Why namespaces?

David Megginson david at megginson.com
Wed Sep 1 15:43:54 BST 1999

Oren Ben-Kiki writes (quoting Paul Prescod, I think):

 > > The important point to note is that to *level two*, htmlstrict:P
 > > is not the same as htmlloose:P.
 > I think this might be the source of our disagreement. I claim it
 > these two are the same; the difference is in the DTD applied to
 > them.

This is the fundamental source of pretty much all of the disagreement
on this thread.  The problem is that a DTD does (and XML Schemas will
do) two entirely different things:

a) supply default values and types; and
b) specify a set of validation rules.

(a) is an essential layer in the interpretation of a document
(assuming that the document author takes advantage of it); (b) is
simply one of many processes that can be applied to an XML document,
and calling it a layer is rather confusing.

As far as I know, the three HTML 4.0 variants differ only in (b), not
in (a), so the differences are not really an essential part of
interpretation -- they affect only one specific type of process,
structural validation.

Applying a structural schema (DTD or otherwise) to a document should
be no different than applying a stylesheet -- in the former case, the
output is a truth value, and in the latter, it is some sort of
rendition.  You can apply many different stylesheets to the same XML
document, and likewise, you should be able to apply many different
structural schemas to an XML document.

This is something that SGML got wrong and XML got right -- SGML
assumed that there was always a *single* DTD that applied to any
existing document (though that never worked in practice, so we always
had to invent kludges for non-trivial systems), so that a document
instance could not exist independently of its schema and vice-versa
(external DTD subsets are not independent objects in SGML, but simply
part of the document that includes them).

By allowing documents without explicit DOCTYPE declarations, XML (and,
eventually, WebSGML) acknowledged that document instances can exist
independently of schemas, and thus, that there can potentially be
*many* schemas applied to any existing document.

All the best,


David Megginson                 david at megginson.com

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