Doctypes – short for ‘document type’ – help browsers to understand the version of HTML the document is written in for better interpretability. Doctype declarations are not HTML tags and belong at the very top of a document. This topic explains the structure and declaration of various doctypes in HTML.
Adding the Doctype
The <!DOCTYPE> declaration should always be included at the top of the HTML document, before the tag.
Version ≥ 5
See HTML 5 Doctype for details on the HTML 5 Doctype.
HTML 5 Doctype
HTML5 is not based on SGML (Standard Generalized Markup Language), and therefore does not require a reference to a DTD (Document Type Definition).
HTML 5 Doctype declaration:
Per the W3.org 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: Uppercase or lowercase doctype?