HTML 5 Doctype

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HTML5 is not based on SGML, and therefore does not require a reference to a DTD.

HTML 5 Doctype declaration:

<!DOCTYPE html>

Case Insensitivity

Per the HTML 5 DOCTYPE Spec:

A DOCTYPE must consist of the following components, in this order:
  1. A string that is an ASCII case-insensitive match for the string "<!DOCTYPE".

therefore the following DOCTYPEs are also valid:

<!doctype html>
<!dOCtyPe html>
<!DocTYpe html>

This SO article discusses the topic extensively:

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