.../Core/Routing/RequestFormatRouteFilter.php      | 27 ++++++++++++++++------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
index 8f34426..9654d4c 100644
--- a/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
+++ b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
@@ -66,7 +66,9 @@ public function filter(RouteCollection $collection, Request $request) {
    *
    * By default, use 'html' as the default format. But when there's only a
    * single route match, and that route specifies a '_format' requirement
-   * listing a single format, then use that as the default format.
+   * listing a single format, then use that as the default format. Also, if
+   * there are multiple routes which all require the same single format then
+   * use it.
    *
    * @param \Symfony\Component\Routing\RouteCollection $collection
    *   The route collection to filter.
@@ -76,14 +78,25 @@ public function filter(RouteCollection $collection, Request $request) {
    */
   protected static function getDefaultFormat(RouteCollection $collection) {
     $default_format = 'html';
-    if ($collection->count() === 1) {
-      $only_route = $collection->getIterator()->current();
-      $required_format = $only_route->getRequirement('_format');
-      if (strpos($required_format, '|') === FALSE) {
-        $default_format = $required_format;
+    $iterator = $collection->getIterator();
+    $first_route = $iterator->current();
+    $required_format = $first_route->getRequirement('_format');
+    if (strpos($required_format, '|') !== FALSE) {
+      return $default_format;
+    }
+    if (!$iterator->valid()) {
+      return $required_format;
+    }
+    else {
+      $iterator->next();
+      for ($iterator; $iterator->valid(); $iterator->next()) {
+        $route = $iterator->current();
+        if ($required_format != $route->getRequirement('_format')) {
+          return $default_format;
+        }
       }
+      return $required_format;
     }
-    return $default_format;
   }
 
 }