diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index 54dba00..1fa736e 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -1090,8 +1090,8 @@ protected function assertNoFieldById($id, $value = '', $message = '', $group = '
   /**
    * Asserts that a checkbox field in the current page is checked.
    *
-   * @param string $id
-   *   ID of field to assert.
+   * @param string $drupal_selector
+   *   Drupal data selector of field to assert.
    * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use format_string() to embed variables in the message text, not
@@ -1105,15 +1105,15 @@ protected function assertNoFieldById($id, $value = '', $message = '', $group = '
    * @return bool
    *   TRUE on pass, FALSE on fail.
    */
-  protected function assertFieldChecked($id, $message = '', $group = 'Browser') {
-    $elements = $this->xpath('//input[@id=:id]', array(':id' => $id));
-    return $this->assertTrue(isset($elements[0]) && !empty($elements[0]['checked']), $message ? $message : SafeMarkup::format('Checkbox field @id is checked.', array('@id' => $id)), $group);
+  protected function assertFieldChecked($drupal_selector, $message = '', $group = 'Browser') {
+    $elements = $this->xpath('//input[@drupal-data-selector=:drupal_data_selector]', array(':drupal_data_selector' => $drupal_selector));
+    return $this->assertTrue(isset($elements[0]) && !empty($elements[0]['checked']), $message ? $message : SafeMarkup::format('Checkbox field @id is checked.', array('@id' => $drupal_selector)), $group);
   }
 
   /**
    * Asserts that a checkbox field in the current page is not checked.
    *
-   * @param string $id
+   * @param string $drupal_selector
    *   ID of field to assert.
    * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
@@ -1128,15 +1128,15 @@ protected function assertFieldChecked($id, $message = '', $group = 'Browser') {
    * @return bool
    *   TRUE on pass, FALSE on fail.
    */
-  protected function assertNoFieldChecked($id, $message = '', $group = 'Browser') {
-    $elements = $this->xpath('//input[@id=:id]', array(':id' => $id));
-    return $this->assertTrue(isset($elements[0]) && empty($elements[0]['checked']), $message ? $message : SafeMarkup::format('Checkbox field @id is not checked.', array('@id' => $id)), $group);
+  protected function assertNoFieldChecked($drupal_selector, $message = '', $group = 'Browser') {
+    $elements = $this->xpath('//input[@drupal-data-selector=:drupal_data_selector]', array(':drupal_data_selector' => $drupal_selector));
+    return $this->assertTrue(isset($elements[0]) && empty($elements[0]['checked']), $message ? $message : SafeMarkup::format('Checkbox field @id is not checked.', array('@id' => $drupal_selector)), $group);
   }
 
   /**
    * Asserts that a select option in the current page exists.
    *
-   * @param string $id
+   * @param string $drupal_selector
    *   ID of select field to assert.
    * @param string $option
    *   Option to assert.
@@ -1153,15 +1153,15 @@ protected function assertNoFieldChecked($id, $message = '', $group = 'Browser')
    * @return bool
    *   TRUE on pass, FALSE on fail.
    */
-  protected function assertOption($id, $option, $message = '', $group = 'Browser') {
-    $options = $this->xpath('//select[@id=:id]//option[@value=:option]', array(':id' => $id, ':option' => $option));
-    return $this->assertTrue(isset($options[0]), $message ? $message : SafeMarkup::format('Option @option for field @id exists.', array('@option' => $option, '@id' => $id)), $group);
+  protected function assertOption($drupal_selector, $option, $message = '', $group = 'Browser') {
+    $options = $this->xpath('//select[@drupal-data-selector=:drupal_data_selector]//option[@value=:option]', array(':drupal_data_selector' => $drupal_selector, ':option' => $option));
+    return $this->assertTrue(isset($options[0]), $message ? $message : SafeMarkup::format('Option @option for field @id exists.', array('@option' => $option, '@id' => $drupal_selector)), $group);
   }
 
