hacks-guide-minimal-mistakes-theme/_sass/vendor/font-awesome/_mixins.scss

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

@mixin fa-icon() {
  display: inline-block;
  font: normal normal normal #{$fa-font-size-base}/#{$fa-line-height-base} FontAwesome; // shortening font declaration
  font-size: inherit; // can't have font-size inherit on line above, so need to override
  text-rendering: auto; // optimizelegibility throws things off #1094
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;

}

@mixin fa-icon-rotate($degrees, $rotation) {
  filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=#{$rotation});
  -webkit-transform: rotate($degrees);
      -ms-transform: rotate($degrees);
          transform: rotate($degrees);
}

@mixin fa-icon-flip($horiz, $vert, $rotation) {
  filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=#{$rotation});
  -webkit-transform: scale($horiz, $vert);
      -ms-transform: scale($horiz, $vert);
          transform: scale($horiz, $vert);
}