function css($flush = true) {
        $css = $this->_css;
        if ($flush) $this->_css = [];
        $html = '';
        foreach ($css as $url => $inc) {
            $html .= '<link rel="stylesheet" href="'.$url.'">'.PHP_EOL;
        }
        if (!empty($this->_styles)) {
            $styles = $this->_styles;
            if ($flush) $this->_styles = [];
            $html .= '<style>'.PHP_EOL;
            foreach ($styles as $selector => $style)
            {
                if ($selector) {
                    $html .= $selector . ' { ' . $style . ' }' . PHP_EOL;
                } else {
                    $html .= $style;
                }
            }
            $html .= '</style>';
        }
        return $html;
    }