code: public function addCss(string $css, array $attributes = []) : self
      {
          if (substr($css, -4) == '.css' ||
              substr($css, 0, 2) == '//' ||
              substr($css, 0, 4) == 'http'
          ) {
              list($path, $hash) = $this->getAssetData($css);
  
              $meta = new HtmlElement('<link />');
  
              if ($hash) {
                  $meta->addAttribute('name', str_replace(['.css', '.', '/', ':'], ['', '_', '_', '_'], $css));
              }
  
              $meta->addAttributes([
                  'type' => 'text/css',
                  'rel' => 'stylesheet',
                  'href' => (string) $path,
              ]);
          } else {
              $meta = new HtmlElement('<style>');
              $meta->addAttributes($attributes);
              $meta->addAttribute('type', 'text/css')
                  ->setContent($css);
          }
  
          $this->head->add($meta);
  
          return $this;
      }
docs: Add Css file inclusion.
  
  @param string $css
  @param array $attributes
  
  @return $this|self