Hmm. I wish I could point you somewhere concrete. This is stuff I gleaned from the xml-dev and xsl-list mailing lists, posted by people such as Tim Bray and Michael Kay. I assume the folks who wrote the specs know what they say. :-) Unfortunately it means I don't have any reference point on hand. Maybe I should look it up on opportunity, or at least find relevant archive links from the lists.

Makeshifts last the longest.