Doctype is required in HTML document for legacy reasons. Its main purpose is to prevent the browser from entering 'Quirks Mode' while rendering the document.
Including the DOCTYPE in a document ensures that the browser makes the best-effort to follow standard specifications. The doctype should be included at the beginning of the HTML document as shown in the example, below.
<!DOCTYPE html>
<html>
<head>
<title>Including Doctype in HTML Document</title>
</head>
<body>
<h1>
Including Doctype in HTML Document
</h1>
<p>
Welcome to hackinbits.com
</p>
</body>
</html>In HTML5, Doctype's only purpose is to activate the full standard mode.