///
/// Generates `background-size` output for a given element.
///
/// @author drublic
///
/// @link http://caniuse.com/background-img-opts caniuse
/// @link http://www.w3.org/TR/css3-background spec
///
/// @param {List} $values
///
/// @output
/// ```css
///   -webkit-background-size: $values;
///           background-size: $values;
/// ```
///
/// @example
///   .selector {
///     @include x-background-size(100% auto);
///   }
///
@mixin x-background-size ($values...) {
    -webkit-background-size: $values; // For Android 2.x
            background-size: $values;
}