[whatwg] itemtypes from same vocabulary

[Ed Summers]

Hi all,

I am looking for some guidance about the use of multiple itemtypes in
microdata [1], specifically the phrase "defined to use the same
vocabulary" in:

"""
The item types must all be types defined in applicable specifications
and must all be defined to use the same vocabulary.
"""

For example, does this mean that I can't say:

<div itemscope itemtype="http://acme.com/Foo http://zenith.com/Bar"> ... </div>

The reason I ask is that there is some desire over in the schema.org
community [2] to provide a mechanism for schema.org to be specialized.
For example, in the case of an audiobook:

<div itemscope itemtype="http://schema.org/Book
http://www.productontology.org/id/Audiobook"> ... </div>

The idea being not to overload schema.org with more vocabulary, and to
let vocabularies grow a bit more organically. This schema.org group is
currently thinking of using a one off property additionalType that
would be used like so:

<div itemscope itemtype="http://schema.org/Book">
  <link itemprop="additionalType"
href="http://www.productontology.org/id/Audiobook">
  ...
</div>

I personally find this to be kind of distasteful since it replicates
the mechanics that microdata's itemtype already offers.

So, my question: is it the case that itemtype cannot reference types
in different vocabularies like the example above? If so, I'm curious
to know what the rationale was, and if perhaps it could be relaxed.

//Ed

[1] http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#items
[2] http://lists.w3.org/Archives/Public/public-schemabibex/2013Feb/0000.html