Alpha mixin

_alpha.scss

@mixin alpha($val: 0) {
  $ie-alpha: $val * 100;
  
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{$ie-alpha})";
  filter: alpha(opacity=#{$ie-alpha});
  @include prefix(opacity, $val);
}

%alpha-0   { @include alpha(0);   }
%alpha-50  { @include alpha(0.5); }
%alpha-100 { @include alpha(1);   }

// usage
.element {
  @include alpha(0.5);
}