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Re: Well-formed and valid

  • From: David Megginson <david@m...>
  • To: "xml-dev" <xml-dev@i...>
  • Date: 08 Jan 2000 09:37:23 -0500

Re: Well-formed and valid
"Frank Boumphrey" <bckman@i...> writes:

> > james anderson <James.Anderson@m...> writes:
> >
> > > > ... it is possible to have a document that is well-formed but
> > > > not valid XML 1.0, but still conforms to Namespaces, RDF, or XLink
> > > > (though not XHTML, which requires validity for strict conformance).
> 
> Well XHTML is no different from any other _document type_.  A document type
> such as docbook or XHTML must conform to a DTD or it would not be a document
> type!

Wrong.  A document type needs to conform to the specification that
defines it: the specification may or may not include a DTD, and may or
may not require the DTD to be used, but the DTD is at best an optional
(if extremely useful) part of a document type specification.

Here's an example of a document type whose specification does not
include a DTD:

  Memo Document Type
  ------------------

  Every memo document consists of a root element named "memo"
  with two element children: "number" and "body", optionally separated
  by whitespace.  The contents of the "number" element shall be a
  number between 1 and 99,999, assigned sequentially; the contents of
  the "body" element shall be text containing a minimum of one and a
  maximum of three sentences, using UK orthography.

Here's a document conforming to that document type:

  <?xml version="1.0"?>

  <memo>
   <number>2,126</number>
   <body>Look, mum, no DTD!</body>
  </memo>

Now, I could have written a DTD and required its use as *part* of the
document type specification:

  <!ELEMENT memo (number, body)>
  <!ELEMENT number (#PCDATA)>
  <!ELEMENT body (#PCDATA)>

However, it would be trivially easy to write documents that conform to 
the DTD but not to the document type:

  <?xml version="1.0"?>

  <!DOCTYPE memo SYSTEM "memo.dtd">

  <memo>
   <number>Let me not to the marriage of true minds</number>
   <body>123</body>
  </memo>



All the best,


David

-- 
David Megginson                 david@m...
           http://www.megginson.com/

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