Optimal CSS preprocessor output

best-case.css

/* flag */
.flag,
.money-back-guarantee {
  display: table;
  width: 100%;
}
.flag__flag,
.money-back-guarantee .money-back-guarantee__messaging {
  padding-left: 30px;
  padding-top: 0px;
  display: table-cell;
}
.flag__mast,
.money-back-guarantee .money-back-guarantee__badge {
  display: table-cell;
  width: 1%;
}
.flag__mast > img,
.money-back-guarantee .money-back-guarantee__badge > img {
  display: inline-block;
  max-width: none;
}
/* flag--middle */
.flag--middle > .flag__mast,
.flag--middle > .flag__flag,
.money-back-guarantee > .money-back-guarantee__badge,
.money-back-guarantee > .money-back-guarantee__messaging {
  vertical-align: middle;
}

input.scss

/* flag */

.flag {
  display: table;
  width: 100%;
}

.flag__flag {
  padding-left: 30px;
  padding-top: 0px;
  display: table-cell;
}

.flag__mast {
  display: table-cell;
  width: 1%;
}
.flag__mast > img {
  display: inline-block;
  max-width: none;
}

/* flag--middle */

.flag--middle > .flag__mast,
.flag--middle > .flag__flag {
  vertical-align: middle;
}

@mixin fafa($a, $b) {
  @extend .flag;

  #{$a} {
    @extend .flag__mast;

  }

  #{$b} {
    @extend .flag__flag;
  }
}

@mixin mama($a, $b) {
  @include fafa($a, $b);

  @extend .flag--middle;
}

.money-back-guarantee {
    @include mama(".money-back-guarantee__badge", ".money-back-guarantee__messaging");
}

input.stylus

/* flag */

.flag, .flag--top,
.flag--middle {
  display: table;
  width: 100%;
}

.flag__flag {
  padding-left: 30px;
  padding-top: 0px;
  display: table-cell;
}

.flag__mast {
  display: table-cell;
  width: 1%;
}
.flag__mast > img {
  display: inline-block;
  max-width: none;
}

/* flag--middle */

.flag--middle > .flag__mast,
.flag--middle > .flag__flag {
  vertical-align: middle;
}

fafa(a, b) {
  @extend .flag;

  {a} {
    @extend .flag__mast;

    > img {
      @extend .flag__mast > img;
    }
  }

  {b} {
    @extend .flag__flag;
  }
}

mama(a, b) {
  fafa(a, b);

  & > {a} {
    @extend .flag--middle > .flag__mast;
  }

  & > {b} {
    @extend .flag--middle > .flag__flag;
  }
}

.money-back-guarantee {
    mama(".money-back-guarantee__badge", ".money-back-guarantee__messaging");
}

output.scss.css

/* modified for easier diff'ing */
/* flag */
.flag,
.money-back-guarantee {
  display: table;
  width: 100%;
}
.flag__flag,
.money-back-guarantee .money-back-guarantee__messaging {
  padding-left: 30px;
  padding-top: 0px;
  display: table-cell;
}
.flag__mast,
.money-back-guarantee .money-back-guarantee__badge {
  display: table-cell;
  width: 1%;
}
.flag__mast > img,
.money-back-guarantee .money-back-guarantee__badge > img {
  display: inline-block;
  max-width: none;
}
/* flag--middle */
.flag--middle > .flag__mast,
.flag--middle > .flag__flag,
.money-back-guarantee > .money-back-guarantee__badge,
.money-back-guarantee > .money-back-guarantee__messaging {
  vertical-align: middle;
}

output.stylus.css

/* flag */
.flag,
.money-back-guarantee {
  display: table;
  width: 100%;
}
.flag__flag,
.money-back-guarantee .money-back-guarantee__messaging {
  padding-left: 30px;
  padding-top: 0px;
  display: table-cell;
}
.flag__mast,
.money-back-guarantee .money-back-guarantee__badge {
  display: table-cell;
  width: 1%;
}
.flag__mast > img,
.money-back-guarantee .money-back-guarantee__badge > img {
  display: inline-block;
  max-width: none;
}
/* flag--middle */
.flag--middle > .flag__mast,
.flag--middle > .flag__flag,
.money-back-guarantee > .money-back-guarantee__badge,
.money-back-guarantee > .money-back-guarantee__messaging {
  vertical-align: middle;
}