!DOCTYPE HTML

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The declaration <!DOCTYPE HTML> must be the very first thing in an HTML5 document, before the tag. The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in.

The doctype in HTML 4.01 required a reference to a DTD, because HTML 4.01 was based on SGML. HTML5 is not based on SGML, and does not require a reference to a DTD, but need the doctype for browsers to behave as they should.