_rem.scss

///
/// Returns a number without unit.
///
/// Borrowed from https://github.com/zurb/foundation/blob/master/scss/foundation/_functions.scss
///
/// @param {Number} $value
///
/// @returns {Number} - 30px -> 30
///
 @function strip-unit($value) {
     @return $value / ($value * 0 + 1);
 }


///
/// This mixin enables you to use the CSS3 value `rem`, which lets you define
/// property sizes based on the root element's font-size.
///
/// Outputs properties that use rem with a px fallback.
/// It also takes px values and converts them to rem.
///
/// @author drublic
///
/// @link http://caniuse.com/rem caniuse
/// @link http://www.w3.org/TR/css3-values/#relative0 spec
///
/// @param {String} $property
/// @param {List} $values
/// @param {Number} $default-font-size [16px] - optional
///
/// @require {Function} strip-unit
///
/// @output
/// ```css
///   <property>: <parsed value>px;
///   <property>: <parsed value>rem;
/// ```
///
/// @example
///   .selector {
///     @include x-rem(font-size, 1.3);
///     @include x-rem(padding, 20px);
///   }
///
@mixin x-rem ($property, $values, $main-font-size: 16px) {

	// Empty list for all values in px
	$px-values: ();
	$rem-values: ();

	// Iterate over entries
	@each $value in $values {

		// If the value is zero or of a type that doesn’t need conversion, return the value untouched
		@if index(auto inherit initial none 0, $value) or type-of($value) != "number" {
			$px-values: append($px-values, $value);
			$rem-values: append($rem-values, $value);

		// Otherwise convert it properly
		} @else {
			$unit: unit($value);

			@if $unit == 'px' {
				$px-values: append($px-values, #{strip-unit($value)} * 1px );
				$rem-values: append($rem-values, (strip-unit($value) / strip-unit($main-font-size) * 1rem));
			} @else {
				$px-values: append($px-values, ($value * $main-font-size) );
				$rem-values: append($rem-values, #{$value} * 1rem);
			}

		}
	}

	// Return the property and its list of converted values
	#{$property}: #{$px-values};
	#{$property}: #{$rem-values};
}