Make HTML the default output format. That is, make HTML the last format to...

Make HTML the default output format.  That is, make HTML the last format to catch output events and apply it to the catch-all accept header.

core/lib/Drupal/Core/EventSubscriber/HtmlSubscriber.php

@@ -31,7 +31,7 @@ class HtmlSubscriber implements EventSubscriberInterface {
    *   True if it is an event we should process as HTML, False otherwise.
    */
   protected function isHtmlRequestEvent(GetResponseEvent $event) {
-    return in_array('text/html', $event->getRequest()->getAcceptableContentTypes());
+    return (boolean)array_intersect(array('text/html', '*/*'), $event->getRequest()->getAcceptableContentTypes());
   }
 
   /**
@@ -96,11 +96,13 @@ public function onView(GetResponseEvent $event) {
    *   An array of event listener definitions.
    */
   static function getSubscribedEvents() {
-    $events[KernelEvents::EXCEPTION][] = array('onNotFoundHttpException');
-    $events[KernelEvents::EXCEPTION][] = array('onAccessDeniedException');
-    $events[KernelEvents::EXCEPTION][] = array('onMethodAllowedException');
+    // Since we want HTML to be our default, catch-all response type, give its
+    // listeners a very low priority so that they always check last.
+    $events[KernelEvents::EXCEPTION][] = array('onNotFoundHttpException', 5);
+    $events[KernelEvents::EXCEPTION][] = array('onAccessDeniedException', 5);
+    $events[KernelEvents::EXCEPTION][] = array('onMethodAllowedException', 5);
 
-    $events[KernelEvents::VIEW][] = array('onView');
+    $events[KernelEvents::VIEW][] = array('onView', 5);
 
     return $events;
   }