DTD is special, sadly.  It got lodged in the XML spec in the early days, too
deeply to be excised now, a legacy of SGML I think?  There seems to have
been an insistence on making validity a property of the document, rather
than a separate concern, as it is w/XSD and others. Probably someone older
and wiser can give you a more complete explanation.

-Mike 

> -----Original Message-----
> From: Mukul Gandhi [mailto:gandhi.mukul@g...] 
> Sent: Saturday, November 07, 2009 5:49 AM
> To: Michael Kay
> Cc: xml-dev@l...
> Subject: Re: [xml-dev] XML spec and XSD
> 
> Thanks, Mike for your remarks.
> 
> On Sat, Nov 7, 2009 at 3:59 PM, Michael Kay <mike@s...> wrote:
> > It's a basic issue of architectural layering. XSD has a 
> dependency on 
> > XML, XML has no dependency on XSD. Nothing in the XML spec 
> is affected 
> > if XSD changes.
> >
> > It's bad enough when you're writing a spec tracking the changes in 
> > technologies you depend on (like Unicode). Introducing unnecessary 
> > dependencies for pedagogic or marketing reasons would be a very bad 
> > thing to do.
> 
> Looking at your view points above, I agree to them as good 
> architectural principles while writing W3C specs.
> 
> But I feel, that mentioning XSD as a validation technology 
> for XML documents, in the XML spec is perhaps a good idea 
> since DTD is also mentioned in the XML spec (which is also a 
> XML validation technology).
> I feel, doing so doesn't promote any pedagogic or marketing 
> attitudes towards XSD.
> 
> Reading the XML spec, gives us a feeling (to me at least, I 
> guess) that DTD is the most important technology for 
> validating XML. Even if we don't mention specific versions of 
> XSD as validating language for XML documents (in specific XML 
> standards, like 1.0 5th edition or XML 1.1), I think it's 
> good to mention in references of the XML spec (I believe, a 
> normative reference to this would also be good in the XML 
> spec), that XSD is also another XML validation technology 
> from W3C, which achieves the same task as DTDs do. I think, 
> referring to the link, http://www.w3.org/XML/Schema in XML 
> specs would serve the purpose I am suggesting.
> 
> 
> 
> --
> Regards,
> Mukul Gandhi
> 
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