Below is the sample code that demonstrate how to write conditional statement tin CSS.
<html>
<head>
<title>Conditional CSS</title>
<style type="text/css">
body
{
color:blue;
}
</style>
<!--[if IE 7]>
<style type="text/css">
body {
background-color:red;
}
</style>
<![endif]-->
</head>
<body>
<p>
SHEO NARAYAHN
</p>
</body>
</html>
If you copy-paste above code as an .html file and browse into IE 7, you will get the background color of the page as red and color of the text as blue but if you will browse the same page in FireFox, the background property of the body tag will not be applicable and it will be default (white) while you will see the color of the text as blue as you were seeing into IE 7.
This is because the color of text has been specified as default style while the background is written as conditional statement.
Thanks
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Edited: Found a detailed article on this topic later on, see here
http://msdn.microsoft.com/library/default.asp?url=/workshop/author/dhtml/overview/ccomment_ovw.asp