HTML5 is not based on SGML, and therefore does not require a reference to a DTD.
HTML 5 Doctype declaration:
Per the W3.org HTML 5
A DOCTYPE must consist of the following components, in this order:
- A string that is an ASCII case-insensitive match for the string
therefore the following
DOCTYPEs are also valid:
<!doctype html> <!dOCtyPe html> <!DocTYpe html>
This SO article discusses the topic extensively: Uppercase or lowercase doctype?