Is validity an option?

David Megginson david at
Thu Apr 1 18:00:26 BST 1999

Paul Prescod writes:

 > In the XSL-List, Ken Sall quoted Tim Bray:

 > > XML evangelists, such as myself, take great glee in pointing out that
 > > XML, unlike SGML, has no optional features; the result, we claim
 > > triumphantly, is that any XML processor in the world should be
 > > able to read any XML document in the world (well, modulo character
 > > encoding issues).

 > Second, the XML specification is quite clear about the fact that
 > different XML processors can legally produce different parse trees
 > for the same data. Heck, they can produce a different parse tree
 > depending on the day of the month.

Paul's right.  There are no options in terms of producing a boolean
value (well-formed/not well-formed), but there are very annoying
options in terms of what information the parser is allowed to ignore
(such as external entities, the external DTD subset, and by extension,
and entity and attribute declarations in the external DTD subset).

 > To be perfectly honest I am a lot more comfortable with the
 > SGML-world's model: some documents are not processable by some
 > parsers but if the parser says it can handle it then you always
 > know what you are getting out.

No, actually, if the parser says that it can handle the SGML
declaration that it happens to have read from some random place on
your system, then you know that if your document happens to match that
SGML declaration you'll get out what you expect.  That model sucks
too (even if it looked good on paper).

 > Perhaps it isn't too late -- maybe the information set group could
 > fix this flaw. After all, they are in the business of ensuring
 > conformance of processors so the next step would be to rigorously
 > specify conformance classes: "validating", "external entity
 > fetching non-validating", "non-external entity fetching
 > non-validating."

The Infoset WG does not intend to rewrite XML 1.0 or redefine
XML-conformance.  SAX2, on the other hand, can take a stab classifying
its parsers (as could the DOM).

All the best,


David Megginson                 david at

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