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