diff --git a/core/includes/form.inc b/core/includes/form.inc
index af1950a..3d76162 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -1710,10 +1710,9 @@ function form_error(&$element, $message = '') {
  *   can also be used to set additional properties or implement special logic
  *   other than adding child elements: for example, for the 'details' element
  *   type, one of the functions in this array is form_process_details(), which
- *   adds the attributes and JavaScript needed to make the details collapsible
- *   if the #collapsible property is set. The #process functions are called in
- *   preorder traversal, meaning they are called for the parent element first,
- *   then for the child elements.
+ *   adds the attributes and JavaScript needed to make the details work in older
+ *   browsers. The #process functions are called in preorder traversal, meaning
+ *   they are called for the parent element first, then for the child elements.
  * - $element['#after_build']: An array of functions called after form_builder()
  *   is done with its processing of the element. These are called in postorder
  *   traversal, meaning they are called for the child elements first, then for
@@ -2817,7 +2816,7 @@ function form_get_options($element, $key) {
  * @param $variables
  *   An associative array containing:
  *   - element: An associative array containing the properties of the element.
- *     Properties used: #attributes, #children, #collapsed, #collapsible,
+ *     Properties used: #attributes, #children, #collapsed,
  *     #description, #id, #title, #value.
  *
  * @ingroup themeable
@@ -2863,7 +2862,7 @@ function theme_fieldset($variables) {
  * @param $variables
  *   An associative array containing:
  *   - element: An associative array containing the properties of the element.
- *     Properties used: #attributes, #children, #collapsed, #collapsible,
+ *     Properties used: #attributes, #children, #collapsed,
  *     #description, #id, #title, #value.
  *
  * @ingroup themeable
@@ -3926,10 +3925,7 @@ function form_pre_render_details($element) {
   }
 
   // Collapsible details.
-  if (!empty($element['#collapsible'])) {
-    $element['#attached']['library'][] = array('system', 'drupal.collapse');
-    $element['#attributes']['class'][] = 'collapsible';
-  }
+  $element['#attached']['library'][] = array('system', 'drupal.collapse');
   if (empty($element['#collapsed'])) {
     $element['#attributes']['open'] = 'open';
   }
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 7f4b26b..b9887da 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -2189,7 +2189,6 @@ function _install_configure_form($form, &$form_state, &$install_state) {
   $form['site_information'] = array(
     '#type' => 'details',
     '#title' => st('Site information'),
-    '#collapsible' => FALSE,
   );
   $form['site_information']['site_name'] = array(
     '#type' => 'textfield',
@@ -2208,7 +2207,6 @@ function _install_configure_form($form, &$form_state, &$install_state) {
   $form['admin_account'] = array(
     '#type' => 'details',
     '#title' => st('Site maintenance account'),
-    '#collapsible' => FALSE,
   );
 
   $form['admin_account']['account']['#tree'] = TRUE;
@@ -2237,7 +2235,6 @@ function _install_configure_form($form, &$form_state, &$install_state) {
   $form['server_settings'] = array(
     '#type' => 'details',
     '#title' => st('Server settings'),
-    '#collapsible' => FALSE,
   );
 
