To convert SGML DTDs to XML:
1.No equivalent of the SGML Declaration. So keywords, character set etc are essentially fixed;
2.Tag minimisation is not allowed, so <!ELEMENT x - O (A,B)> becomes <!ELEMENT X (A,B)> and <!ELEMENT x - O EMPTY> becomes <!ELEMENT X EMPTY>;
3.#PCDATA must only occur at the extreme left (ie first) in an OR model, eg <!ELEMENT x - - (A|B|#PCDATA|C)> (in SGML) becomes <!ELEMENT x (#PCDATA|A|B|C)*>, and <!ELEMENT x (A,#PCDATA)> is illegal;
4. No CDATA, RCDATA elements [declared content];
5.Some SGML attribute types are not allowed in XML eg NUTOKEN;
6.Some SGML attribute defaults are not allowed in XML eg CONREF;
7.Comments cannot be inline to declarations like <!ELEMENT x - - (A,B) -- an SGML comment in a declaration -->;
8.A whole bunch of SGML optional features are not present in XML: all forms of tag minimisation (OMITTAG, DATATAG, SHORTREF, etc); Link Process Definitions; Multiple DTDs per document; and many more
9.And [nearly] last but not least, no CONCUR!
10.There are some important differences between the internal and external subset portion of a DTD in XML: Marked Sections can only occur in the external subset; and Parameter Entities must be used to replace entire declarations in the internal subset portion of a DTD, eg the following is invalid XML:
<!DOCTYPE x [
<!ENTITY % modelx "(A|B)*">
<!ELEMENT x %modelx;>
]>
<x></x>
