [1] http://www.w3.org/2011/rdfa-context/html-rdfa-1.1.html
>> [2] http://www.iana.org/assignments/link-relations/link-relations.xml
> [3] http://microformats.org/wiki/existing-rel-values
> -- 
> Jeni Tennison
> http://www.jenitennison.com
> 
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Thursday, 10 November 2011 09:16:44 GMT

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