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   Re: [xml-dev] XML-appropriate editing data structures

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At 4:36 PM +0200 4/9/04, Henrik Martensson wrote:

>Allowing authors to add tags not supported by the DTD (and therefore not
>by the processing systems that are designed to support what the DTD
>supports, nothing less, nothing more) renders the use of XML

A common fallacy, and one I'm not surprised to hear from you given 
your avowed SGML background. The fundamental difference between XML 
and SGML is not the restricted syntax, and the one that means XML is 
not just a subset of SGML, is that XML separates the notion of 
well-formedness from validity. In XML, well-formedness is required. 
Validity isn't, and normally shouldn't be. A processing system that 
can only handle what the DTD mandates is flawed. Instead a processing 
system needs to ask if the document provides what it needs to do its 
job, not if it adheres to some arbitrary schema. And if the document 
doesn't provide what it needs, then the system should notice and 
handle the case gracefully, perhaps alerting a human, perhaps not, 
depending on circumstances.

The DocBook XSL stylesheets, for example, still work quite well with 
invalid markup. Indeed, XPath and XSLT 1.0 based systems generally do 
work quite well with documents regardless of validity because those 
languages were designed without the assumption of validity. Sadly, 
the same cannot be said for many other systems that were needlessly 
tightly coupled to particular formats. Built here it is the systems 
that are broken. The X stands for extensible. When objects are 
encountered that are not represented in the current DTD/schema it is 
the right and duty of the author to invent new markup to describe 
what is actually there. Systems that cannot deal sensibly with the 
extensibility of XML are broken.


   Elliotte Rusty Harold
   Effective XML (Addison-Wesley, 2003)


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