   $countries = country_get_list();
@@ -2264,7 +2261,6 @@ function _install_configure_form($form, &$form_state, &$install_state) {
   $form['update_notifications'] = array(
     '#type' => 'details',
     '#title' => st('Update notifications'),
-    '#collapsible' => FALSE,
   );
   $form['update_notifications']['update_status_module'] = array(
     '#type' => 'checkboxes',
diff --git a/core/lib/Drupal/Core/Database/Install/Tasks.php b/core/lib/Drupal/Core/Database/Install/Tasks.php
index 2c8f006..ab1f771 100644
--- a/core/lib/Drupal/Core/Database/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Install/Tasks.php
@@ -234,7 +234,6 @@ public function getFormOptions($database) {
     $form['advanced_options'] = array(
       '#type' => 'details',
       '#title' => st('Advanced options'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#description' => st("These options are only necessary for some sites. If you're not sure what you should enter here, leave the default settings or check with your hosting provider."),
       '#weight' => 10,
diff --git a/core/lib/Drupal/Core/FileTransfer/FileTransfer.php b/core/lib/Drupal/Core/FileTransfer/FileTransfer.php
index 72a6751..823e17e 100644
--- a/core/lib/Drupal/Core/FileTransfer/FileTransfer.php
+++ b/core/lib/Drupal/Core/FileTransfer/FileTransfer.php
@@ -408,7 +408,6 @@ public function getSettingsForm() {
     $form['advanced'] = array(
       '#type' => 'details',
       '#title' => t('Advanced settings'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
     );
     $form['advanced']['hostname'] = array(
diff --git a/core/misc/collapse.js b/core/misc/collapse.js
index 3d99bce..77eb9ff 100644
--- a/core/misc/collapse.js
+++ b/core/misc/collapse.js
@@ -174,7 +174,7 @@ Drupal.behaviors.collapse = {
     if (isDetailsSupported) {
       return;
     }
-    var $collapsibleDetails = $(context).find('details.collapsible').once('collapse');
+    var $collapsibleDetails = $(context).find('details').once('collapse');
     if ($collapsibleDetails.length) {
       for (var i = 0; i < $collapsibleDetails.length; i++) {
         CollapsibleDetails.instances.push(new CollapsibleDetails($collapsibleDetails[i], settings.collapsibleDetails));
diff --git a/core/misc/vertical-tabs.js b/core/misc/vertical-tabs.js
index 6d902e9..a9450a8 100644
--- a/core/misc/vertical-tabs.js
+++ b/core/misc/vertical-tabs.js
@@ -44,7 +44,7 @@ Drupal.behaviors.verticalTabs = {
         });
         tab_list.append(vertical_tab.item);
         $this
-          .removeClass('collapsible collapsed')
+          .removeClass('collapsed')
           .attr('open', true)
           .addClass('vertical-tabs-pane')
           .data('verticalTab', vertical_tab);
diff --git a/core/modules/aggregator/aggregator.admin.inc b/core/modules/aggregator/aggregator.admin.inc
index 3612e0c..fcd0a48 100644
--- a/core/modules/aggregator/aggregator.admin.inc
+++ b/core/modules/aggregator/aggregator.admin.inc
@@ -550,7 +550,6 @@ function aggregator_admin_form($form, $form_state) {
       '#type' => 'details',
       '#title' => t('Basic configuration'),
       '#description' => t('For most aggregation tasks, the default settings are fine.'),
-      '#collapsible' => TRUE,
       '#collapsed' => FALSE,
       );
     $form['basic_conf'] += $basic_conf;
diff --git a/core/modules/aggregator/aggregator.processor.inc b/core/modules/aggregator/aggregator.processor.inc
index f585a3c..c98d5a3 100644
--- a/core/modules/aggregator/aggregator.processor.inc
+++ b/core/modules/aggregator/aggregator.processor.inc
@@ -79,13 +79,12 @@ function aggregator_form_aggregator_admin_form_alter(&$form, $form_state) {
     $period = drupal_map_assoc(array(3600, 10800, 21600, 32400, 43200, 86400, 172800, 259200, 604800, 1209600, 2419200, 4838400, 9676800), 'format_interval');
     $period[AGGREGATOR_CLEAR_NEVER] = t('Never');
 
-    // Only wrap into a collapsible details if there is a basic configuration.
+    // Only wrap into details if there is a basic configuration.
     if (isset($form['basic_conf'])) {
       $form['modules']['aggregator'] = array(
         '#type' => 'details',
         '#title' => t('Default processor settings'),
         '#description' => $info['description'],
-        '#collapsible' => TRUE,
         '#collapsed' => !in_array('aggregator', $aggregator_processors),
       );
     }
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index af436d4..cc741ba 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -423,7 +423,6 @@ function _book_add_form_elements(&$form, &$form_state, Node $node) {
     '#type' => 'details',
     '#title' => t('Book outline'),
     '#weight' => 10,
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#group' => 'advanced',
     '#attributes' => array(
diff --git a/core/modules/book/book.pages.inc b/core/modules/book/book.pages.inc
index f8ec0dc..64b39f2 100644
--- a/core/modules/book/book.pages.inc
+++ b/core/modules/book/book.pages.inc
@@ -146,8 +146,6 @@ function book_outline_form($form, &$form_state, Node $node) {
   $form['#id'] = 'book-outline';
   _book_add_form_elements($form, $form_state, $node);
 
