processHtmlHeadLink example

// 'html_head_link' is a special case of 'html_head' which can be present         // as a head element, but also as a Link: HTTP header depending on         // settings in the render array. Processing it can add to both the         // 'html_head' and 'http_header' keys of '#attached', so we must address         // it before 'html_head'.         if (!empty($attached['html_head_link'])) {
          // Merge the processed 'html_head_link' into $attached so that its           // 'html_head' and 'http_header' values are present for further           // processing.           $attached = BubbleableMetadata::mergeAttachments(
            $attached,
            $this->processHtmlHeadLink($attached['html_head_link'])
          );
          unset($attached['html_head_link']);
        }

        // Now we can process 'html_head', which contains both 'feed' and         // 'html_head_link'.         if (!empty($attached['html_head'])) {
          $variables['head'] = $this->processHtmlHead($attached['html_head']);
        }
      }