diff --git a/core/includes/common.inc b/core/includes/common.inc
index 9589aa1..3afb1b7 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2883,8 +2883,9 @@ 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 ebab63f..8180467 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;
@@ -791,7 +792,6 @@ public function doBuildForm($form_id, &$element, FormStateInterface &$form_state
       $element[$key] = $this->doBuildForm($form_id, $element[$key], $form_state);
       $count++;
     }
-
     // The #after_build flag allows any piece of a form to be altered
     // after normal input parsing has been completed.
     if (isset($element['#after_build']) && !isset($element['#after_build_done'])) {
@@ -857,6 +857,33 @@ public function doBuildForm($form_id, &$element, FormStateInterface &$form_state
         $form_state->setValue($triggering_element['#name'], $triggering_element['#value']);
       }
     }
+    // Make sure each form element is checked for safe markup and
+    // they are properly escaped.
+    $element = $this->formSafeCheck($element);
+    return $element;
+  }
+
+  /**
+   * Method to ensure every form element pass the safe check.
+   *
+   * @param array $element
+   *   - The form element.
+   *
+   * @return array
+   *   - The form element marked as safe.
+   */
+  protected function formSafeCheck(array $element) {
+    // 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]);
+      }
+    }
     return $element;
   }
 
diff --git a/core/lib/Drupal/Core/Render/Element/HtmlTag.php b/core/lib/Drupal/Core/Render/Element/HtmlTag.php
index 851bbf9..5f81e93 100644
--- a/core/lib/Drupal/Core/Render/Element/HtmlTag.php
+++ b/core/lib/Drupal/Core/Render/Element/HtmlTag.php
@@ -139,7 +139,8 @@ public static function preRenderConditionalComments($element) {
       $expression = '!IE';
     }
     else {
-      $expression = $browsers['IE'];
+      // Need to filter at least for admin usage.
+      $expression = SafeMarkup::checkAdminXss($browsers['IE']);
     }
 
     // Wrap the element's potentially existing #prefix and #suffix properties with
@@ -151,15 +152,23 @@ public static function preRenderConditionalComments($element) {
       '#prefix' => '',
       '#suffix' => '',
     );
+
+    // Ensure what we are dealing with is safe.
+    // This would be done later anyway in drupal_render().
+    $prefix = SafeMarkup::checkAdminXss($element['#prefix']);
+    $suffix = SafeMarkup::checkAdminXss($element['#suffix']);
+
+    // Now calling SafeMarkup::set is safe, because we ensured the
+    // data coming in was at least admin escaped.
     if (!$browsers['!IE']) {
       // "downlevel-hidden".
-      $element['#prefix'] = "\n<!--[if $expression]>\n" . $element['#prefix'];
-      $element['#suffix'] .= "<![endif]-->\n";
+      $element['#prefix'] = SafeMarkup::set("\n<!--[if $expression]>\n" . $prefix);
+      $element['#suffix'] = SafeMarkup::set($suffix . "<![endif]-->\n");
     }
     else {
       // "downlevel-revealed".
-      $element['#prefix'] = "\n<!--[if $expression]><!-->\n" . $element['#prefix'];
-      $element['#suffix'] .= "<!--<![endif]-->\n";
+      $element['#prefix'] = SafeMarkup::set("\n<!--[if $expression]><!-->\n" . $prefix);
+      $element['#suffix'] = SafeMarkup::set($suffix . "<!--<![endif]-->\n");
     }
 
     return $element;
diff --git a/core/modules/field_ui/src/Tests/FieldUiTestBase.php b/core/modules/field_ui/src/Tests/FieldUiTestBase.php
index 962acec..314cf5b 100644
--- a/core/modules/field_ui/src/Tests/FieldUiTestBase.php
+++ b/core/modules/field_ui/src/Tests/FieldUiTestBase.php
@@ -105,6 +105,7 @@ function fieldUIAddExistingField($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('&amp;lt;', '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('&amp;lt;', 'The page does not have double escaped HTML tags.');
 
     if (is_string($result)) {
       $this->assertText($result, $message);
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index 371be2c..aa01232 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -505,7 +505,7 @@ function rdf_preprocess_comment(&$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 15da0e5..bf66e00 100644
--- a/core/modules/system/src/Tests/Common/RenderTest.php
+++ b/core/modules/system/src/Tests/Common/RenderTest.php
@@ -808,10 +808,10 @@ 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;
@@ -852,7 +852,7 @@ 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(
@@ -895,11 +895,11 @@ 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;
@@ -943,7 +943,7 @@ 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(
@@ -969,7 +969,7 @@ 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(
@@ -999,7 +999,7 @@ 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(