Hi Friends,
<!DOCTYPE> is tag for HTML Declaration

<! DOCTYPE html> goes at the top of every HTML5 page.

Unlike earlier versions of HTML, in HTML5 the doctype declaration is case-insensitive.

In other words <! doctype html> works as well as <! DOCTYPE html>

If you need a doctype matching a specific version of (X)HTML, the doctype declaration must be exact (both in spelling and in case) to have the desired effect, which makes it sometimes difficult. To ease the work, below is a list of recommended doctype declarations that you can use in your Web documents.

This needs to be the first thing in your document, before the <html> or >head> elements. Also, there is no closing tag.