core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
index 8119efa..3c171d2 100644
--- a/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
+++ b/core/lib/Drupal/Core/Routing/RequestFormatRouteFilter.php
@@ -23,7 +23,20 @@ public function applies(Route $route) {
    * {@inheritdoc}
    */
   public function filter(RouteCollection $collection, Request $request) {
-    $format = $request->getRequestFormat('html');
+    // Determine the request format. 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.
+    $default_format = 'html';
+    if ($collection->count() === 1) {
+      $only_route = $collection->getIterator()->current();
+      $required_format = $only_route->getRequirement('_format');
+      if (strpos($required_format, '|') === FALSE) {
+        $default_format = $required_format;
+      }
+    }
+    $format = $request->getRequestFormat($default_format);
+
     /** @var \Symfony\Component\Routing\Route $route */
     foreach ($collection as $name => $route) {
       // If the route has no _format specification, we move it to the end. If it