REM / PX mixin http://sassmeister.com/gist/9a7d1d33b263073a16c3

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

// Modified without base 10 reset
// From https://css-tricks.com/snippets/css/less-mixin-for-rem-font-sizing/

@function strip-unit($num) {
  @return $num / ($num * 0 + 1);
}


$base-font-size:   16px !default;

// Supply px value for fonts
@mixin font-size($sizeValue) {
  font-size: $sizeValue + px;
  font-size: ($sizeValue / strip-unit($base-font-size)) + rem;
}
// Example in use
h1 {
  @include font-size(90);
}




@mixin rem-fallback($property, $values...) {
  $max: length($values);
  $pxValues: '';
  $remValues: '';

  @for $i from 1 through $max {
    $value: strip-unit(nth($values, $i));
    $pxValues: #{$pxValues + $value*16}px;

    @if $i < $max {
      $pxValues: #{$pxValues + " "};
    }
  }

  @for $i from 1 through $max {
    $value: strip-unit(nth($values, $i));
    $remValues: #{$remValues + $value}rem;

    @if $i < $max {
      $remValues: #{$remValues + " "};
    }
  }

  #{$property}: $pxValues;
  #{$property}: $remValues;
}

// Example in use
h2 {
  @include rem-fallback(margin, 15, 20);
}

SassMeister-output.css

h1 {
  font-size: 90px;
  font-size: 5.625rem;
}

h2 {
  margin: 240px 320px;
  margin: 15rem 20rem;
}