public function addCss($css)
    {
        if (!is_array($css)) {

            // Clean css argument from unnecessary spaces
            $css = preg_replace('/[ ]+/', ' ', $css);

            // Do not trust the programmer and convert a possible
            // string of multiple css class notations to array
            $css = explode(' ', $css);
        }

        foreach ($css as $class) {
            $this->css[$class] = $class;
        }
    }