external subset syntax

David Megginson ak117 at freenet.carleton.ca
Wed Dec 17 01:16:04 GMT 1997

Peter Murray-Rust writes:

 > At 17:30 15/12/97 +0100, Per-Ake Ling wrote:

 > >Not only that, it is an underexploited feature in SGML that this is the
 > >case. The only indication of real use of this feature in SGML comes from
 > >Eliot Kimber, but I believe that it would be even more valuable in XML.
 > >
 > I agree. I have only just discovered a week ago that something like:
 > <!DOCTYPE BODY SYSTEM "html20.dtd">
 > <BODY>
 > <P>Hello world!</P>
 > </BODY>
 > could be an allowable use of SGML. If I had realised this earlier I could
 > have saved weeks of work in my CML DTDs.

Actually, this is by no means an underexploited technique in SGML; on
the contrary, it's standard practice in larger projects.  Some
industry-standard DTDs like DocBook even repeat inclusion exceptions
on many different element types (book, chapter, section, glossary,
etc) so that any one of them can be used as the document element with
identical results.

Of course, each application (in the SGML sense) has its own rules.
For example,

  <!DOCTYPE a SYSTEM "html20.dtd">
  <a href="http://www.microstar.com/">Microstar</a>

is valid SGML, but it is not correct HTML.

All the best,


David Megginson                 ak117 at freenet.carleton.ca
Microstar Software Ltd.         dmeggins at microstar.com

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