The < !DOCTYPE> is an instruction to the web browser about what version of HTML the page is written in. AND The < !DOCTYPE> tag does not have an end tag and It is not case sensitive.
The < !DOCTYPE> declaration must be the very first thing in HTML5 document, before the tag. As In HTML 4.01, all < ! DOCTYPE > declarations require a reference to a Document Type Definition (DTD), because HTML 4.01 was based on Standard Generalized Markup Language (SGML). Where as HTML5 is not based on SGML, and therefore does not require a reference to a Document Type Definition (DTD).