   /**
    * Asserts that a select option in the current page does not exist.
    *
-   * @param string $id
+   * @param string $drupal_selector
    *   ID of select field to assert.
    * @param string $option
    *   Option to assert.
@@ -1178,16 +1178,16 @@ protected function assertOption($id, $option, $message = '', $group = 'Browser')
    * @return bool
    *   TRUE on pass, FALSE on fail.
    */
-  protected function assertNoOption($id, $option, $message = '', $group = 'Browser') {
-    $selects = $this->xpath('//select[@id=:id]', array(':id' => $id));
-    $options = $this->xpath('//select[@id=:id]//option[@value=:option]', array(':id' => $id, ':option' => $option));
-    return $this->assertTrue(isset($selects[0]) && !isset($options[0]), $message ? $message : SafeMarkup::format('Option @option for field @id does not exist.', array('@option' => $option, '@id' => $id)), $group);
+  protected function assertNoOption($drupal_selector, $option, $message = '', $group = 'Browser') {
+    $selects = $this->xpath('//select[@drupal-data-selector=:drupal_data_selector]', array(':id' => $drupal_selector));
+    $options = $this->xpath('//select[@drupal-data-selector=:drupal_data_selector]//option[@value=:option]', array(':drupal_data_selector' => $drupal_selector, ':option' => $option));
+    return $this->assertTrue(isset($selects[0]) && !isset($options[0]), $message ? $message : SafeMarkup::format('Option @option for field @id does not exist.', array('@option' => $option, '@id' => $drupal_selector)), $group);
   }
 
   /**
    * Asserts that a select option in the current page is checked.
    *
-   * @param string $id
+   * @param string $drupal_selector
    *   ID of select field to assert.
    * @param string $option
    *   Option to assert.
@@ -1206,15 +1206,15 @@ protected function assertNoOption($id, $option, $message = '', $group = 'Browser
    *
    * @todo $id is unusable. Replace with $name.
    */
-  protected function assertOptionSelected($id, $option, $message = '', $group = 'Browser') {
-    $elements = $this->xpath('//select[@id=:id]//option[@value=:option]', array(':id' => $id, ':option' => $option));
-    return $this->assertTrue(isset($elements[0]) && !empty($elements[0]['selected']), $message ? $message : SafeMarkup::format('Option @option for field @id is selected.', array('@option' => $option, '@id' => $id)), $group);
+  protected function assertOptionSelected($drupal_selector, $option, $message = '', $group = 'Browser') {
+    $elements = $this->xpath('//select[@drupal-data-selector=:drupal_data_selector]//option[@value=:option]', array(':drupal_data_selector' => $drupal_selector, ':option' => $option));
+    return $this->assertTrue(isset($elements[0]) && !empty($elements[0]['selected']), $message ? $message : SafeMarkup::format('Option @option for field @id is selected.', array('@option' => $option, '@id' => $drupal_selector)), $group);
   }
 
   /**
    * Asserts that a select option in the current page is not checked.
    *
-   * @param string $id
+   * @param string $drupal_selector
    *   ID of select field to assert.
    * @param string $option
    *   Option to assert.
@@ -1231,16 +1231,16 @@ protected function assertOptionSelected($id, $option, $message = '', $group = 'B
    * @return bool
    *   TRUE on pass, FALSE on fail.
    */
-  protected function assertNoOptionSelected($id, $option, $message = '', $group = 'Browser') {
-    $elements = $this->xpath('//select[@id=:id]//option[@value=:option]', array(':id' => $id, ':option' => $option));
-    return $this->assertTrue(isset($elements[0]) && empty($elements[0]['selected']), $message ? $message : SafeMarkup::format('Option @option for field @id is not selected.', array('@option' => $option, '@id' => $id)), $group);
+  protected function assertNoOptionSelected($drupal_selector, $option, $message = '', $group = 'Browser') {
+    $elements = $this->xpath('//select[@drupal-data-selector=:drupal_data_selector]//option[@value=:option]', array(':drupal_data_selector' => $drupal_selector, ':option' => $option));
+    return $this->assertTrue(isset($elements[0]) && empty($elements[0]['selected']), $message ? $message : SafeMarkup::format('Option @option for field @id is not selected.', array('@option' => $option, '@id' => $drupal_selector)), $group);
   }
 
