Sass mixin copied from a fiddle by csswizardry http://jsfiddle.net/csswizardry/h3eG8/

Sass rgba() fallback mixin

/**
 * A very simple, pragmatic Sass mixin for generating any property with an rgba() colour
 * and its associated fallback (sourced from the original rgba() colour).
 *
 * Do not use if you require a fallback vastly different from the rgba().
 *
 * 1. Strip the alpha value and write out a solid rgb() color.
 * 2. Drop the red, green, blue, and alpha paramaters into the relevant places.
 */
@mixin rgba($property, $red, $green, $blue, $alpha) {
    #{$property}:  rgb(#{$red}, #{$green}, #{$blue}); /* [1] */
    #{$property}: rgba(#{$red}, #{$green}, #{$blue}, #{$alpha}); /* [2] */
}



h1 {
    @include rgba(color, 255, 0, 0, 0.5)
}

.box {
    padding: 1em;
    color: #fff;
    @include rgba(background-color, 0, 0, 0, 0.5);
    border: 2px solid;
    @include rgba(border-color, 0, 0, 0, 0.5)
}