<!DOCTYPE> declaration tells the browser that a given file is a markup .html file and that a set of web standards must be applied to it. It is the first line of code that go at the top of every HTML or XHTML file.
<!DOCTYPE> is not an element or a tag.
<!DOCTYPE> declaration is added to the top of an HTML file.
<!DOCTYPE html><html><head><title>Title of the document</title></head><body>The content of the document...</body></html>