The IFRAME solution works like this:
In your main HTML file, you'll have your DIV:
<div id="myspecialdiv">
<iframe width="100%" height="100%" frameborder="0" src="divcontent.html"></iframe>
</div>
Style that as you need it. The divcontent.html file should be a complete HTML file, including the content of the DIV tag, and a LINK using your external stylesheet:
<html>
<head>
<link rel="stylesheet" href="path/to/external/stylesheet.css" />
</head>
<body>
<!-- The contents of your DIV -->
</body>
</html>
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