Controller.php

<?php

/**
 * @file
 *
 * Contains Drupal\newsletter_view\Controller\NewsletterViewNodeController
 */

namespace Drupal\newsletter_view\Controller;

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Controller\TitleResolverInterface;
use Drupal\Core\Extension\ModuleHandlerInterface;
use Drupal\Core\Render\BubbleableMetadata;
use Drupal\Core\Render\HtmlResponse;
use Drupal\Core\Render\RenderCacheInterface;
use Drupal\Core\Render\RenderContext;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\node\Controller\NodeViewController;
use Drupal\Core\Entity\EntityInterface;
use Symfony\Component\HttpFoundation\Response;
use Pelago\Emogrifier;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Entity\EntityManagerInterface;
use Drupal\Core\Render\RendererInterface;

class NewsletterViewNodeController extends NodeViewController {
  protected $emogrifier;
  protected $titleResolver;
  protected $routeMatch;
  protected $moduleHandler;
  protected $renderCache;

  public function __construct(EntityManagerInterface $entity_manager, RendererInterface $renderer, Emogrifier $emogrifier, TitleResolverInterface $title_resolver, RouteMatchInterface $route_match, ModuleHandlerInterface $module_handler, RenderCacheInterface $render_cache) {
    parent::__construct($entity_manager, $renderer);

    $this->emogrifier = $emogrifier;
    $this->titleResolver = $title_resolver;
    $this->routeMatch = $route_match;
    $this->moduleHandler = $module_handler;
    $this->renderCache = $render_cache;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('entity.manager'),
      $container->get('renderer'),
      $container->get('emogrifier'),
      $container->get('title_resolver'),
      $container->get('current_route_match'),
      $container->get('module_handler'),
      $container->get('render_cache')
    );
  }

  public function view(EntityInterface $node, $view_mode = 'full', $langcode = NULL) {
    $render_array = array();
    $render_context = new RenderContext();
    $this->renderer->executeInRenderContext($render_context, function() use (&$render_array, $node, $view_mode, $langcode) {
      $render_array = $this->viewHelper($node, $view_mode, $langcode);
    });

    $bubbleable_metadata = $render_context->pop();
    $bubbleable_metadata->applyTo($render_array);
    $content = $this->renderCache->getCacheableRenderArray($render_array);

    $content['#cache']['tags'][] = 'rendered';

    $response = new HtmlResponse($content, 200, [
      'Content-Type' => 'text/html; charset=UTF-8',
    ]);

    return $response;
  }
    
  protected function viewHelper(EntityInterface $node, $view_mode = 'full', $langcode = NULL)
    $page = $this->entityManager
      ->getViewBuilder($node->getEntityTypeId())
      ->view($node, $view_mode);

    $page['#pre_render'][] = [$this, 'buildTitle'];
    $page['#entity_type'] = $node->getEntityTypeId();
    $page['#' . $page['#entity_type']] = $node;

    $render_array = [
      '#theme' => 'newsletter',
      '#title' => $this->titleResolver->getTitle(\Drupal::request(), $this->routeMatch->getRouteObject()),
      '#content' => $page,
    ];

    $this->renderer->render($render_array);
    $content = $this->renderCache->getCacheableRenderArray($render_array);

    $path = drupal_get_path('theme', 'newsletter') . '/css/newsletter.css';
    if (file_exists($path)) {
      $this->emogrifier->setCss(file_get_contents($path));
    }

    $this->emogrifier->enableCssToHtmlMapping();
    $this->emogrifier->setHtml($content['#markup']);

    $content['#markup'] = preg_replace('/^\s+|\n|\r|\s+$/m', '', $this->emogrifier->emogrify());

    $this->moduleHandler->alter('emogrify_html', $content['#markup']);

    return $content;
  }

  public function access(AccountInterface $account, EntityInterface $node) {
    return AccessResult::allowedIf($node->isPublished() || $account->hasPermission('bypass node access'));
  }
}