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Marcus Carr wrote:
>
>...
>
> We accept that HTML and XML are designed for different purposes from each
> other, so why is there so much resistance to accepting SGML as a third facet?
I can define the difference between HTML and XML in one relatively easy
to understand sentence: "XML allows you to invent your own element type
names which allows it to more descriptively represent a much wider range
of data than HTML, with its fixed list of element types. Therefore XML
is more appropriate when the HTML element type set is not sufficient.
HTML has a predefined set of hypertext-oriented element types supported
by many software products such as web browsers. Therefore HTML is more
appropriate than generic XML when you are trying to achieve a high level
of interoperability in hypertext publishing."
Now what would we say to differentiate XML from SGML? How would I decide
when my customers should use SGML instead of XML? What sort of problems
are uniquely suited to non-XML SGML? SGML's survival as a technology
separate from XML will depend on the ability of its proponents to
articulate its appropriate problem domain and to ground this
articulation in technical features that XML lacks.
> Another turnaround - if XML matters and SGML doesn't, why haven't we seen the
> demise of SGML?
The functional difference between SGML and XML is sufficiently minor
that it is seldom worth the effort to switch one way or another.
Furthermore, insofar as XML is officially a subset of SGML, SGML must
(in theory) live at least as long as XML.
But is anyone using SGML features that were left out of XML? CONCUR,
LINK, DATATAG, OMITTAG, RANK, SHORTTAG? I still use some for my personal
projects but I don't encourage my customers to. If the only difference
between SGML and XML in common usage is the empty-tag and processing
instruction syntax then SGML survives but not in a sense that I find
very meaningful.
Did SGML matter? Yes, of course, without it XML would not exist or might
well be radically different (s-expressions?). Is SGML a thriving,
growing technology separate from its XML incarnation? I personally do
not think so. SGML has achieved its success under the name XML.
Paul Prescod
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