I just tried to validate an epub (3) and received a notice that in file OEBPS/toc.ncx:
External identifiers must not appear in the document type declaration.
So I used oxygen to delete the line as recommended.
This created more problems:
FATAL OEBPS/toc.ncx 2 2 Fatal Error while parsing file: Content is not allowed in prolog.
ERROR OEBPS/toc.ncx - - Error while parsing file: Content is not allowed in prolog.
How do I fix the External identifier issue?