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

[Ed Summers]

Ian,

Thanks very much for the guidance re: using <meta> and <link>. I like
both solutions quite a bit better than leaning more on itemid for this
use case.

Was the "id" itemprop you used in your examples a hypothetical
property that would need to be defined at schema.org or elsewhere, or
did you find it defined already?

//Ed