public function viewport($content = null)
    {
        if (empty($content)) {
            $content = 'width=device-width, initial-scale=1.0';
        }
        if (is_array($content)) {
            $content = implode(', ', $content);
        }
        return $this->meta(array('name' => 'viewport', 'content' => $content));
    }