A DOCTYPE is an instruction to the web browser about the version of HTML in which the web page is written. A DOCTYPE declaration appears at the top of a web page before all other elements. According to the HTML specification or standards, every HTML document requires a valid document type declaration to insure that your web pages are displayed the way they are intended to be displayed.
The DOCTYPE for HTML5
<!DOCTYPE html>
Doctypes for earlier versions of HTML were longer because the HTML language was SGML-based and therefore required a reference to a DTD, but they are obsolete now.
With HTML5 this is no longer the case and the doctype declaration is only needed to enable the standard mode for documents written using the HTML syntax.
HTML4
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
You can use the following markup as a template to create a new HTML5 document.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Hello World</title>
</head>
<body>
<h1>Hello World!</h1>
</body>
</html>