The SGML community was very forthcoming in working with us to avoid the existence of any real or perceived split between SGML and XML. To that end, they designed, proposed, and voted into the ISO standard a set of changes that removed many potential irritants. This was a good thing all around.
Fortunately or unfortunately, there is a large installed base of SGML products that don't know about these changes to the standard and will thus (quite properly) complain about some things that can appear in XML documents. Wherever this can happen, the XML spec contains a "for interoperability" warning, and if you think you might need to feed your XML documents to existing SGML products, you should take these seriously.
Copyright © 1998, Tim Bray. All rights reserved.