   /**
-   * Asserts that a field exists with the given name or ID.
+   * Asserts that a field exists with the given name or ID or drupal-selector.
    *
    * @param string $field
-   *   Name or ID of field to assert.
+   *   Name or ID or data-drupal-selector of field to assert.
    * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use format_string() to embed variables in the message text, not
@@ -1255,14 +1255,14 @@ protected function assertNoOptionSelected($id, $option, $message = '', $group =
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertField($field, $message = '', $group = 'Other') {
-    return $this->assertFieldByXPath($this->constructFieldXpath('name', $field) . '|' . $this->constructFieldXpath('id', $field), NULL, $message, $group);
+    return $this->assertFieldByXPath($this->constructFieldXpath('name', $field) . '|' . $this->constructFieldXpath('id', $field) . '|' . $this->constructFieldXpath('data-drupal-selector', $field), NULL, $message, $group);
   }
 
   /**
-   * Asserts that a field does not exist with the given name or ID.
+   * Asserts that a field does not exist with the given name or ID or data-drupal-selector.
    *
    * @param string $field
-   *   Name or ID of field to assert.
+   *   Name or ID or data-drupal-selector of field to assert.
    * @param string $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use format_string() to embed variables in the message text, not
@@ -1277,7 +1277,7 @@ protected function assertField($field, $message = '', $group = 'Other') {
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertNoField($field, $message = '', $group = 'Other') {
-    return $this->assertNoFieldByXPath($this->constructFieldXpath('name', $field) . '|' . $this->constructFieldXpath('id', $field), NULL, $message, $group);
+    return $this->assertNoFieldByXPath($this->constructFieldXpath('name', $field) . '|' . $this->constructFieldXpath('id', $field) . '|' . $this->constructFieldXpath('data-drupal-selector', $field), NULL, $message, $group);
   }
 
   /**
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index c8cced7..eef5241 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1505,13 +1505,10 @@ protected function drupalGetAjax($path, array $options = array(), array $headers
    * @param $headers
    *   An array containing additional HTTP request headers, each formatted as
    *   "name: value".
-   * @param $form_html_id
-   *   (optional) HTML ID of the form to be submitted. On some pages
+   * @param $xpath
+   *   (optional) The xpath selector of the form to be submitted. On some pages
    *   there are many identical forms, so just using the value of the submit
    *   button is not enough. For example: 'trigger-node-presave-assign-form'.
-   *   Note that this is not the Drupal $form_id, but rather the HTML ID of the
-   *   form, which is typically the same thing but with hyphens replacing the
-   *   underscores.
    * @param $extra_post
    *   (optional) A string of additional data to append to the POST submission.
    *   This can be used to add POST data for which there are no HTML fields, as
@@ -1519,7 +1516,7 @@ protected function drupalGetAjax($path, array $options = array(), array $headers
    *   POST data, so it must already be urlencoded and contain a leading "&"
    *   (e.g., "&extra_var1=hello+world&extra_var2=you%26me").
    */
-  protected function drupalPostForm($path, $edit, $submit, array $options = array(), array $headers = array(), $form_html_id = NULL, $extra_post = NULL) {
+  protected function drupalPostForm($path, $edit, $submit, array $options = array(), array $headers = array(), $xpath = '//form', $extra_post = NULL) {
     $submit_matches = FALSE;
     $ajax = is_array($submit);
     if (isset($path)) {
@@ -1529,10 +1526,6 @@ protected function drupalPostForm($path, $edit, $submit, array $options = array(
     if ($this->parse()) {
       $edit_save = $edit;
       // Let's iterate over all the forms.
-      $xpath = "//form";
-      if (!empty($form_html_id)) {
-        $xpath .= "[@id='" . $form_html_id . "']";
-      }
       $forms = $this->xpath($xpath);
       foreach ($forms as $form) {
         // We try to set the fields of this form as specified in $edit.
@@ -1635,11 +1628,10 @@ protected function drupalPostForm($path, $edit, $submit, array $options = array(
    * @param $headers
    *   (optional) An array containing additional HTTP request headers, each
    *   formatted as "name: value". Forwarded to drupalPostForm().
-   * @param $form_html_id
-   *   (optional) HTML ID of the form to be submitted, use when there is more
-   *   than one identical form on the same page and the value of the triggering
-   *   element is not enough to identify the form. Note this is not the Drupal
-   *   ID of the form but rather the HTML ID of the form.
+   * @param $form_xpath
+   *   (optional) The xpath selector of the form to be submitted. On some pages
+   *   there are many identical forms, so just using the value of the submit
+   *   button is not enough. For example: 'trigger-node-presave-assign-form'.
    * @param $ajax_settings
    *   (optional) An array of Ajax settings which if specified will be used in
    *   place of the Ajax settings of the triggering element.
@@ -1651,7 +1643,7 @@ protected function drupalPostForm($path, $edit, $submit, array $options = array(
    * @see drupalProcessAjaxResponse()
    * @see ajax.js
    */
-  protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_path = NULL, array $options = array(), array $headers = array(), $form_html_id = NULL, $ajax_settings = NULL) {
+  protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_path = NULL, array $options = array(), array $headers = array(), $form_xpath = NULL, $ajax_settings = NULL) {
 
