diff --git a/core/includes/form.inc b/core/includes/form.inc
index 9e8551c..d4430da 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -7,6 +7,8 @@
 
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\NestedArray;
+use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\Xss;
 use Drupal\Core\Form\FormInterface;
 use Drupal\Core\Form\BaseFormIdInterface;
 use Drupal\Core\Database\Database;
@@ -3615,7 +3617,6 @@ function form_process_tableselect($element) {
  *   The processed element.
  *
  * @see form_process_tableselect()
- * @see theme_tableselect()
  */
 function form_process_table($element, &$form_state) {
   if ($element['#tableselect']) {
@@ -4058,7 +4059,7 @@ function form_pre_render_vertical_tabs($element) {
 }
 
 /**
- * Preapres an element's children details for vertical tabs templates.
+ * Prepares an element's children details for vertical tabs templates.
  *
  * Default template: vertical-tabs.html.twig.
  *
@@ -4516,8 +4517,6 @@ function form_pre_render_color($element) {
  *   An associative array containing:
  *   - element: An associative array containing the properties of the element.
  *     Properties used: #action, #method, #attributes, #children
- *
- * @ingroup themeable
  */
 function template_preprocess_form(&$variables) {
   $element = $variables['element'];
@@ -4558,7 +4557,7 @@ function template_preprocess_textarea(&$variables) {
   }
 
   $variables['attributes'] = $element['#attributes'];
-  $variables['value'] = check_plain($element['#value']);
+  $variables['value'] = String::checkPlain($element['#value']);
 }
 
 /**
@@ -4755,7 +4754,7 @@ function template_preprocess_form_required_marker(&$variables) {
  *
  * Form element labels include the #title and a #required marker. The label is
  * associated with the element itself by the element #id. Labels may appear
- * before or after elements, depending on theme_form_element() and
+ * before or after elements, depending on form_element template and
  * #title_display.
  *
  * This function will not be called for elements with no labels, depending on
@@ -4781,7 +4780,7 @@ function template_preprocess_form_element_label(&$variables) {
   }
   else {
     // If the element is required, a required marker is appended to the label.
-    $variables['label'] = (isset($element['#title'])) ? filter_xss_admin($element['#title']) : NULL;
+    $variables['label'] = (isset($element['#title'])) ? Xss::filterAdmin($element['#title']) : NULL;
     $variables['required'] = (!empty($element['#required'])) ? array('#theme' => 'form_required_marker', '#element' => $element) : '';
     // Associate the label with the field it is for.
     if (!empty($element['#id'])) {
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 1f258f0..d7d9068 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2356,8 +2356,6 @@ function theme_indentation($variables) {
  *   An associative array containing:
  *   - element: An associative array containing the properties of the element.
  *     Properties used: #id, #attributes, #children.
- *
- * @ingroup themeable
  */
 function template_preprocess_container(&$variables) {
   $element = $variables['element'];
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index d366e01..d1d8d7a 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -235,7 +235,7 @@ function hook_queue_info_alter(&$queues) {
  *  - "#post_render": array of callables taking $children and $element.
  *  - "#submit": array of callback functions taking $form and $form_state.
  *  - "#title_display": optional string indicating if and how #title should be
- *    displayed, see theme_form_element() and theme_form_element_label().
+ *    displayed, see templates for form_element and form_element_label.
  *
  * @see hook_element_info_alter()
  * @see system_element_info()
diff --git a/core/modules/system/templates/vertical-tabs.html.twig b/core/modules/system/templates/vertical-tabs.html.twig
index bbd76d9..aec5da5 100644
--- a/core/modules/system/templates/vertical-tabs.html.twig
+++ b/core/modules/system/templates/vertical-tabs.html.twig
@@ -10,5 +10,7 @@
  *
  * @ingroup themeable
  */
+ @todo: remove this file once http://drupal.org/node/1819284 is resolved.
+ This is identical to core/modules/system/templates/container.html.twig
 #}
 <div{{ attributes }}>{{ children }}</div>
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 60d153a..d271e7e 100644
--- a/core/modules/system/tests/modules/design_test/form/details.inc
+++ b/core/modules/system/tests/modules/design_test/form/details.inc
@@ -6,7 +6,7 @@
  */
 
 /**
- * Form constructor for testing theme_details().
+ * Form constructor for testing details template.
  */
 function design_test_form_details($form, &$form_state) {
   $form['collapsible'] = array(
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 f55bfe5..d1e47a6 100644
--- a/core/modules/system/tests/modules/design_test/form/fieldset.inc
+++ b/core/modules/system/tests/modules/design_test/form/fieldset.inc
@@ -6,7 +6,7 @@
  */
 
 /**
- * Form constructor for testing theme_fieldset().
+ * Form constructor for testing fieldset template.
  */
 function design_test_form_fieldset($form, &$form_state) {
   $form['regular'] = array(