diff -u b/core/misc/vertical-tabs.es6.js b/core/misc/vertical-tabs.es6.js
--- b/core/misc/vertical-tabs.es6.js
+++ b/core/misc/vertical-tabs.es6.js
@@ -80,7 +80,7 @@
           const vertical_tab = new Drupal.verticalTab({
             title: $that.find('> summary').html(),
             details: $that,
-            child_error_count: $that.data('child-error-count'),
+            childErrorCount: $that.data('child-error-count'),
           });
           tab_list.append(vertical_tab.item);
           $that
@@ -127,6 +127,8 @@
    *   The name of the tab.
    * @param {jQuery} settings.details
    *   The jQuery object of the details element that is the tab pane.
+   * @param {jQuery} settings.childErrorCount
+   *   The number of form errors contained inside the tab pane.
    *
    * @fires event:summaryUpdated
    *
@@ -250,6 +252,10 @@
    *   An object with the following keys:
    * @param {string} settings.title
    *   The name of the tab.
+   * @param {jQuery} settings.details
+   *   The jQuery object of the details element that is the tab pane.
+   * @param {jQuery} settings.childErrorCount
+   *   The number of form errors contained inside the tab pane.
    *
    * @return {object}
    *   This function has to return an object with at least these keys:
@@ -267,7 +273,7 @@
         ),
       );
 
-    if (settings.child_error_count) {
+    if (settings.childErrorCount) {
       tab.item.addClass('vertical-tab__menu-item--child-error');
     }
 
diff -u b/core/misc/vertical-tabs.js b/core/misc/vertical-tabs.js
--- b/core/misc/vertical-tabs.js
+++ b/core/misc/vertical-tabs.js
@@ -41,7 +41,7 @@
           var vertical_tab = new Drupal.verticalTab({
             title: $that.find('> summary').html(),
             details: $that,
-            child_error_count: $that.data('child-error-count')
+            childErrorCount: $that.data('child-error-count')
           });
           tab_list.append(vertical_tab.item);
           $that.removeClass('collapsed').attr('open', true).addClass('vertical-tabs__pane').data('verticalTab', vertical_tab);
@@ -141,7 +141,7 @@
     var tab = {};
     tab.item = $('<li class="vertical-tabs__menu-item" tabindex="-1"></li>').append(tab.link = $('<a href="#"></a>').append(tab.title = $('<strong class="vertical-tabs__menu-item-title"></strong>').html(settings.title)).append(tab.summary = $('<span class="vertical-tabs__menu-item-summary"></span>')));
 
-    if (settings.child_error_count) {
+    if (settings.childErrorCount) {
       tab.item.addClass('vertical-tab__menu-item--child-error');
     }