Re: C.4 Undeclared entities?

Charles F. Goldfarb wrote:
> I am *not* proposing that there be an explicit "DTD" for merely "well-formed"
> XML. I am proposing, as Eliot has pointed out, that there is a legitimate 8879
> DOCTYPE declaration for the case where there is no explicit DTD, viz:
> <!DOCTYPE DocumentTypeName SYSTEM>
> If XML uses this to introduce a well-formed DTD-less document, it will satisfy
> SGML conformance as well. Therefore, there is no reason to break SGML
> conformance for well-formed DTD-less XML documents.

Suppose the user doesn't include the DOCTYPE.  The system implementor 
puts that on their list of exceptions to handle and either whacks the 
users kneecaps with a "Foreswear bad XML" or says nothing and goes on 
doing what they will do if it is there but no DTD is specified.  What 
is the big deal?  OTH, tell them that the DOCTYPE is optional and they 
will never do it.  It isn't the laziness of the author that is at issue.
It is the laziness of the programmer and the conformance of the system.
Even HTML application users are learning it's a good idea to put the 
DOCTYPE in there.  The large numbers that don't says more about the 
levels of practicioners out there and the systems they use, not the 
validity of the practice.

What is the technical reason for optionality on this issue?