/**
 * converts normal font size to rem and gives a
 * fallback for browsers that do not support rem
 *
 * html font-size must be set to 62.5%
 *
 * i.e: @include font-size(16);
 * output: font-size: 1.6rem;
 *         font-size: 16px;
 */
@mixin font-size($sizeValue: 16) {
    font-size: $sizeValue + px;
    font-size: ($sizeValue / 10) + rem;
}