inside/includes/tiny_mce/plugins/preview/example.html

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script language="javascript" src="../../tiny_mce_popup.js"></script>
<title>Example of a custom preview page</title>
<link href="{$content_css}" rel="stylesheet" type="text/css" />
</head>
<body>

Editor contents: <br />
{$content}

</body>
</html>