-  $form['book']['#collapsible'] = FALSE;
-
   $form['update'] = array(
     '#type' => 'submit',
     '#value' => $node->book['original_bid'] ? t('Update book outline') : t('Add to book outline'),
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 3bcabfe..7869209 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -1005,7 +1005,6 @@ function comment_form_node_type_form_alter(&$form, $form_state) {
     $form['comment'] = array(
       '#type' => 'details',
       '#title' => t('Comment settings'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#group' => 'additional_settings',
       '#attributes' => array(
@@ -1112,7 +1111,6 @@ function comment_form_node_form_alter(&$form, $form_state) {
     '#type' => 'details',
     '#access' => user_access('administer comments'),
     '#title' => t('Comment settings'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#group' => 'advanced',
     '#attributes' => array(
diff --git a/core/modules/comment/lib/Drupal/comment/CommentFormController.php b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
index fca09ba..bae6458 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentFormController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
@@ -53,7 +53,6 @@ public function form(array $form, array &$form_state, EntityInterface $comment)
       $form['author'] += array(
         '#type' => 'details',
         '#title' => t('Administration'),
-        '#collapsible' => TRUE,
         '#collapsed' => TRUE,
       );
     }
diff --git a/core/modules/contact/contact.module b/core/modules/contact/contact.module
index 44a6f5a..7f99326 100644
--- a/core/modules/contact/contact.module
+++ b/core/modules/contact/contact.module
@@ -334,7 +334,6 @@ function contact_form_user_profile_form_alter(&$form, &$form_state) {
     '#type' => 'details',
     '#title' => t('Contact settings'),
     '#weight' => 5,
-    '#collapsible' => TRUE,
   );
   $account = $form_state['controller']->getEntity($form_state);
   $account_data = drupal_container()->get('user.data')->get('contact', $account->id(), 'enabled');
diff --git a/core/modules/dblog/dblog.admin.inc b/core/modules/dblog/dblog.admin.inc
index d6b921a..fd056c2 100644
--- a/core/modules/dblog/dblog.admin.inc
+++ b/core/modules/dblog/dblog.admin.inc
@@ -320,7 +320,6 @@ function dblog_filter_form($form) {
   $form['filters'] = array(
     '#type' => 'details',
     '#title' => t('Filter log messages'),
-    '#collapsible' => TRUE,
     '#collapsed' => empty($_SESSION['dblog_overview_filter']),
   );
   foreach ($filters as $key => $filter) {
@@ -398,7 +397,6 @@ function dblog_clear_log_form($form) {
     '#type' => 'details',
     '#title' => t('Clear log messages'),
     '#description' => t('This will permanently remove the log messages from the database.'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
   );
   $form['dblog_clear']['clear'] = array(
diff --git a/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php b/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
index 0477096..30836a3 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
@@ -467,7 +467,6 @@ function multiple_options_form(&$form, &$form_state) {
     $form['multiple_field_settings'] = array(
       '#type' => 'details',
       '#title' => t('Multiple field settings'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#weight' => 5,
     );
diff --git a/core/modules/field_ui/field_ui.admin.inc b/core/modules/field_ui/field_ui.admin.inc
index 5764343..a867056 100644
--- a/core/modules/field_ui/field_ui.admin.inc
+++ b/core/modules/field_ui/field_ui.admin.inc
@@ -938,7 +938,6 @@ function field_ui_default_value_widget($field, $instance, &$form, &$form_state)
   $element = array(
     '#type' => 'details',
     '#title' => t('Default value'),
-    '#collapsible' => FALSE,
     '#tree' => TRUE,
     '#description' => t('The default value for this field, used when creating new content.'),
     // Stick to an empty 'parents' on this form in order not to breaks widgets
diff --git a/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php b/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php
index 4ead6e7..f474aa6 100644
--- a/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php
+++ b/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php
@@ -353,7 +353,6 @@ public function form(array $form, array &$form_state) {
         $form['modes'] = array(
           '#type' => 'details',
           '#title' => t('Custom display settings'),
-          '#collapsible' => TRUE,
           '#collapsed' => TRUE,
         );
         // Collect options and default values for the 'Custom display settings'
diff --git a/core/modules/language/language.admin.inc b/core/modules/language/language.admin.inc
index 1e6cdc4..e9100c6 100644
--- a/core/modules/language/language.admin.inc
+++ b/core/modules/language/language.admin.inc
@@ -828,7 +828,6 @@ function language_negotiation_configure_browser_form($form, &$form_state) {
   $form['new_mapping'] = array(
     '#type' => 'details',
     '#title' => t('Add a new mapping'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#tree' => TRUE,
   );
diff --git a/core/modules/locale/locale.bulk.inc b/core/modules/locale/locale.bulk.inc
index 5420ac9..f49100b 100644
--- a/core/modules/locale/locale.bulk.inc
+++ b/core/modules/locale/locale.bulk.inc
@@ -177,7 +177,6 @@ function locale_translate_export_form($form, &$form_state) {
     $form['content_options'] = array(
       '#type' => 'details',
       '#title' => t('Export options'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#tree' => TRUE,
