No document type declaration; will parse without validation

Cause:

No document type specified at the begin of the document.

Example:

Good
<title>my_title</title>
<body>my_body</body>
Good
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
  "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>my_title</title>
<head> ...

Solution:

Add a doctype at the begining of the HTML

References: