<iframe name="info" src="main.html" width="561" height="479" scrolling="auto" align="top" frameborder="0" marginwidth="0"
marginheight="0" allowtransparency="true">
// some code for browsers who are not able to view the <iframe> tag
</iframe>
Second, you have to put the transparency to the pages that should load into the <iframe>, with CSS like this:
<style type="text/css">
</style>