       '#states' => array(
diff --git a/core/modules/locale/locale.pages.inc b/core/modules/locale/locale.pages.inc
index 42e5ad3..7e52285 100644
--- a/core/modules/locale/locale.pages.inc
+++ b/core/modules/locale/locale.pages.inc
@@ -169,7 +169,6 @@ function locale_translate_filter_form($form, &$form_state) {
   $form['filters'] = array(
     '#type' => 'details',
     '#title' => t('Filter translatable strings'),
-    '#collapsible' => TRUE,
     '#collapsed' => FALSE,
   );
   foreach ($filters as $key => $filter) {
diff --git a/core/modules/menu/menu.module b/core/modules/menu/menu.module
index 6acb232..b0f3e97 100644
--- a/core/modules/menu/menu.module
+++ b/core/modules/menu/menu.module
@@ -579,7 +579,6 @@ function menu_form_node_form_alter(&$form, $form_state) {
     '#type' => 'details',
     '#title' => t('Menu settings'),
     '#access' => user_access('administer menu'),
-    '#collapsible' => TRUE,
     '#collapsed' => !$link['link_title'],
     '#group' => 'advanced',
     '#attached' => array(
@@ -685,7 +684,6 @@ function menu_form_node_type_form_alter(&$form, $form_state) {
   $form['menu'] = array(
     '#type' => 'details',
     '#title' => t('Menu settings'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#attached' => array(
       'library' => array(array('menu', 'drupal.menu.admin')),
diff --git a/core/modules/node/content_types.inc b/core/modules/node/content_types.inc
index c958f27..9d71ff0 100644
--- a/core/modules/node/content_types.inc
+++ b/core/modules/node/content_types.inc
@@ -158,7 +158,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
   $form['submission'] = array(
     '#type' => 'details',
     '#title' => t('Submission form settings'),
-    '#collapsible' => TRUE,
     '#group' => 'additional_settings',
   );
   $form['submission']['title_label'] = array(
@@ -193,7 +192,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
   $form['workflow'] = array(
     '#type' => 'details',
     '#title' => t('Publishing options'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#group' => 'additional_settings',
   );
@@ -212,7 +210,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
     $form['language'] = array(
       '#type' => 'details',
       '#title' => t('Language settings'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#group' => 'additional_settings',
     );
@@ -232,7 +229,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
   $form['display'] = array(
     '#type' => 'details',
     '#title' => t('Display settings'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#group' => 'additional_settings',
   );
diff --git a/core/modules/node/lib/Drupal/node/NodeFormController.php b/core/modules/node/lib/Drupal/node/NodeFormController.php
index 47e1f30..cf38c63 100644
--- a/core/modules/node/lib/Drupal/node/NodeFormController.php
+++ b/core/modules/node/lib/Drupal/node/NodeFormController.php
@@ -199,7 +199,6 @@ public function form(array $form, array &$form_state, EntityInterface $node) {
       '#type' => 'details',
       '#access' => user_access('administer nodes'),
       '#title' => t('Authoring information'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#group' => 'advanced',
       '#attributes' => array(
@@ -240,7 +239,6 @@ public function form(array $form, array &$form_state, EntityInterface $node) {
       '#type' => 'details',
       '#access' => user_access('administer nodes'),
       '#title' => t('Publishing options'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#group' => 'advanced',
       '#attributes' => array(
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index eb56876..8553ff8 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -1995,7 +1995,6 @@ function node_form_block_form_alter(&$form, &$form_state) {
   $form['visibility']['node_type'] = array(
     '#type' => 'details',
     '#title' => t('Content types'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#group' => 'visibility',
     '#weight' => 5,
@@ -2325,7 +2324,6 @@ function node_form_search_form_alter(&$form, $form_state) {
     $form['advanced'] = array(
       '#type' => 'details',
       '#title' => t('Advanced search'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#attributes' => array('class' => array('search-advanced')),
     );
diff --git a/core/modules/overlay/overlay.module b/core/modules/overlay/overlay.module
index 632d270..383f08a 100644
--- a/core/modules/overlay/overlay.module
+++ b/core/modules/overlay/overlay.module
@@ -92,7 +92,6 @@ function overlay_form_user_profile_form_alter(&$form, &$form_state) {
       '#type' => 'details',
       '#title' => t('Administrative overlay'),
       '#weight' => 4,
-      '#collapsible' => TRUE,
     );
     $form['overlay_control']['overlay'] = array(
       '#type' => 'checkbox',
diff --git a/core/modules/path/path.module b/core/modules/path/path.module
index 7521dee..7d3fadd 100644
--- a/core/modules/path/path.module
+++ b/core/modules/path/path.module
@@ -122,7 +122,6 @@ function path_form_node_form_alter(&$form, $form_state) {
   $form['path'] = array(
     '#type' => 'details',
     '#title' => t('URL path settings'),
-    '#collapsible' => TRUE,
     '#collapsed' => empty($path['alias']),
     '#group' => 'advanced',
     '#attributes' => array(
diff --git a/core/modules/poll/poll.module b/core/modules/poll/poll.module
index 85d29b4..b5308d6 100644
--- a/core/modules/poll/poll.module
+++ b/core/modules/poll/poll.module
@@ -289,7 +289,6 @@ function poll_form(Node $node, &$form_state) {
 
