missing <!DOCTYPE> declaration

Cause:

A DOCTYPE Declaration is mandatory for most current markup languages and without one it is impossible to reliably validate a document. The DOCTYPE defines the version of the document like : HTML 4.0.1, XHTML 1.0, 1.1, ...

Solution:

Please place a DOCTYPE declaration as the very first thing in an HTML document. For example, for a typical XHTML 1.0 document:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
  <title>Title</title>
</head>

<body>
  ... body of document ...
</body>

</html>


For HTML 4.0.1, there are several one. Here is the most typical one:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
  "http://www.w3.org/TR/html4/loose.dtd">


References:

W3C spec : http://www.w3.org/TR/html4/struct/global.html#h-7.2
W3C faq : http://validator.w3.org/docs/help.html#faq-doctype
List of valid doctype: http://www.w3.org/QA/2002/04/valid-dtd-list.html
Choosing a doctype : http://htmlhelp.com/tools/validator/doctype.html