EPUB Content Documents 3.0, Draft at 15 Feb 2011, section 2.1.1: > An XHTML Content Document must meet all of the following > criteria: > ... > It must meet the conformance constraints for XML ...

Problem is likely that our version of HTML Tidy is outdated. Or we need to default to using html5lib for syntax checking.

In the older version of HTML, we need to write a long like to declare DOCTYPE because HTML is SGML(Standard Generalized Markup Language) based so its need a DTD (document type definition).