Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="neat-background box">
  I'm have a transparent background y'all
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.8)
// Compass (v1.0.0.alpha.19)
// ----

// RGBA w/Fallback
@mixin rgba($color, $alpha) {
  $rgba: rgba($color, $alpha);
  $ie-hex-str: ie-hex-str($rgba);
  background: $rgba;
  .ie & {
    background: transparent;
    -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#{$ie-hex-str}, endColorstr=#{$ie-hex-str})";
    filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#{$ie-hex-str},endColorstr=#{$ie-hex-str});
    zoom: 1
  }
}

// Use it like this
.neat-background {
  @include rgba(#333,.5);
}

SassMeister-output.css

.neat-background {
  background: rgba(51, 51, 51, 0.5);
}
.ie .neat-background {
  background: transparent;
  -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#80333333, endColorstr=#80333333)";
  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#80333333,endColorstr=#80333333);
  zoom: 1;
}

SassMeister-rendered.html

<div class="neat-background box">
  I'm have a transparent background y'all
</div>