.opacity(@opacity: 0.5) {
  -moz-opacity: @opacity;
  -khtml-opacity: @opacity;
  -webkit-opacity: @opacity;
  opacity: @opacity;
  @opperc: @opacity * 100;
  -ms-filter: ~"progid:DXImageTransform.Microsoft.Alpha(opacity=@{opperc})";
  filter: ~"alpha(opacity=@{opperc})";
}