SASS rem fallback.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.10)
// ----

$em-base: 16px;
@function strip-unit($num) {
  @return $num / ($num * 0 + 1);
}
@function convert-to-rem($value, $base-value: $em-base)  {
  $value: strip-unit($value) / strip-unit($base-value) * 1rem;
  @if ($value == 0rem) { $value: 0; } // Turn 0em into 0
  @return $value;
}
@function print-px($values) {
  $max: length($values);

  @if $max == 1 { @return strip-unit(nth($values, 1)) + px; }

  $pxValues: ();
  @for $i from 1 through $max {
    $pxValues: append($pxValues, strip-unit(nth($values, $i)) + px);
  }
  @return $pxValues;
}
@function rem-calc($values, $base-value: $em-base) {
  $max: length($values);

  @if $max == 1 { @return convert-to-em(nth($values, 1), $base-value); }

  $remValues: ();
  @for $i from 1 through $max {
    $remValues: append($remValues, convert-to-rem(nth($values, $i), $base-value));
  }
  @return $remValues;
}
@mixin prop($prop, $values) {
  #{$prop}: print-px($values);
  #{$prop}: rem-calc($values);
}
.test {
    @include prop(margin, 10 16);
}

SassMeister-output.css

.test {
  margin: 10px 16px;
  margin: 0.625rem 1rem;
}