external dtd subset content

David Megginson ak117 at freenet.carleton.ca
Fri Dec 12 15:14:09 GMT 1997

james anderson writes:

 > we're trying to understand the necessary form for the external dtd
 > subset.
 > in particular two questions have arisen.
 > 1?  since the external subset contains markup declarations only, it
 > would appear that it establishes no constraint on the root element. is
 > it legitimate to use the same dtd for various xml documents, each with a
 > different root element?

Yes -- that's standard practice in the SGML world (you can use the
same external DTD for an entire book or for just one chapter of it).

 > 2? among the example DTD's we've found, some begin with an <?xml ... >
 > form. others don't. isn't that form excluded from being a PI and thus
 > from being a markupdecl?

No, on two counts:

1) The grammatical production for markupdecl [30] explicitly includes
   processing instructions.

2) The <?xml...?> that you see at the beginning of the external subset
   is not a processing instruction but a text declaration (which is
   similar but not identical to an XML declaration).  For example, if
   my external subset were encoded in ISO Latin 1, I would be required
   to put the following declaration at the top:

     <?xml encoding="ISO-8859-1"?>

   If it were in ASCII, however, I could just let the encoding default
   to UTF-8.

All the best,


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

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