In order to validate an HTML document as being compliant with HTML standard 4.01 the document must include a Document Type Definition and a specification as to what character set should be used.
The DTD tells the browser what type of document it is about to attempt to process and to what standard the document will conform. The character set tells the browser whether to use English lettering or the lettering systems of an alternative language such as Icelandic, Norse, Korean, Japanese, Hebrew, Arabic, Keralan, or any of the other many languages on the planet.
The easy way to add these to your page is to copy and paste them from this page online.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
" http://www.w3.org/TR/html4/strict.dtd ">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;
charset=iso-8859-1">
<title>The baby that ate
Chicago!</title>
<style type="text/css">
body
{background:black;color:lightyellow;font-family: garamond,"book
antiqua",serif}
h1 {color:goldenrod}
a:link {color:yellow;
text-decoration:none}
a:visited {color:darkgoldenrod}
a:active
{color:chocolate}
a:hover
{background:maroon}
</style>
</head>
<body>
.
.
.
A sample page using a DTD and Character Set
File Upload Validator at W3C