   $form['settings'] = array(
     '#type' => 'details',
-    '#collapsible' => TRUE,
     '#title' => t('Poll settings'),
     '#weight' => -3,
     '#access' => $admin,
diff --git a/core/modules/simpletest/simpletest.pages.inc b/core/modules/simpletest/simpletest.pages.inc
index d5e8c5f..6a83674 100644
--- a/core/modules/simpletest/simpletest.pages.inc
+++ b/core/modules/simpletest/simpletest.pages.inc
@@ -42,7 +42,6 @@ function simpletest_test_form($form) {
   );
   $form['clean'] = array(
     '#type' => 'details',
-    '#collapsible' => FALSE,
     '#collapsed' => FALSE,
     '#title' => t('Clean test environment'),
     '#description' => t('Remove tables with the prefix "simpletest" and temporary directories that are left over from tests that crashed. This is intended for developers when creating tests.'),
@@ -244,7 +243,6 @@ function simpletest_result_form($form, &$form_state, $test_id) {
       '#type' => 'details',
       '#title' => $info['name'],
       '#description' => $info['description'],
-      '#collapsible' => TRUE,
     );
     $form['result']['results'][$group]['summary'] = $summary;
     $group_summary = &$form['result']['results'][$group]['summary'];
@@ -457,7 +455,6 @@ function simpletest_settings_form($form, &$form_state) {
     '#type' => 'details',
     '#title' => t('HTTP authentication'),
     '#description' => t('HTTP auth settings to be used by the SimpleTest browser during testing. Useful when the site requires basic HTTP authentication.'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
   );
   $form['httpauth']['simpletest_httpauth_method'] = array(
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
index 485dc50..8e053f9 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
@@ -250,11 +250,8 @@ function testDrupalRenderFormElements() {
     $element = array(
       '#type' => 'details',
       '#title' => $this->randomName(),
-      '#collapsible' => TRUE,
     );
-    $this->assertRenderedElement($element, '//details[contains(@class, :class)]', array(
-      ':class' => 'collapsible',
-    ));
+    $this->assertRenderedElement($element, '//details');
 
