HTML is based on SGML. XML is a subset of SGML. XHTML is HTML converted to valid XML. IANAE, but I suspect most problems originate with SGML, which was designed

HTML is based on SGML. XML is a subset of SGML. XHTML is HTML converted to valid XML. IANAE, but I suspect most problems originate with SGML, which was designed for flexibility and extensibility rather than security. That’s compounded by the sprawling complexities of both HTML and XML that easily masks weaknesses and holes and makes implementation very hard to verify. Road to Hell, etc.