diff --git a/core/core.libraries.yml b/core/core.libraries.yml
index d14e978..5f87068 100644
--- a/core/core.libraries.yml
+++ b/core/core.libraries.yml
@@ -125,6 +125,7 @@ drupal.collapse:
   version: VERSION
   js:
     misc/collapse.js: {}
+    misc/details-aria.js: {}
   dependencies:
     - core/jquery
     - core/modernizr
diff --git a/core/includes/form.inc b/core/includes/form.inc
index 70dc953..4b88461 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -226,7 +226,7 @@ function template_preprocess_details(&$variables) {
       $variables['summary_attributes']['aria-controls'] = $element['#attributes']['id'];
     }
     $variables['summary_attributes']['aria-expanded'] = !empty($element['#attributes']['open']) ? 'true' : 'false';
-    $variables['summary_attributes']['aria-pressed'] = $variables['summary_attributes']['aria-expanded'];
+    $variables['summary_attributes']['aria-pressed'] = !empty($element['#attributes']['open']) ? 'true' : 'false';
   }
   $variables['title'] = (!empty($element['#title'])) ? $element['#title'] : '';
   $variables['description'] = (!empty($element['#description'])) ? $element['#description'] : '';