     $element['item'] = array(
       '#type' => 'item',
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index c3c2ce4..e22a4ba 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -917,7 +917,6 @@ function system_modules($form, $form_state = array()) {
     $form['modules'][$package] += array(
       '#type' => 'details',
       '#title' => t($package),
-      '#collapsible' => TRUE,
       '#theme' => 'system_modules_details',
       '#header' => array(
         array('data' => t('<span class="element-invisible">Enabled</span>'), 'class' => array('checkbox')),
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 336f84b..c4e1ad8 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -548,7 +548,6 @@ function system_element_info() {
     '#theme_wrappers' => array('fieldset'),
   );
   $types['details'] = array(
-    '#collapsible' => FALSE,
     '#collapsed' => FALSE,
     '#value' => NULL,
     '#process' => array('form_process_group', 'ajax_process_form'),
@@ -1353,9 +1352,9 @@ function system_library_info() {
     ),
   );
 
-  // Drupal's collapsible details.
+  // Collapsible details.
   $libraries['drupal.collapse'] = array(
-    'title' => 'Drupal collapsible details',
+    'title' => 'Collapsible details',
     'version' => VERSION,
     'js' => array(
       'core/misc/collapse.js' => array('group' => JS_DEFAULT),
@@ -2606,7 +2605,6 @@ function system_user_timezone(&$form, &$form_state) {
     '#type' => 'details',
     '#title' => t('Locale settings'),
     '#weight' => 6,
-    '#collapsible' => TRUE,
   );
   $form['timezone']['timezone'] = array(
     '#type' => 'select',
diff --git a/core/modules/system/tests/modules/design_test/form/details.inc b/core/modules/system/tests/modules/design_test/form/details.inc
index 186945f..60d153a 100644
--- a/core/modules/system/tests/modules/design_test/form/details.inc
+++ b/core/modules/system/tests/modules/design_test/form/details.inc
@@ -13,7 +13,6 @@ function design_test_form_details($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Collapsible details',
     '#description' => 'Details description',
-    '#collapsible' => TRUE,
     '#collapsed' => FALSE,
     '#states' => array(
       'collapsed' => array(
@@ -26,7 +25,6 @@ function design_test_form_details($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Collapsed details',
     '#description' => 'Details description',
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
   );
   $form['collapsed']['textfield'] = array(
@@ -48,35 +46,30 @@ function design_test_form_details($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Details',
     '#description' => 'Details description',
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
   );
   $form['collapsed2']['collapsible'] = array(
     '#type' => 'details',
     '#title' => 'Collapsible details',
     '#description' => 'Details description',
-    '#collapsible' => TRUE,
     '#collapsed' => FALSE,
   );
   $form['collapsed2']['collapsed'] = array(
     '#type' => 'details',
     '#title' => 'Collapsed details',
     '#description' => 'Details description',
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
   );
   $form['collapsed2']['regular'] = array(
     '#type' => 'details',
     '#title' => 'Details',
     '#description' => 'Details description',
-    '#collapsible' => FALSE,
   );
 
   $form['regular'] = array(
     '#type' => 'details',
     '#title' => 'Details',
     '#description' => 'Details description',
-    '#collapsible' => FALSE,
   );
 
   #$form['#attributes'] = array('class' => array('search-form'));
diff --git a/core/modules/system/tests/modules/design_test/form/fieldset.inc b/core/modules/system/tests/modules/design_test/form/fieldset.inc
index 4ab07fd..f55bfe5 100644
--- a/core/modules/system/tests/modules/design_test/form/fieldset.inc
+++ b/core/modules/system/tests/modules/design_test/form/fieldset.inc
@@ -19,7 +19,6 @@ function design_test_form_fieldset($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Collapsible details',
     '#description' => 'Details description',
-    '#collapsible' => TRUE,
     '#collapsed' => FALSE,
   );
   $form['collapsible']['fieldset'] = array(
@@ -43,10 +42,6 @@ function design_test_form_fieldset($form, &$form_state) {
   $form['collapsed']['#title'] = 'Collapsed details';
   $form['collapsed']['#collapsed'] = TRUE;
 
