[whatwg] Proposal: allow itemid to record multiple identifiers for an item

[Ed Summers]

Hi WHATWG,

Over on a schema.org related list [1] there has been a discussion
about making identifiers for bibliographic items (books, articles,
etc) available in microdata using itemid. The use case is well
described in the dev edition, with this example [2]:

<dl itemscope
    itemtype="http://vocab.example.net/book"
    itemid="urn:isbn:0-330-34032-8">
 <dt>Title
 <dd itemprop="title">The Reality Dysfunction
 <dt>Author
 <dd itemprop="author">Peter F. Hamilton
 <dt>Publication date
 <dd><time itemprop="pubdate" datetime="1996-01-26">26 January 1996</time>
</dl>

Our use case is when a publisher wants to record multiple identifiers
for an item. For example The Reality Dysfunction also has an OCLC
number 225870439, which can be expressed as a URI,
info:oclcnum/225870439. It would be convenient if we could say:

<dl itemscope
    itemtype="http://vocab.example.net/book"
    itemid="urn:isbn:0-330-34032-8 info:oclcnum/225870439">
 <dt>Title
 <dd itemprop="title">The Reality Dysfunction
 <dt>Author
 <dd itemprop="author">Peter F. Hamilton
 <dt>Publication date
 <dd><time itemprop="pubdate" datetime="1996-01-26">26 January 1996</time>
</dl>

But it appears that the current specification does not allow for this [3]

"""
The itemid attribute, if specified, must have a value that is a valid
URL potentially surrounded by spaces.
"""

One alternative to using itemid would be to define an oclcnum itemprop
to the relevant schema.org types. This solution is a bit unwieldy
because there are so many identifier types in the bibliographic world.
I imagine the same is similar in other domains.

Another solution would be to come up with some generic identifier
mechanism, but I feel like this would then duplicate what itemid is
already doing.

So, is it feasible for the Microdata specification to allow for
multiple space separated identifiers in an itemid, similar to the set
of space-separated tokens microsyntax [4]? Or is there an alternate
solution that I'm missing?

Thanks for your attention,
//Ed

[1] http://lists.w3.org/Archives/Public/public-schemabibex/2012Dec/0000.html
[2] http://developers.whatwg.org/links.html#global-identifiers-for-items
[3] http://www.whatwg.org/specs/web-apps/current-work/multipage/microdata.html#items
[4] http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#set-of-space-separated-tokens