The HTML <!DOCTYPE> tag is used for specifying which language and version the document is using. This is referred to as the document type declaration (DTD).

With the introduction of HTML 5, the <!DOCTYPE> tag is mostly useless, as HTML 5 does not require a reference to a DTD. Having said that, for legacy reasons, the HTML 5 specification recommends including the <!DOCTYPE> header at the top of all HTML documents.

Example Code:
<!DOCTYPE html>
<title>Document Title</title>

<p>Document Contents</p>