Missing Pieces, New Pieces
XML struck SGML options
XML's simplification process struck pretty dramatically at SGML's entity features. SDATA probably wasn't critical in XML's Unicode-centric world, but CDATA and SUBDOC were removed in a way thorough enough that it took an entirely different mechanism to restore their functionality.
XInclude for SUBDOC
The W3C's XInclude specification, thanks to its discussion of Infosets, already-processed XML documents, could easily combine documents that had their own DTDs.
XInclude for CDATA
By including an option to treat a document as text, XInclude made it easy to include XML documents (and other markup-unfriendly bits) as plain text, skipping the XML parser's interpretation.
Even updated for the Web
The W3C is all about the Web, right? Of course they included functionality to support Web features like content negotiation, right? Well, of course they did after being reminded that these issues are part of the Web's underlying architecture. SGML (so far as I know) never offered this kind of extra detail beyond the identifier
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