Font size mixin for rem values with pixel fallback for old browsers

_font-size.scss

// REM output based on the $em-base variable with pixel fallback
@mixin font-size($pxVal) {
    @if not unitless($pxVal) {
        $pxVal: strip-unit($pxVal);
    }

    font-size: $pxVal * 1px;
    font-size: rem($pxVal);
}