For static text just use an HTML-file and the url in place.
For dynamic content you could do smth like
Code:
<script type="text/javascript">
var popup = window.open('', 'popup', '');
popup.document.open('text/html', 'replace');
popup.document.write('$stylevar[htmldoctype]<HTML><HEAD><TITLE>$vboptions[bbname] Greeting</TITLE></HEAD><BODY>Welcome $bbuserinfo[username]</BODY></HTML>');
popup.document.close();