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

Philip Taylor excors+whatwg at gmail.com
Thu May 14 18:07:40 PDT 2009

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

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

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