CSS3 PIE is an HTC file that allows you to use standard CSS3 (plus vendor prefixes) to get features such as linear background gradients, rounded corners and drop-shadows in all browsers. HTC files are applied using a behavior in your CSS, which is IE-only, and does not validate.

#example1 {
	-moz-box-shadow: 5px 5px 0 #666; /* FF3.5+ */
	-webkit-box-shadow: 5px 5px 0 #666; /* Saf3.0+, Chrome */
	box-shadow: 5px 5px 0 #666; /* Opera 10.5, IE9+, W3C  */
}
html.lte8 #example1 {
	behavior: url('../../wp-content/themes/practicalcss3/css3-pie.htc'); /* IE6-8 */
	zoom: 1; /* 'hasLayout' required for HTC method */
}

Another example with a “connected”, “soft edge” shadow look:

#example2 {
	-moz-box-shadow: 5px 5px  5px #666; /* FF3.5+ */
	-webkit-box-shadow: 5px 5px  5px #666; /* Saf3.0+, Chrome */
	box-shadow: 5px 5px  5px #666; /* Opera 10.5, IE9+, W3C  */
}
html.lte8 #example2 {
	behavior: url('../../wp-content/themes/practicalcss3/css3-pie.htc'); /* IE6-8 */
	zoom: 1; /* 'hasLayout' required for HTC method */
}