diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index 60b83df..cde2a74 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -35,11 +35,11 @@ class Html {
   protected static $seenIds;
 
   /**
-   * Contains the current AJAX HTML IDs.
+   * Contains if the current request was send via AJAX.
    *
-   * @var string
+   * @var bool
    */
-  protected static $ajaxHTMLIDs;
+  protected static $isAjax = FALSE;
 
   /**
    * Prepares a string for use as a valid class name.
@@ -102,13 +102,13 @@ public static function cleanCssIdentifier($identifier, array $filter = array(
   }
 
   /**
-   * Sets the AJAX HTML IDs.
+   * Sets if this request is an Ajax request.
    *
-   * @param string $ajax_html_ids
-   *   The AJAX HTML IDs, probably coming from the current request.
+   * @param bool $is_ajax
+   *   TRUE if this request is an Ajax request, FALSE otherwise.
    */
-  public static function setAjaxHtmlIds($ajax_html_ids = '') {
-    static::$ajaxHTMLIDs = $ajax_html_ids;
+  public static function setIsAjax($is_ajax) {
+    static::$isAjax = $is_ajax;
   }
 
   /**
@@ -142,43 +142,14 @@ public static function setAjaxHtmlIds($ajax_html_ids = '') {
   public static function getUniqueId($id) {
     // If this is an Ajax request, then content returned by this page request
     // will be merged with content already on the base page. The HTML IDs must
-    // be unique for the fully merged content. Therefore, initialize $seen_ids
-    // to take into account IDs that are already in use on the base page.
+    // be unique for the fully merged content. Therefore use unique IDs.
+    if (static::$isAjax) {
+      return static::getId($id) . '--' . uniqid();
+    }
+
+    // seenIdsInit is only useful for tests anymore.
     if (!isset(static::$seenIdsInit)) {
-      // Ideally, Drupal would provide an API to persist state information about
-      // prior page requests in the database, and we'd be able to add this
-      // function's $seen_ids static variable to that state information in order
-      // to have it properly initialized for this page request. However, no such
-      // page state API exists, so instead, ajax.js adds all of the in-use HTML
-      // IDs to the POST data of Ajax submissions. Direct use of $_POST is
-      // normally not recommended as it could open up security risks, but
-      // because the raw POST data is cast to a number before being returned by
-      // this function, this usage is safe.
-      if (empty(static::$ajaxHTMLIDs)) {
-        static::$seenIdsInit = array();
-      }
-      else {
-        // This function ensures uniqueness by appending a counter to the base
-        // id requested by the calling function after the first occurrence of
-        // that requested id. $_POST['ajax_html_ids'] contains the ids as they
-        // were returned by this function, potentially with the appended
-        // counter, so we parse that to reconstruct the $seen_ids array.
-        $ajax_html_ids = explode(' ', static::$ajaxHTMLIDs);
-        foreach ($ajax_html_ids as $seen_id) {
-          // We rely on '--' being used solely for separating a base id from the
-          // counter, which this function ensures when returning an id.
-          $parts = explode('--', $seen_id, 2);
-          if (!empty($parts[1]) && is_numeric($parts[1])) {
-            list($seen_id, $i) = $parts;
-          }
-          else {
-            $i = 1;
-          }
-          if (!isset(static::$seenIdsInit[$seen_id]) || ($i > static::$seenIdsInit[$seen_id])) {
-            static::$seenIdsInit[$seen_id] = $i;
-          }
-        }
-      }
+      static::$seenIdsInit = array();
     }
     if (!isset(static::$seenIds)) {
       static::$seenIds = static::$seenIdsInit;
diff --git a/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
index 8996b5d..ab2ab8b 100644
--- a/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/AjaxSubscriber.php
@@ -20,13 +20,15 @@
 class AjaxSubscriber implements EventSubscriberInterface {
 
   /**
-   * Sets the AJAX HTML IDs from the current request.
+   * Sets the AJAX parameter from the current request.
    *
    * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
    *   The response event, which contains the current request.
    */
   public function onRequest(GetResponseEvent $event) {
-    Html::setAjaxHtmlIds($event->getRequest()->request->get('ajax_html_ids', ''));
+    if ($event->getRequest()->request->get('ajax', '') == 1) {
+      Html::setIsAjax(TRUE);
+    }
   }
 
   /**
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index d80c2c5..39d6173 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -394,16 +394,8 @@
       Drupal.detachBehaviors(this.$form.get(0), settings, 'serialize');
     }
 
-    // Prevent duplicate HTML ids in the returned markup.
-    // @see \Drupal\Component\Utility\Html::getUniqueId()
-    var ids = document.querySelectorAll('[id]');
-    var ajaxHtmlIds = [];
-    var il = ids.length;
-    for (var i = 0; i < il; i++) {
-      ajaxHtmlIds.push(ids[i].id);
-    }
-    // Join IDs to minimize request size.
-    options.data.ajax_html_ids = ajaxHtmlIds.join(' ');
+    // Set that this is an drupal ajax request.
+    options.data.ajax = 1;
 
