Separate instructions for testing well formed XML files and validation against DTD

Related to #943 , #945

Would it possible to make two different instructions: one to test if a xml file is well formed and other to validate it against DTD file. Thats the way XML copy editor works, and many online validators.

Or at least in problems output, make a clear difference ( ideally separated by horizontal line or something like that ) between the xml file ( errors of not well formed file) and errors that came from DTD ( from validation of the xml file against DTD). Because mixing the two diferent source of errors, would be some kind of mess....specially to my students.

As @angelozerr asked me, i open a issue to separate, the bug from this feature request or enchancement.

The validation check is performed against any XML schema or DTD declared inside the XML document.
If neither an XML schema nor a DTD is declared, only a syntax check is performed.

Another example:
https://www.liquid-technologies.com/online-xml-validator

But in this case it misses the XML vs DTD validator...

My students and me were very grateful.... ;)