private function elementClasses(string $element) : string
    {
        if (in_array($element, $this->elements) === true) {
            $class = '';

            if (count($this->classes[$element]) > 0) {
                $class = ' ' . implode(' ', $this->classes[$element]);
            }

            return $class;
        } else {
            return '';
        }
    }