-  $form['uncollapsible'] = $form['collapsible'];
-  $form['uncollapsible']['#title'] = 'Uncollapsible details';
-  $form['uncollapsible']['#collapsible'] = FALSE;
-
   $form['nested'] = $form['regular'];
   $form['nested']['#title'] = 'Parent fieldset';
   $form['nested']['nested'] = $form['nested'];
diff --git a/core/modules/system/tests/modules/form_test/form_test.module b/core/modules/system/tests/modules/form_test/form_test.module
index 8e1992c..be55dcf 100644
--- a/core/modules/system/tests/modules/form_test/form_test.module
+++ b/core/modules/system/tests/modules/form_test/form_test.module
@@ -824,7 +824,6 @@ function _form_test_vertical_tabs_form($form, &$form_state) {
   $form['tab1'] = array(
     '#type' => 'details',
     '#title' => t('Tab 1'),
-    '#collapsible' => TRUE,
     '#group' => 'vertical_tabs',
     '#access' => user_access('access vertical_tab_test tabs')
   );
@@ -835,7 +834,6 @@ function _form_test_vertical_tabs_form($form, &$form_state) {
   $form['tab2'] = array(
     '#type' => 'details',
     '#title' => t('Tab 2'),
-    '#collapsible' => TRUE,
     '#group' => 'vertical_tabs',
     '#access' => user_access('access vertical_tab_test tabs')
   );
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php b/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php
index e2b0b59..3735ad6 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php
@@ -53,7 +53,6 @@ public function form(array $form, array &$form_state, EntityInterface $term) {
     $form['relations'] = array(
       '#type' => 'details',
       '#title' => t('Relations'),
-      '#collapsible' => TRUE,
       '#collapsed' => ($vocabulary->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE),
       '#weight' => 10,
     );
diff --git a/core/modules/translation/translation.module b/core/modules/translation/translation.module
index 805b58f..a864120 100644
--- a/core/modules/translation/translation.module
+++ b/core/modules/translation/translation.module
@@ -215,7 +215,6 @@ function translation_form_node_form_alter(&$form, &$form_state) {
         '#type' => 'details',
         '#title' => t('Translation settings'),
         '#access' => translation_user_can_translate_node($node),
-        '#collapsible' => TRUE,
         '#collapsed' => !$node->translate,
         '#tree' => TRUE,
         '#weight' => 30,
diff --git a/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php b/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php
index 36202eb..fb7eb71 100644
--- a/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php
+++ b/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php
@@ -148,7 +148,6 @@ public function entityFormAlter(array &$form, array &$form_state, EntityInterfac
       $form['source_langcode'] = array(
         '#type' => 'details',
         '#title' => t('Source language: @language', array('@language' => $languages[$source_langcode]->name)),
-        '#collapsible' => TRUE,
         '#collapsed' => TRUE,
         '#tree' => TRUE,
         '#weight' => -100,
@@ -216,7 +215,6 @@ public function entityFormAlter(array &$form, array &$form_state, EntityInterfac
       $form['translation'] = array(
         '#type' => 'details',
         '#title' => t('Translation'),
-        '#collapsible' => TRUE,
         '#collapsed' => TRUE,
         '#tree' => TRUE,
         '#weight' => 10,
diff --git a/core/modules/user/user.admin.inc b/core/modules/user/user.admin.inc
index 1689f62..c06ca00 100644
--- a/core/modules/user/user.admin.inc
+++ b/core/modules/user/user.admin.inc
@@ -400,7 +400,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_admin_created'] = array(
     '#type' => 'details',
     '#title' => t('Welcome (new user created by administrator)'),
-    '#collapsible' => TRUE,
     '#collapsed' => ($config->get('register') != USER_REGISTER_ADMINISTRATORS_ONLY),
     '#description' => t('Edit the welcome e-mail messages sent to new member accounts created by an administrator.') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -421,7 +420,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_pending_approval'] = array(
     '#type' => 'details',
     '#title' => t('Welcome (awaiting approval)'),
-    '#collapsible' => TRUE,
     '#collapsed' => ($config->get('register') != USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL),
     '#description' => t('Edit the welcome e-mail messages sent to new members upon registering, when administrative approval is required.') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -442,7 +440,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_no_approval_required'] = array(
     '#type' => 'details',
     '#title' => t('Welcome (no approval required)'),
-    '#collapsible' => TRUE,
     '#collapsed' => ($config->get('register') != USER_REGISTER_VISITORS),
     '#description' => t('Edit the welcome e-mail messages sent to new members upon registering, when no administrator approval is required.') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -463,7 +460,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_password_reset'] = array(
     '#type' => 'details',
     '#title' => t('Password recovery'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#description' => t('Edit the e-mail messages sent to users who request a new password.') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -485,7 +481,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_activated'] = array(
     '#type' => 'details',
     '#title' => t('Account activation'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#description' => t('Enable and edit e-mail messages sent to users upon account activation (when an administrator activates an account of a user who has already registered, on a site where administrative approval is required).') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -520,7 +515,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_blocked'] = array(
     '#type' => 'details',
     '#title' => t('Account blocked'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#description' => t('Enable and edit e-mail messages sent to users when their accounts are blocked.') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -555,7 +549,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_cancel_confirm'] = array(
     '#type' => 'details',
     '#title' => t('Account cancellation confirmation'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#description' => t('Edit the e-mail messages sent to users when they attempt to cancel their accounts.') . ' ' . $email_token_help,
     '#group' => 'email',
@@ -576,7 +569,6 @@ function user_admin_settings($form, &$form_state) {
   $form['email_canceled'] = array(
     '#type' => 'details',
     '#title' => t('Account canceled'),
-    '#collapsible' => TRUE,
     '#collapsed' => TRUE,
     '#description' => t('Enable and edit e-mail messages sent to users when their accounts are canceled.') . ' ' . $email_token_help,
     '#group' => 'email',
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
index 7c789e6..f7559e4 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
@@ -291,7 +291,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['more'] = array(
       '#type' => 'details',
       '#title' => t('More'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#weight' => 150,
     );
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
index e6c2551..34c4f12 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
@@ -309,7 +309,6 @@ public function globalTokenForm(&$form, &$form_state) {
     $form['global_tokens'] = array(
       '#type' => 'fieldset',
       '#title' => t('Available global token replacements'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
     );
     $form['global_tokens']['list'] = array(
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php
index 8ce189a..82d00ad 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php
@@ -116,7 +116,6 @@ public function tokenForm(&$form, &$form_state) {
       $form['tokens'] = array(
         '#type' => 'details',
         '#title' => t('Replacement patterns'),
-        '#collapsible' => TRUE,
         '#collapsed' => TRUE,
         '#id' => 'edit-options-token-help',
         '#states' => array(
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php
index e3af73a..a7bd33e 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php
@@ -192,7 +192,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['exception'] = array(
       '#type' => 'details',
       '#title' => t('Exceptions'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#fieldset' => 'no_argument',
     );
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
index 2c866d3..0cfb780 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
@@ -532,7 +532,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['style_settings'] = array(
       '#type' => 'details',
       '#title' => t('Style settings'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#weight' => 99,
     );
@@ -682,7 +681,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['alter'] = array(
       '#title' => t('Rewrite results'),
       '#type' => 'details',
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#weight' => 100,
     );
@@ -886,7 +884,6 @@ public function buildOptionsForm(&$form, &$form_state) {
       $form['alter']['help'] = array(
         '#type' => 'details',
         '#title' => t('Replacement patterns'),
-        '#collapsible' => TRUE,
         '#collapsed' => TRUE,
         '#value' => $output,
         '#states' => array(
@@ -1033,7 +1030,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['empty_field_behavior'] = array(
       '#type' => 'details',
       '#title' => t('No results behavior'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#weight' => 100,
     );
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/pager/Full.php b/core/modules/views/lib/Drupal/views/Plugin/views/pager/Full.php
index cc2db4e..cd22d10 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/pager/Full.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/pager/Full.php
@@ -105,7 +105,6 @@ public function buildOptionsForm(&$form, &$form_state) {
 
