Mine looks like this
<html>
<head>
<meta http-equiv="Content-Type"
content="text/html; charset=windows-1250">
<meta name="GENERATOR" content="VB">
<title>Overview</title>
</head>
<body bgcolor="#ffffff">
<br>
<h2 align="center"><a name="Main">Lookout!</a></h2>
I don't know what any of it means but it works. From what I can gather the Overview and Briefing use html (obviously) but as they are not strictly html docs - the game converts them to its own format - simple html may not be enough. I suspect you may need
<title>Overview</title>
and
<a name="Main">
in there somewhere to let the game know what it's supposed to do with this file.
Edit: When I first started I got nothing up either until I started editing it in a text file.