xml-dev - "XML is a subset of SGML" - True or False?

[AndrewWatt2000@aol.com]

[This message was first sent to the list on 30th June but I didn't see it or 
any responses to  it on the list. My guess is that it did not reach the list. 
It is also absent from the list archive, hence this re-send.]

The XML 1.0 Recommendation starts off with a statement to the effect that XML 
is a subset of SGML.

My question is whether that statement was true at the time it was published 
and whether it is true now.

My perception is that XML never was truly a "subset" of SGML.

Take for example the optional nature of a DTD in XML. Such a document is 
legal "XML" but is not, as far as I understand it, legal "SGML". Thus part of 
the XML domain lies outwith the SGML domain. Therefore on that point alone 
XML is not a subset of SGML.

Similarly, XML is case sensitive whereas SGML is not. Thus the way that a 
document will be handled in an SGML-compliant tool and and XML-compliant tool 
are different.

On these two points alone it seems to me that XML is NOT and NEVER WAS a 
"subset" of SGML, in any strict sense. Am I missing something fundamental 
here or is the opening statement of the XML 1.0 Recommendation inaccurate?

Of course, I am not attempting to deny similarities between XML and SGML. But 
if the XML WG wished to say there were many similarities between the two it 
seems to me that there were more precise ways of expressing that available to 
the authors.

The secondary part of my question relates to the implications of XML Schema 
for the assertion that "XML is a subset of SGML". As far as I am aware SGML 
has no such feature. If so, then (yet another) aspect of XML lies outwith the 
SGML domain.

Of course, if the statement "XML is a subset of SGML" was never true XML 
Schema simply confirms that situation. If some believe that XML was 
originally a subset of SGML what is the impact of XML Schema on their 
perception?

Andrew Watt

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