I decided to simplify the page even further to see if it would work.
The HTML code of my Frame-Test.html is below

[FONT="Courier New"]<head>
<link href="_css/Test_cart.css" rel="stylesheet" type="text/css" media="all">
</head>
<body>
<div id="apDiv1">
<!-- START EMBED -->
<!-- END EMBED -->
</div>

</body>
</html>[/FONT]

I removed the doctype and title tags (the php scripts will remove them later anyway) but still no luck... none of the CSS that should pass trough the link is considered...

How did you manage to make yours work?