core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php | 2 +-
 core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php          | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php b/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
index 9e8c0af..a8e6ba1 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
@@ -79,7 +79,7 @@ public function __construct(AccountInterface $current_user, CurrentPathStack $cu
    */
   public function onResponse(FilterResponseEvent $event) {
     // Only care about HTML responses.
-    if ($event->getRequest()->getRequestFormat() !== 'html') {
+    if (FALSE === stripos($event->getResponse()->headers->get('Content-Type'), 'text/html')) {
       return;
     }
 
diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index 9f73f11..68d8e33 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -178,6 +178,7 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
     list($version) = explode('.', \Drupal::VERSION, 2);
 
     $response = new Response($content, 200,[
+      'Content-Type' => 'text/html; charset=UTF-8',
       'X-Drupal-Cache-Tags' => implode(' ', $cache_tags),
       'X-Drupal-Cache-Contexts' => implode(' ', $this->cacheContextsManager->optimizeTokens($cache_contexts)),
       'X-Generator' => 'Drupal ' . $version . ' (https://www.drupal.org)'