In various places the text says something like:
An Annotation should have exactly 1 http/https URI that identifies it.
I do not think we should restrict anywhere the identification to be HTTP(S) (even with a SHOULD); we should be silent on that issue and stick to URI-s. There are other URI schemes around, and communities do use those, whether we like it or not. A typical example is the usage of the DOI scheme in the scientific publishing domain (even if there is a standard 'mapping' of those to HTTP); we do not want to give the impression that those are not usable.
I know that the Linked Data community has the extra constraint, when using RDF, to insist on HTTP(S). We can add a (non-normative) note that it is advised to use HTTP(S) for a better linkage to other data. But that is different than requiring it in a normative text.
(as an aside, it is probably better to capitalize on http(s), that is the usual usage).
Ivan
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Ivan Herman, W3C
Digital Publishing Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
ORCID ID: http://orcid.org/0000-0003-0782-2704