Nested DTDs

John Cowan cowan at
Fri Jun 18 18:44:57 BST 1999

Paul Prescod wrote:

> XML ended up breaking too many other SGML rules for this to happen. The
> empty end tag syntax was the most serious example but there are others.

This is a myth. As says:

# NET delimiters can be used only to close an empty element. In SGML
# without the Web SGML Adaptations Annex, the NET delimiter
# is declared as />. With this approach, XML is not allowing null
# end-tags and is allowing net-enabling start-tags only for elements
# with no end-tag.
# In SGML with the Web SGML Adaptations Annex, there
# is a separate NESTC (net-enabling start tag close) delimiter.
# This allows the XML <e/> syntax to be handled as a combination
# of a net-enabling start-tag <e/ and a null end-tag >.
# With this approach, XML is allowing a net-enabling start-tag only
# when immediately followed by a null end-tag.

In short, XML empty tags are proper SGML, including pre-1996 SGML,
with some side constraints eliminating non-XML flexibilities
like "<foo/bogus content>", which "nsgmls -xml" duly complains about.

John Cowan		cowan at
	You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
	You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
		Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)

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