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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.

Section 2.1: Adding the Doctype

The <!DOCTYPE> declaration should always be included at the top of the HTML document, before the <html> tag.

Version ≥ 5

See HTML 5 Doctype for details on the HTML 5 Doctype.

<!DOCTYPE html>

Section 2.2: 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:

<!DOCTYPE html>

Case Insensitivity

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>