px/rem mixin, from http://hugogiraudel.com/2013/03/18/ultimate-rem-mixin/

px2rem.scss

html {
    font-size: 62.5%; /* 1 */
}

@function parseInt($n) { /* 2 */
  @return $n / ($n * 0 + 1);
}

@mixin rem($property, $values) {
  $px : (); /* 3 */
  $rem: (); /* 3 */
  
  @each $value in $values { /* 4 */
   
    @if $value == 0 or $value == auto { /* 5 */
      $px : append($px , $value);
      $rem: append($rem, $value);
    }
    
    @else { 
      $unit: unit($value);    /* 6 */
      $val: parseInt($value); /* 6 */
      
      @if $unit == "px" {  /* 7 */
        $px : append($px,  $value);
        $rem: append($rem, ($val / 10 + rem));
      }
      
      @if $unit == "rem" { /* 7 */
        $px : append($px,  ($val * 10 + px));
        $rem: append($rem, $value);
      }
    }
  }
  
  @if $px == $rem {     /* 8 */
    #{$property}: $px;  /* 9 */
  } @else {
    #{$property}: $px;  /* 9 */
    #{$property}: $rem; /* 9 */
  }
}