Access: [3.2.1.1]: <doctype> missing
(Priority 2)

Cause:

All documents must begin with a Document Type Declaration <doctype> or DTD. The informs a validator which version of HTML to use when verifying a document's syntax. The most commonly used DTD is HTML 4.01 Transitional.

Testfile:

<html lang="en">
<head>
<title>Document missing doctype</title>
</head>
<body>
</body>
</html>

References:

W3C WAI: http://www.w3.org/WAI//
HTML Techniques for WAI 1.0: http://www.w3.org/TR/WCAG10-HTML-TECHS/