Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/misc/details-aria.js

@@ -0,0 +1,29 @@
+/**
+ * @file
+ * Add aria attribute handling for details and summary elements.
+ */
+
+(function ($, Drupal) {
+
+  "use strict";
+
+  /**
+   * Handles `aria-expanded` and `aria-pressed` attributes on details elements.
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.detailsAria = {
+    attach: function () {
+      $('body').once('detailsAria').on('click.detailsAria', 'summary', function (event) {
+        var $summary = $(event.currentTarget);
+        var open = $(event.currentTarget.parentNode).attr('open') === 'open' ? 'false' : 'true';
+
+        $summary.attr({
+          'aria-expanded': open,
+          'aria-pressed': open
+        });
+      });
+    }
+  };
+
+})(jQuery, Drupal);