code: public function css($group = '*')
      {
          $this->sort();
  
          $cssList = $this->getLoadedCssList($group);
  
          list($cssList) = $this->fireEvent('load.css', [ & $cssList ]);
  
          $cssList = $this->transformListToLinkHtmlNodes($cssList);
  
          return implode("\n", $cssList);
      }
docs: Returns CSS tags from given group name.
  If group name is asterisk (*), will return from all loaded groups.
  @param  string $group Group name.
  @return string HTML tags as string.