     $form['tags'] = array(
       '#type' => 'details',
-      '#collapsible' => FALSE,
       '#collapsed' => FALSE,
       '#tree' => TRUE,
       '#title' => t('Pager link labels'),
@@ -138,7 +137,6 @@ public function buildOptionsForm(&$form, &$form_state) {
 
     $form['expose'] = array(
       '#type' => 'details',
-      '#collapsible' => FALSE,
       '#collapsed' => FALSE,
       '#tree' => TRUE,
       '#title' => t('Exposed options'),
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php b/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php
index c140305..b2348d9 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php
@@ -92,7 +92,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['guid_field_options'] = array(
       '#type' => 'details',
       '#title' => t('GUID settings'),
-      '#collapsible' => FALSE,
       '#collapsed' => FALSE,
     );
     $form['guid_field_options']['guid_field'] = array(
diff --git a/core/modules/views/views_ui/css/views-admin.theme.css b/core/modules/views/views_ui/css/views-admin.theme.css
index 8bf02e5..8248f34 100644
--- a/core/modules/views/views_ui/css/views-admin.theme.css
+++ b/core/modules/views/views_ui/css/views-admin.theme.css
@@ -390,7 +390,7 @@ td.group-title {
  * details need to be floated as well so that the margin above the details interacts with
  * the float edit option columns.
  */
-#edit-options .collapsible {
+#edit-options details {
   float: left;
   width: 100%;
 }
@@ -1061,7 +1061,7 @@ ul#views-display-menu-tabs li.add ul.action-list li{
   float: left;
 }
 
-#edit-options-exception.collapsible {
+#edit-options-exception {
   float: right;
   width: 250px;
   margin-top: -2px;
diff --git a/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php b/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php
index 4e6a746..4f61d72 100644
--- a/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php
+++ b/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php
@@ -462,7 +462,6 @@ public function getDisplayDetails($view, $display) {
     $build['columns']['third'] = array(
       '#type' => 'details',
       '#title' => t('Advanced'),
-      '#collapsible' => TRUE,
       '#collapsed' => TRUE,
       '#theme_wrappers' => array('details', 'container'),
       '#attributes' => array(
diff --git a/core/themes/seven/style.css b/core/themes/seven/style.css
index 445730f..f1d660f 100644
--- a/core/themes/seven/style.css
+++ b/core/themes/seven/style.css
@@ -1386,7 +1386,7 @@ details.fieldset-no-legend {
   top: -1px;
 }
 
-.views-ui-dialog details.collapsible:not(.collapsed) {
+.views-ui-dialog details:not(.collapsed) {
   padding-top: 1.5em;
 }
 
diff --git a/core/update.php b/core/update.php
index 17b44fd..2367369 100644
--- a/core/update.php
+++ b/core/update.php
@@ -70,7 +70,6 @@ function update_script_selection_form($form, &$form_state) {
     '#tree' => TRUE,
     '#type' => 'details',
     '#collapsed' => TRUE,
-    '#collapsible' => TRUE,
   );
 
   // Ensure system.module's updates appear first.