Why? HTML5 is not SGML-based, and there will be no official DTD for it. SGML can express only a rather limited set of rules for syntax. Yet, a DTD is useful for validation. SGML validation helps to detect unintentional low-level mistakes in code, such as a forgotten end tag (e.g., a missing </i> may turn large parts of text to italic) or a mistyped attribute name (e.g., aling for align). SGML vali