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 ...

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).

HTML5 is the latest version of the Hypertext Markup Language. HTML is a standard programming language for creating websites and web applications. HTML5 has become the most widely used version and is ...