I’m trying to wrap my little bonce around HTML5 microdata, not least because Opera 12 pre-alpha has support for it.
I’m quite discouraged because the two articles I’ve read tell me that it’s easy, but I’m still stuck (although they are by noted brain-boxes Oli Studholme and Tab Atkins).
I’m befuddled over
Sometimes, an item gives information about a topic that has a global identifier. For example, books can be identified by their ISBN number.
Vocabularies (as identified by the itemtype attribute) can be designed such that items get associated with their global identifier in an unambiguous way by expressing the global identifiers as URLs given in an itemid attribute.
The exact meaning of the URLs given in
itemidattributes depends on the vocabulary used.
What actually does this mean? How do I know if a particular vocabulary supports global identifiers for items?
In the spec, some Microdata vocabularies are listed. vCard is one, and the spec says “This vocabulary supports global identifiers for items.” The URL defining the itemtype for vCard doesn’t seem to tell me, and the examples in the spec make no use of
And, because I understand real examples rather than the theoretical, what’s the practical benefit of
Specifically, what would I gain by using
<div itemscope itemtype="http://vocab.example.net/book" itemid="urn:isbn:0867193719"> Rebekah Brooks' Self-portraits (ISBN 0867193719) </div>
[example simplified from an example in the spec which says "The "http://vocab.example.net/book" vocabulary in this example would define that the itemid attribute takes a urn: URL pointing to the ISBN of the book."]
over Schema.org’s Book schema (which doesn’t seem to use
itemid – in fact, schema.org seems to make no mention of it):
<div itemscope itemtype="http://schema.org/Book"> Rebekah Brooks' Self-portraits (ISBN <span itemprop="isbn">0867193719</span>) </div>
Double points if you can answer the question without baffling me with mentions of SPARQLy OWLs and Don’tologies.