What is the <!DOCTYPE>? Is it necessary in HTML5?

The <!DOCTYPE> is an instruction to web browser that tells what version of HTML the page is written in.And it 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 <html> tag. As In HTML 4.01, all <! DOCTYPE > declarations require a reference to a Document Type Definition , 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).