     // Get the content of the initial page prior to calling drupalPostForm(),
     // since drupalPostForm() replaces $this->content.
@@ -1679,8 +1671,8 @@ protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_p
       else {
         $xpath = '//*[@name="' . $triggering_element . '"]';
       }
-      if (isset($form_html_id)) {
-        $xpath = '//form[@id="' . $form_html_id . '"]' . $xpath;
+      if (isset($form_xpath)) {
+        $xpath = '//' . $form_xpath . $xpath;
       }
       $element = $this->xpath($xpath);
       $element_id = (string) $element[0]['id'];
@@ -1724,7 +1716,7 @@ protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_p
     $ajax_path = $this->container->get('unrouted_url_assembler')->assemble('base://' . $ajax_path, $options);
 
     // Submit the POST request.
-    $return = Json::decode($this->drupalPostForm(NULL, $edit, array('path' => $ajax_path, 'triggering_element' => $triggering_element), $options, $headers, $form_html_id, $extra_post));
+    $return = Json::decode($this->drupalPostForm(NULL, $edit, array('path' => $ajax_path, 'triggering_element' => $triggering_element), $options, $headers, $form_xpath, $extra_post));
 
     // Change the page content by applying the returned commands.
     if (!empty($ajax_settings) && !empty($return)) {
diff --git a/core/modules/system/src/Tests/Form/RebuildTest.php b/core/modules/system/src/Tests/Form/RebuildTest.php
index e993b49..272338b 100644
--- a/core/modules/system/src/Tests/Form/RebuildTest.php
+++ b/core/modules/system/src/Tests/Form/RebuildTest.php
@@ -96,7 +96,7 @@ function testPreserveFormActionAfterAJAX() {
     // submission and verify it worked by ensuring the updated page has two text
     // field items in the field for which we just added an item.
     $this->drupalGet('node/add/page');
-    $this->drupalPostAjaxForm(NULL, array(), array('field_ajax_test_add_more' => t('Add another item')), 'system/ajax', array(), array(), 'node-page-form');
+    $this->drupalPostAjaxForm(NULL, array(), array('field_ajax_test_add_more' => t('Add another item')), 'system/ajax', array(), array(), '/form[@data-drupal-selector=node-page-form]');
     $this->assert(count($this->xpath('//div[contains(@class, "field-name-field-ajax-test")]//input[@type="text"]')) == 2, 'AJAX submission succeeded.');
 
     // Submit the form with the non-Ajax "Save" button, leaving the title field