public function setViewport($width = 'device-width', $initial_scale = '1', $user_scalable = '', $minimum_scale = '', $maximum_scale = '')
    {
        $tag = [
            'name' => 'viewport',
            'content' => 'width=' . $width . ', initial-scale=' . $initial_scale
        ];

        if ($user_scalable) {
            $tag['content'] .= ', user-scalable=' . $user_scalable;
        }

        if ($minimum_scale) {
            $tag['content'] .= ', minimum-scale=' . $minimum_scale;
        }

        if ($maximum_scale) {
            $tag['content'] .= ', maximum_scale=' . $maximum_scale;
        }

        $this->tags['viewport'] = $tag;
    }