balkian.github.com/font-awesome/scss/_mixins.scss

// Mixins
// --------------------------

@mixin icon($icon) {
  @include icon-FontAwesome();
  content: $icon;
}

@mixin icon-FontAwesome() {
  font-family: FontAwesome;
  font-weight: normal;
  font-style: normal;
  text-decoration: inherit;
  -webkit-font-smoothing: antialiased;
  *margin-right: .3em; // fixes ie7 issues
}

@mixin border-radius($radius) {
  -webkit-border-radius: $radius;
  -moz-border-radius: $radius;
  border-radius: $radius;
}

@mixin icon-stack($width: 2em, $height: 2em, $top-font-size: 1em, $base-font-size: 2em) {
  .icon-stack {
    position: relative;
    display: inline-block;
    width: $width;
    height: $height;
    line-height: $width;
    vertical-align: -35%;
    [class^="icon-"],
    [class*=" icon-"] {
      display: block;
      text-align: center;
      position: absolute;
      width: 100%;
      height: 100%;
      font-size: $top-font-size;
      line-height: inherit;
      *line-height: $height;
    }
    .icon-stack-base {
      font-size: $base-font-size;
      *line-height: #{$height / $base-font-size}em;
    }
  }
}