diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index 659cb27..b62ba69 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -35,7 +35,7 @@ class Html {
   protected static $seenIds;
 
   /**
-   * Contains if the current request was sent via AJAX.
+   * Stores whether the current request was sent via AJAX.
    *
    * @var bool
    */
@@ -147,7 +147,7 @@ public static function getUniqueId($id) {
       return static::getId($id) . '--' . Crypt::randomBytesBase64(8);
     }
 
-    // seenIdsInit is only useful for tests anymore.
+    // seenIdsInit is now only useful during testing.
     if (!isset(static::$seenIdsInit)) {
       static::$seenIdsInit = array();
     }
diff --git a/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
index 83262f8..87f89d1 100644
--- a/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
@@ -20,7 +20,7 @@
 class AjaxSubscriber implements EventSubscriberInterface {
 
   /**
-   * Request parameter to indicate that a request is a drupal ajax request.
+   * Request parameter to indicate that a request is a Drupal Ajax request.
    */
   const AJAX_REQUEST_PARAMETER = '_drupal_ajax';
 
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 76ee918..39c7c37 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -377,7 +377,7 @@
   Drupal.ajax.WRAPPER_FORMAT = '_wrapper_format';
 
   /**
-   * Request parameter to indicate that a request is a drupal ajax request.
+   * Request parameter to indicate that a request is a Drupal Ajax request.
    */
    Drupal.ajax.AJAX_REQUEST_PARAMETER = '_drupal_ajax';
 
diff --git a/core/modules/file/src/Element/ManagedFile.php b/core/modules/file/src/Element/ManagedFile.php
index 740791c..a6fe4f8 100644
--- a/core/modules/file/src/Element/ManagedFile.php
+++ b/core/modules/file/src/Element/ManagedFile.php
@@ -142,7 +142,7 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
     $element['#files'] = !empty($fids) ? File::loadMultiple($fids) : FALSE;
     $element['#tree'] = TRUE;
 
-    // Generate a wrapper html ID. All we need is that its unique.
+    // Generate a unique wrapper HTML ID.
     $ajax_wrapper_id = Html::getUniqueId('ajax-wrapper');
 
     $ajax_settings = [
@@ -260,7 +260,7 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
       $element['upload']['#attached']['drupalSettings']['file']['elements']['#' . $element['#id']] = $extension_list;
     }
 
-    // Let #id point of the file element, so the field label's 'for' corresponds
+    // Let #id point to the file element, so the field label's 'for' corresponds
     // with it.
     $element['#id'] = &$element['upload']['#id'];
 
diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index d6ac7bd..54dba00 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -822,12 +822,12 @@ protected function assertThemeOutput($callback, array $variables = array(), $exp
   }
 
   /**
-   * Asserts that a field exists in the current page with a given xpath result.
+   * Asserts that a field exists in the current page with a given Xpath result.
    *
    * @param \SimpleXmlElement[] $fields
    *   Xml elements.
    * @param string $value
-   *   Value of the field to assert. You may pass in NULL (default) to skip
+   *   (optional) Value of the field to assert. You may pass in NULL (default) to skip
    *   checking the actual value, while still checking that the field exists.
    * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
diff --git a/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
index 3026a17..b634354 100644
--- a/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
@@ -151,8 +151,9 @@ public function providerTestHtmlGetUniqueId() {
   public function testHtmlGetUniqueIdWithAjaxIds($expected, $source) {
     Html::setIsAjax(TRUE);
     $id = Html::getUniqueId($source);
-    $t = explode('--', $id);
-    $expected .= $t[1];
+
+    $random_suffix = substr($id, strlen($source) + 2);
+    $expected = $expected . $random_suffix;
     $this->assertSame($expected, $id);
   }
 
@@ -165,6 +166,9 @@ public function testHtmlGetUniqueIdWithAjaxIds($expected, $source) {
   public function providerTestHtmlGetUniqueIdWithAjaxIds() {
     return array(
       array('test-unique-id1--', 'test-unique-id1'),
+      // Note, we truncate two hyphens at the end.
+      // @see \Drupal\Component\Utility\Html::getId()
+      array('test-unique-id1---', 'test-unique-id1--'),
       array('test-unique-id2--', 'test-unique-id2'),
     );
   }