diff --git a/core/includes/common.inc b/core/includes/common.inc
index 4a789c8..35802ac 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2894,8 +2894,9 @@ function drupal_render(&$elements, $is_recursive_call = FALSE) {
   // with how render cached output gets stored. This ensures that
   // #post_render_cache callbacks get the same data to work with, no matter if
   // #cache is disabled, #cache is enabled, there is a cache hit or miss.
-  $prefix = isset($elements['#prefix']) ? $elements['#prefix'] : '';
-  $suffix = isset($elements['#suffix']) ? $elements['#suffix'] : '';
+  $prefix = isset($elements['#prefix']) ? SafeMarkup::checkAdminXss($elements['#prefix']) : '';
+  $suffix = isset($elements['#suffix']) ? SafeMarkup::checkAdminXss($elements['#suffix']) : '';
+
   $elements['#markup'] = $prefix . $elements['#children'] . $suffix;
 
   // We've rendered this element (and its subtree!), now update the stack.
diff --git a/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
index dc0a6a1..7f8a756 100644
--- a/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Component\Utility;
 
+use Drupal\Component\Utility\Xss;
+
 /**
  * Manages known safe strings for rendering at the theme layer.
  *
@@ -137,6 +139,22 @@ public static function escape($string) {
   }
 
   /**
+   *  Applies a very permissive XSS/HTML filter for admin-only use.
+   *
+   * @param $string
+   *   A string.
+   *
+   * @return string
+   *   The escaped string. If $string was already set as safe with
+   *   SafeString::set, it won't be escaped again.
+   *
+   *  @see \Drupal\Component\Utility\Xss\filterAdmin
+   */
+  public static function checkAdminXss($string) {
+    return static::isSafe($string) ? $string : Xss::filterAdmin($string);
+  }
+
+  /**
   * Retrieves all strings currently marked as safe.
   *
   * This is useful for the batch and form APIs, where it is important to
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 6fb6467..67af81f 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\NestedArray;
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Access\CsrfTokenGenerator;
@@ -679,6 +680,18 @@ public function doBuildForm($form_id, &$element, FormStateInterface &$form_state
       '#errors' => NULL,
     );
 
+    // Filtering keys which are expected to contain HTML.
+    $markup_keys = array(
+      '#description',
+      '#field_prefix',
+      '#field_suffix',
+    );
+    foreach ($markup_keys as $key) {
+      if (!empty($element[$key]) && is_scalar($element[$key])) {
+        $element[$key] = SafeMarkup::checkAdminXss($element[$key]);
+      }
+    }
+
     // Special handling if we're on the top level form element.
     if (isset($element['#type']) && $element['#type'] == 'form') {
       if (!empty($element['#https']) && Settings::get('mixed_mode_sessions', FALSE) &&
diff --git a/core/modules/field_ui/src/Tests/FieldUiTestBase.php b/core/modules/field_ui/src/Tests/FieldUiTestBase.php
index 962acec..314cf5b 100644
--- a/core/modules/field_ui/src/Tests/FieldUiTestBase.php
+++ b/core/modules/field_ui/src/Tests/FieldUiTestBase.php
@@ -105,6 +105,7 @@ function fieldUIAddExistingField($bundle_path, $initial_edit, $field_edit = arra
 
     // First step : 'Re-use existing field' on the 'Manage fields' page.
     $this->drupalPostForm("$bundle_path/fields", $initial_edit, t('Save'));
+    $this->assertNoRaw('<', 'The page does not have double escaped HTML tags.');
 
     // Second step : 'Field settings' form.
     $this->drupalPostForm(NULL, $field_edit, t('Save settings'));
diff --git a/core/modules/locale/src/Form/ImportForm.php b/core/modules/locale/src/Form/ImportForm.php
index eda1567..d9a811c 100644
--- a/core/modules/locale/src/Form/ImportForm.php
+++ b/core/modules/locale/src/Form/ImportForm.php
@@ -103,18 +103,22 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       'file_validate_extensions' => array('po'),
       'file_validate_size' => array(file_upload_max_size()),
     );
+
+    $file_description = array(
+      '#theme' => 'file_upload_help',
+      '#description' => $this->t('A Gettext Portable Object file.'),
+      '#upload_validators' => $validators,
+    );
+
     $form['file'] = array(
       '#type' => 'file',
       '#title' => $this->t('Translation file'),
-      '#description' => array(
-        '#theme' => 'file_upload_help',
-        '#description' => $this->t('A Gettext Portable Object file.'),
-        '#upload_validators' => $validators,
-      ),
+      '#description' => drupal_render($file_description),
       '#size' => 50,
       '#upload_validators' => $validators,
       '#attributes' => array('class' => array('file-import-input')),
     );
+
     $form['langcode'] = array(
       '#type' => 'select',
       '#title' => $this->t('Language'),
diff --git a/core/modules/options/src/Tests/OptionsFieldUITest.php b/core/modules/options/src/Tests/OptionsFieldUITest.php
index e3b4684..40343bf 100644
--- a/core/modules/options/src/Tests/OptionsFieldUITest.php
+++ b/core/modules/options/src/Tests/OptionsFieldUITest.php
@@ -278,6 +278,7 @@ protected function createOptionsField($type) {
   function assertAllowedValuesInput($input_string, $result, $message) {
     $edit = array('field_storage[settings][allowed_values]' => $input_string);
     $this->drupalPostForm($this->admin_path, $edit, t('Save field settings'));
+    $this->assertNoRaw('<', 'The page does not have double escaped HTML tags.');
 
