Issue #2002336 by LewisNyman, mgifford, sun: Introduce a CSS class to hide borders of fieldset elements.

core/includes/install.core.inc

@@ -2567,7 +2567,7 @@ function install_display_requirements($install_state, $requirements) {
  */
 function _install_configure_form($form, &$form_state, &$install_state) {
   $form['site_information'] = array(
-    '#type' => 'details',
+    '#type' => 'fieldgroup',
     '#title' => t('Site information'),
   );
   $form['site_information']['site_name'] = array(
@@ -2584,11 +2584,11 @@ function _install_configure_form($form, &$form_state, &$install_state) {
     '#required' => TRUE,
     '#weight' => -15,
   );
+
   $form['admin_account'] = array(
-    '#type' => 'details',
+    '#type' => 'fieldgroup',
     '#title' => t('Site maintenance account'),
   );
-
   $form['admin_account']['account']['#tree'] = TRUE;
   $form['admin_account']['account']['name'] = array('#type' => 'textfield',
     '#title' => t('Username'),
@@ -2598,7 +2598,6 @@ function _install_configure_form($form, &$form_state, &$install_state) {
     '#weight' => -10,
     '#attributes' => array('class' => array('username')),
   );
-
   $form['admin_account']['account']['mail'] = array(
     '#type' => 'email',
     '#title' => t('E-mail address'),
@@ -2613,10 +2612,9 @@ function _install_configure_form($form, &$form_state, &$install_state) {
   );
 
   $form['regional_settings'] = array(
-    '#type' => 'details',
+    '#type' => 'fieldgroup',
     '#title' => t('Regional settings'),
   );
-
   $countries = \Drupal::service('country_manager')->getList();
   $form['regional_settings']['site_default_country'] = array(
     '#type' => 'select',
@@ -2627,7 +2625,6 @@ function _install_configure_form($form, &$form_state, &$install_state) {
     '#description' => t('Select the default country for the site.'),
     '#weight' => 0,
   );
-
   $form['regional_settings']['date_default_timezone'] = array(
     '#type' => 'select',
     '#title' => t('Default time zone'),
@@ -2639,7 +2636,7 @@ function _install_configure_form($form, &$form_state, &$install_state) {
   );
 
   $form['update_notifications'] = array(
-    '#type' => 'details',
+    '#type' => 'fieldgroup',
     '#title' => t('Update notifications'),
   );
   $form['update_notifications']['update_status_module'] = array(

core/modules/system/css/system.module.css

@@ -26,6 +26,14 @@
   background-position: 0% center;
 }
 
+/**
+ * Unboxed fieldsets for grouping form elements.
+ */
+.fieldgroup {
+  border-width: 0;
+  padding: 0;
+}
+
 /**
  * Collapsible details.
  *

core/modules/system/system.module

@@ -551,6 +551,9 @@ function system_element_info() {
     '#pre_render' => array('form_pre_render_group'),
     '#theme_wrappers' => array('fieldset'),
   );
+  $types['fieldgroup'] = $types['fieldset'] + array(
+    '#attributes' => array('class' => array('fieldgroup')),
+  );
   $types['details'] = array(
     '#collapsed' => FALSE,
     '#value' => NULL,