public function MarkupLink($rel, $href, $type = null, $sizes = null)
    {
        // start fragment
        $return = '<link rel="' . $rel . '" href="' . $href . '"';
        // if type
        if ($type !== null) {
            $return .= ' type="' . $type . '"';
        }
        // if sizes
        if ($sizes !== null) {
            $return .= ' sizes="' . $sizes . '"';
        }
        // end fragment
        $return .= ' />' . PHP_EOL;
        // return
        return $return;
    }