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.