问题
http://www.w3schools.com/tags/tag_doctype.asp
HTML5 is not based on SGML, and therefore does not require a reference to a DTD.
On what standard is HTML 5 based on if not on SGML?
回答1:
The HTML5 standard specifies two serializations of HTML5: "html" and "xml". "xml" is a valid XML serialization (which in turn is a subset of SGML). "html" is not based on any specific serialization standard anymore, it has its own complete serialization. Herein lies the difference: HTML4 has a "sgml" serialization and "xml" serialization (called XHTML 1.0)
Of course HTML5 is for a large part based on HTML4 (based on SGML) and XHTML (based on HTML4 and XML).
Also see the history section of the HTML5 specification
来源：https://stackoverflow.com/questions/16185880/html5-is-not-based-on-sgml-so-what-is-it-based-on-then