Re: C.4 Undeclared entities?
| Questions like this are more easily analyzed if we avoid the confusing
| term "optional DTD". A missing DTD is really an implied DTD (like
| SGML's implied SGML Declaration) in which all element types have mixed
| content consisting of "(#pcdata | any element type)*" and all
| attributes are CDATA #REQUIRED (except for special conventions for ID,
| etc. that we might adopt).
There is no "implied DTD" of this nature in XML.
It is possible for a well-formed XML document to be parsed in the
absence of a DTD. In such a case, the DTD does not maintain a
noumenal existence; it just really isn't there at all.