[whatwg] Annotating structured data that HTML has no semantics for

[Philip Taylor]

On Thu, May 14, 2009 at 2:54 PM, Philip Taylor <excors+whatwg at gmail.com> wrote:
> [...]
>  <urn:subject> <urn:predicate> _:X .
> [...]
>  <div item>
>    <link itemprop="about" href="urn:subject">
>    <meta itemprop="urn:predicate" item id="X">
>  </div>
> [...]
> So, I can't see any limits on expressivity other than that literals
> must be strings.

Hmm, I think I'm wrong here. 'id' has to be unique, which means this
pattern won't work if _:X is the object for triples with two different
subjects.

Additionally, there must be a chain from every blank node back to <>
via <http://www.w3.org/1999/xhtml/vocab#item>, else it won't get
serialised (since serialisation starts from top-level items and
recurses down the correspondence chains). As a consequence of this and
the previous point, it is impossible to express cycles (e.g. "_:X
<urn:predicate> _:X", or any longer cycles) unless the cycle contains
<>.

So there are these two restrictions on the shapes of expressible RDF
graphs. (I can't think of any other restrictions, though...)

-- 
Philip Taylor
excors at gmail.com