diff -u b/core/misc/collapse.js b/core/misc/collapse.js
--- b/core/misc/collapse.js
+++ b/core/misc/collapse.js
@@ -8,7 +8,6 @@
 (function ($, Modernizr, Drupal) {
   function CollapsibleDetails(node) {
     this.$node = $(node);
-    this.$node.data('details', this);
 
     if (!Modernizr.details) {
       this.$node.addClass('collapse-processed');
@@ -34,14 +33,13 @@
       this.$node.find('> summary').append(this.$summary);
 
       this.$node.on('summaryUpdated', this.onSummaryUpdated.bind(this)).trigger('summaryUpdated');
-     },
+    },
 
     onSummaryUpdated: function onSummaryUpdated() {
       var text = $.trim(this.$node.drupalGetSummary());
       this.$summary.html(text ? ' (' + text + ')' : '');
     },
 
-    },
     setupLegend: function setupLegend() {
       var $legend = this.$node.find('> summary');
       $('<span class="details-summary-prefix visually-hidden"></span>').append(this.$node.attr('open') ? Drupal.t('Hide') : Drupal.t('Show')).prependTo($legend).after(document.createTextNode(' '));
only in patch2:
unchanged:
--- a/core/profiles/demo_umami/themes/umami/css/classy/components/collapse-processed.css
+++ b/core/profiles/demo_umami/themes/umami/css/classy/components/collapse-processed.css
@@ -30,3 +30,6 @@
   transform: rotate(90deg);
   background-position: 75% 35%;
 }
+.collapse-processed .details-title {
+  color: #333;
+}
only in patch2:
unchanged:
--- a/core/tests/Drupal/KernelTests/Core/Theme/ConfirmClassyCopiesTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Theme/ConfirmClassyCopiesTest.php
@@ -681,7 +681,7 @@ protected function getClassyHash($type, $file) {
         'book-navigation.css' => 'e8219368d360bd4a10763610ada85a1c',
         'breadcrumb.css' => '14268f8071dffd40ce7a39862b8fbc56',
         'button.css' => '3abebf58e144fd4150d80facdbe5d10f',
-        'collapse-processed.css' => 'e928df55485662a4499c9ba12def22e6',
+        'collapse-processed.css' => 'a37822b887d80917ba465cb8b9bbee60',
         'container-inline.css' => 'ae9caee6071b319ac97bf0bb3e14b542',
         'details.css' => 'fdd0606ea856072f5e6a19ab1a2e850e',
         'dialog.css' => 'f30e4423380f5f01d02ef0a93e010c53',
only in patch2:
unchanged:
--- a/core/themes/bartik/css/classy/components/collapse-processed.css
+++ b/core/themes/bartik/css/classy/components/collapse-processed.css
@@ -30,3 +30,6 @@
   transform: rotate(90deg);
   background-position: 75% 35%;
 }
+.collapse-processed .details-title {
+  color: #333;
+}
only in patch2:
unchanged:
--- a/core/themes/seven/css/classy/components/collapse-processed.css
+++ b/core/themes/seven/css/classy/components/collapse-processed.css
@@ -30,3 +30,6 @@
   transform: rotate(90deg);
   background-position: 75% 35%;
 }
+.collapse-processed .details-title {
+  color: #333;
+}