diff -u b/core/modules/outside_in/tests/src/FunctionalJavascript/OutsideInBlockFormTest.php b/core/modules/outside_in/tests/src/FunctionalJavascript/OutsideInBlockFormTest.php
--- b/core/modules/outside_in/tests/src/FunctionalJavascript/OutsideInBlockFormTest.php
+++ b/core/modules/outside_in/tests/src/FunctionalJavascript/OutsideInBlockFormTest.php
@@ -90,8 +90,9 @@
 
     if (isset($new_page_text)) {
       $page->pressButton($button_text);
-      // Make sure the changes are present.
-      $this->assertJsCondition('jQuery("' . $block_selector . ' ' . $label_selector . '").html() == "' . $new_page_text . '"');
+      // Make sure the changes are present. Use this method to give the browser
+      // some time to reload the page.
+      $this->assertElementHtmlEquals($block_selector . ' ' . $label_selector, $new_page_text);
     }
 
     $this->openBlockForm($block_selector);
only in patch2:
unchanged:
--- a/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/JavascriptTestBase.php
@@ -94,6 +94,36 @@ protected function assertElementNotVisible($css_selector, $message = '') {
   }
 
   /**
+   * Asserts that an element with the given css selector contains the given html.
+   *
+   * The assertion will wait the default timeout for the element to contain the
+   * given HTML.
+   *
+   * @param string $css_selector
+   *   The CSS selector identifying the element to check.
+   * @param string $html
+   *   The expected HTML.
+   */
+  protected function assertElementHtmlEquals($css_selector, $html) {
+    $this->assertJsCondition('jQuery("' . $css_selector . '").html() == "' . $html . '"');
+  }
+
+  /**
+   * Asserts that an element with the given css selector equals the given html.
+   *
+   * The assertion will wait the default timeout for the element to not contain
+   * the given HTML.
+   *
+   * @param string $css_selector
+   *   The CSS selector identifying the element to check.
+   * @param string $html
+   *   The expected HTML.
+   */
+  protected function assertElementHtmlNotEquals($css_selector, $html) {
+    $this->assertJsCondition('jQuery("' . $css_selector . '").html() != "' . $html . '"');
+  }
+
+  /**
    * Waits for the given time or until the given JS condition becomes TRUE.
    *
    * @param string $condition