Well-formed and valid

David Megginson david at megginson.com
Sat Jan 8 15:51:06 GMT 2000

"Frank Boumphrey" <bckman at ix.netcom.com> writes:

> > james anderson <James.Anderson at mecomnet.de> 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"?>

   <body>Look, mum, no DTD!</body>

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">

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

All the best,


David Megginson                 david at megginson.com

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