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.opacity {
  /* Theoretically for IE 8 & 9 (more valid) */
  /* ...but not required as filter works too */
  /* should come BEFORE filter */
  -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)"; // IE8

  /* This works in IE 8 & 9 too */
  /* ... but also 5, 6, 7 */
  filter: alpha(opacity=50); // IE 5-7
  
  /* Modern Browsers */
  opacity: 0.5;
}