public function css( $cssClasses )
    {
        if ( ! $cssClasses )
        {
            return $this;
        }

        $cssClasses = explode( ' ', $cssClasses );

        foreach ( $cssClasses as $cssClass )
        {
            if ( $cssClass && false === in_array( $cssClass, $this->cssClasses ) )
            {
                $this->cssClasses[] = $cssClass;
            }
        }

        return $this;
    }