     if (is_string($result)) {
       $this->assertText($result, $message);
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index f407e83..5ffd203 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -506,7 +506,7 @@ function rdf_preprocess_comment(&$variables) {
       '#theme' => 'rdf_metadata',
       '#metadata' => $variables['rdf_metadata_attributes'],
     );
-    $variables['content']['comment_body']['#prefix'] = drupal_render($rdf_metadata) . $variables['content']['comment_body']['#prefix'];
+    $variables['content']['comment_body']['#prefix'] = SafeMarkup::set(drupal_render($rdf_metadata) . $variables['content']['comment_body']['#prefix']);
   }
 }
 
diff --git a/core/modules/system/src/Tests/Common/RenderTest.php b/core/modules/system/src/Tests/Common/RenderTest.php
index d07085c..6ce9eae 100644
--- a/core/modules/system/src/Tests/Common/RenderTest.php
+++ b/core/modules/system/src/Tests/Common/RenderTest.php
@@ -782,10 +782,10 @@ function testDrupalRenderRenderCachePlaceholder() {
         ),
       ),
       '#markup' => $placeholder,
-      '#prefix' => '<foo>',
-      '#suffix' => '</foo>'
+      '#prefix' => '<pre>',
+      '#suffix' => '</pre>',
     );
-    $expected_output = '<foo><bar>' . $context['bar'] . '</bar></foo>';
+    $expected_output = '<pre><bar>' . $context['bar'] . '</bar></pre>';
 
     // #cache disabled.
     $element = $test_element;
@@ -826,7 +826,7 @@ function testDrupalRenderRenderCachePlaceholder() {
     $this->assertIdentical($token, $expected_token, 'The tokens are identical');
     // Verify the token is in the cached element.
     $expected_element = array(
-      '#markup' => '<foo><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></foo>',
+      '#markup' => '<pre><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></pre>',
       '#attached' => array(),
       '#post_render_cache' => array(
         'common_test_post_render_cache_placeholder' => array(
@@ -869,11 +869,11 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
           ],
         ],
         '#markup' => $placeholder,
-        '#prefix' => '<foo>',
-        '#suffix' => '</foo>'
+        '#prefix' => '<pre>',
+        '#suffix' => '</pre>'
       ],
     ];
-    $expected_output = '<foo><bar>' . $context['bar'] . '</bar></foo>' . "\n";
+    $expected_output = '<pre><bar>' . $context['bar'] . '</bar></pre>' . "\n";
 
     // #cache disabled.
     $element = $test_element;
@@ -917,7 +917,7 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
     $this->assertIdentical($token, $expected_token, 'The tokens are identical for the child element');
     // Verify the token is in the cached element.
     $expected_element = array(
-      '#markup' => '<foo><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></foo>',
+      '#markup' => '<pre><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></pre>',
       '#attached' => array(),
       '#post_render_cache' => array(
         'common_test_post_render_cache_placeholder' => array(
@@ -943,7 +943,7 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
     $this->assertIdentical($token, $expected_token, 'The tokens are identical for the parent element');
     // Verify the token is in the cached element.
     $expected_element = array(
-      '#markup' => '<foo><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></foo>' . "\n",
+      '#markup' => '<pre><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></pre>' . "\n",
       '#attached' => array(),
       '#post_render_cache' => array(
         'common_test_post_render_cache_placeholder' => array(
@@ -973,7 +973,7 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
     $this->assertIdentical($token, $expected_token, 'The tokens are identical for the child element');
     // Verify the token is in the cached element.
     $expected_element = array(
-      '#markup' => '<foo><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></foo>',
+      '#markup' => '<pre><drupal-render-cache-placeholder callback="common_test_post_render_cache_placeholder" token="'. $expected_token . '"></drupal-render-cache-placeholder></pre>',
       '#attached' => array(),
       '#post_render_cache' => array(
         'common_test_post_render_cache_placeholder' => array(