An element's opacity can be set using the opacity property. Values can be anywhere from 0.0 (transparent) to 1.0 (opaque).

Example Usage

<div style="opacity:0.8;">
    This is a partially transparent element
</div>

To use opacity in all versions of IE, the order is:

.transparent-element {
  /* for IE 8 & 9 */
  -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=60)"; // IE8
  /* works in IE 8 & 9 too, but also 5, 6, 7 */
  filter: alpha(opacity=60); // IE 5-7
  /* Modern Browsers */
  opacity: 0.6;
}