It is the possibility of ids being in schemes like
HTTP that would introduce the possibility of scheme specific URI
comparisons, not the ability for URIs to be expressed as being relative
to xml:base.
If the consensus is that HTTP URLs should be allowed for IDs then relative URIs should be banned. If a
specific URI scheme is mandated then it should be one
that doesn't have a relative URI scheme. Either way
relative URIs for IDs are a bad idea.
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