I am just trying DTDDoc for the first. It works great on some simple DTDs I've got lying around (nice job!). Then I tried generating documentation from the DITA (an OASIS standard, see http://dita.xml.org/\) DTDs, at which point DTDDoc choked.
The error was:
[DTDDoc] ERROR: com.wutka.dtd.DTDParseException: URI topic.mod at
line 179, column 25: Invalid token in attribute declaration: CDATA
Which seems to be caused by the contents of this entity:
<!ENTITY % arch-atts "
xmlns:%DITAArchNSPrefix;
CDATA #FIXED
'http://dita.oasis-open.org/architecture/2005/'
%DITAArchNSPrefix;:DITAArchVersion
CDATA '1.1'" >