     // Allow Drupal to return new JavaScript and CSS files to load without
     // returning the ones already loaded.
diff --git a/core/modules/field/src/Tests/Boolean/BooleanFieldTest.php b/core/modules/field/src/Tests/Boolean/BooleanFieldTest.php
index 28f6334..647180c 100644
--- a/core/modules/field/src/Tests/Boolean/BooleanFieldTest.php
+++ b/core/modules/field/src/Tests/Boolean/BooleanFieldTest.php
@@ -171,7 +171,7 @@ function testBooleanField() {
       t('Display setting checkbox is available')
     );
     $this->assertFieldByXPath(
-      '*//input[@id="edit-fields-' . $field_name . '-settings-edit-form-settings-display-label" and @value="1"]',
+      '*//input[starts-with(@id, "edit-fields-' . $field_name . '-settings-edit-form-settings-display-label") and @value="1"]',
       TRUE,
       t('Display label changes label of the checkbox')
     );
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 99cb227..0f97787 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1694,13 +1694,7 @@ protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_p
         $extra_post[$key] = $value;
       }
     }
-    $ajax_html_ids = array();
-    foreach ($this->xpath('//*[@id]') as $element) {
-      $ajax_html_ids[] = (string) $element['id'];
-    }
-    if (!empty($ajax_html_ids)) {
-      $extra_post['ajax_html_ids'] = implode(' ', $ajax_html_ids);
-    }
+    $extra_post['ajax'] = 1;
     $extra_post += $this->getAjaxPageStatePostData();
     // Now serialize all the $extra_post values, and prepend it with an '&'.
     $extra_post = '&' . $this->serializePostValues($extra_post);
diff --git a/core/modules/views/src/Controller/ViewAjaxController.php b/core/modules/views/src/Controller/ViewAjaxController.php
index bd5f956..70c9d4e 100644
--- a/core/modules/views/src/Controller/ViewAjaxController.php
+++ b/core/modules/views/src/Controller/ViewAjaxController.php
@@ -133,7 +133,7 @@ public function ajaxView(Request $request) {
 
       // Remove all of this stuff from the query of the request so it doesn't
       // end up in pagers and tablesort URLs.
-      foreach (array('view_name', 'view_display_id', 'view_args', 'view_path', 'view_dom_id', 'pager_element', 'view_base_path', 'ajax_html_ids') as $key) {
+      foreach (array('view_name', 'view_display_id', 'view_args', 'view_path', 'view_dom_id', 'pager_element', 'view_base_path', 'ajax') as $key) {
         $request->query->remove($key);
         $request->request->remove($key);
       }
diff --git a/core/modules/views_ui/src/ViewUI.php b/core/modules/views_ui/src/ViewUI.php
index 75bbbdf..2a46832 100644
--- a/core/modules/views_ui/src/ViewUI.php
+++ b/core/modules/views_ui/src/ViewUI.php
@@ -582,7 +582,7 @@ public function renderPreview($display_id, $args = array()) {
       // have some input in the query parameters, so we merge request() and
       // query() to ensure we get it all.
       $exposed_input = array_merge(\Drupal::request()->request->all(), \Drupal::request()->query->all());
-      foreach (array('view_name', 'view_display_id', 'view_args', 'view_path', 'view_dom_id', 'pager_element', 'view_base_path', 'ajax_html_ids', 'ajax_page_state', 'form_id', 'form_build_id', 'form_token') as $key) {
+      foreach (array('view_name', 'view_display_id', 'view_args', 'view_path', 'view_dom_id', 'pager_element', 'view_base_path', 'ajax', 'ajax_page_state', 'form_id', 'form_build_id', 'form_token') as $key) {
         if (isset($exposed_input[$key])) {
           unset($exposed_input[$key]);
         }
diff --git a/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
index 18b07f7..c1b463c 100644
--- a/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
@@ -143,19 +143,17 @@ public function providerTestHtmlGetUniqueId() {
    *   The expected result.
    * @param string $source
    *   The string being transformed to an ID.
-   * @param bool $reset
-   *   (optional) If TRUE, reset the list of seen IDs. Defaults to FALSE.
    *
    * @dataProvider providerTestHtmlGetUniqueIdWithAjaxIds
    *
    * @covers ::getUniqueId
    */
-  public function testHtmlGetUniqueIdWithAjaxIds($expected, $source, $reset = FALSE) {
-    if ($reset) {
-      Html::resetSeenIds();
-    }
-    Html::setAjaxHtmlIds('test-unique-id1 test-unique-id2--3');
-    $this->assertSame($expected, Html::getUniqueId($source));
+  public function testHtmlGetUniqueIdWithAjaxIds($expected, $source) {
+    Html::setIsAjax(TRUE);
+    $id = Html::getUniqueId($source);
+    $t = explode('--', $id);
+    $expected .= $t[1];
+    $this->assertSame($expected, $id);
   }
 
   /**
@@ -166,10 +164,10 @@ public function testHtmlGetUniqueIdWithAjaxIds($expected, $source, $reset = FALS
    */
   public function providerTestHtmlGetUniqueIdWithAjaxIds() {
     return array(
-      array('test-unique-id1--2', 'test-unique-id1', TRUE),
-      array('test-unique-id1--3', 'test-unique-id1'),
-      array('test-unique-id2--4', 'test-unique-id2', TRUE),
-      array('test-unique-id2--5', 'test-unique-id2'),
+      array('test-unique-id1--', 'test-unique-id1'),
+      array('test-unique-id1--', 'test-unique-id1'),
+      array('test-unique-id2--', 'test-unique-id2'),
+      array('test-unique-id2--', 'test-unique-id2'),
     );
   }
 
@@ -186,6 +184,7 @@ public function providerTestHtmlGetUniqueIdWithAjaxIds() {
    * @covers ::getId
    */
   public function testHtmlGetId($expected, $source) {
+    Html::setIsAjax(FALSE);
     $this->assertSame($expected, Html::getId($source));
   }