Two versions: with and without `!important`

Hard Cross Browser Opacity

// Inspired in:
// https://css-tricks.com/snippets/css/cross-browser-opacity/
// http://zerosixthree.se/8-sass-mixins-you-must-have-in-your-toolbox/
 
// Usage: `@include hardOpacity(1);`, `@include hardOpacity(.5);`

@mixin hardOpacity($valueOpacity) {
 
  $unitOp: unit($valueOpacity);
  $valOp: parseInt($valueOpacity);
  
  $opacity-ie: $valOp * 100;
  
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{$opacity-ie})"; // IE 8
  filter: alpha(opacity=$opacity-ie); //IE 5-7
  -moz-opacity: $valOp; // Netscape (not a joke...)
  -khtml-opacity: $valOp; // Safari 1.x
   opacity: $valOp; // Good browsers
}

Hard Cross Browser Opacity with boolean `!important`

// Usage: `@include hardOpacity-imp(1,true);`, Will give `!important`
// `@include hardOpacity(.5, false);` Will not give `!important`

@mixin hardOpacity-imp($valueOpacity, $imp) {

  $imp: false !default;

  $unitOp: unit($valueOpacity);
  $valOp: parseInt($valueOpacity);

  $opacity-ie: $valOp * 100;

  @if $imp {
    //@debug "$boolean is #{$imp}";
    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{$opacity-ie})" !important; // IE 8
    filter: alpha(opacity=$opacity-ie) !important; //IE 5-7
    -moz-opacity: $valOp !important; // Netscape (not a joke)
    -khtml-opacity: $valOp !important; // Safari 1.x
     opacity: $valOp !important; // Good browsers
 }
  @else {
    //@debug "$boolean is #{$imp}";
    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{$opacity-ie})"; // IE 8
    filter: alpha(opacity=$opacity-ie); //IE 5-7
    -moz-opacity: $valOp; // Netscape (not a joke)
    -khtml-opacity: $valOp; // Safari 1.x
     opacity: $valOp; // Good browsers
  }
}