.../src/Unit/Plugin/DisplayVariant/BlockPageVariantTest.php      | 9 +++++++--
 core/modules/comment/src/Tests/CommentTestBase.php               | 9 +--------
 core/modules/system/templates/container--main-content.html.twig  | 2 +-
 3 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/core/modules/block/tests/src/Unit/Plugin/DisplayVariant/BlockPageVariantTest.php b/core/modules/block/tests/src/Unit/Plugin/DisplayVariant/BlockPageVariantTest.php
index 22f3a3e..d83c2ca 100644
--- a/core/modules/block/tests/src/Unit/Plugin/DisplayVariant/BlockPageVariantTest.php
+++ b/core/modules/block/tests/src/Unit/Plugin/DisplayVariant/BlockPageVariantTest.php
@@ -186,7 +186,10 @@ public function providerBuild() {
         // The main content, messages and title are rendered via the fallback in case
         // there are no blocks rendering them.
         'content' => [
-          'system_main' => ['#markup' => 'Hello kittens!'],
+          'system_main' => [
+            '#markup' => 'Hello kittens!',
+            '#theme_wrappers' => ['container__main_content'],
+          ],
           'messages' => [
             '#weight' => -1000,
             '#type' => 'status_messages',
@@ -264,7 +267,9 @@ public function testBuildWithoutMainContent() {
         'max-age' => -1,
       ],
       'content' => [
-        'system_main' => [],
+        'system_main' => [
+          '#theme_wrappers' => ['container__main_content'],
+        ],
         'messages' => [
           '#weight' => -1000,
           '#type' => 'status_messages',
diff --git a/core/modules/comment/src/Tests/CommentTestBase.php b/core/modules/comment/src/Tests/CommentTestBase.php
index 12ea894..f78a8bd 100644
--- a/core/modules/comment/src/Tests/CommentTestBase.php
+++ b/core/modules/comment/src/Tests/CommentTestBase.php
@@ -188,14 +188,7 @@ public function postComment($entity, $comment, $subject = '', $contact = NULL, $
    */
   function commentExists(CommentInterface $comment = NULL, $reply = FALSE) {
     if ($comment) {
-      $regex = '!' . ($reply ? '<div class="indented">(.*?)' : '');
-      $regex .= '<a id="comment-' . $comment->id() . '"(.*?)';
-      $regex .= $comment->getSubject() . '(.*?)';
-      $regex .= $comment->comment_body->value . '(.*?)';
-      $regex .= ($reply ? '</article>\s</div>(.*?)' : '');
-      $regex .= '!s';
-
-      return (boolean) preg_match($regex, $this->getRawContent());
+      return (bool) $this->cssSelect('.comment-wrapper ' . ($reply ? '.indented ' : '') . '#comment-' . $comment->id());
     }
     else {
       return FALSE;
diff --git a/core/modules/system/templates/container--main-content.html.twig b/core/modules/system/templates/container--main-content.html.twig
index 9db3e4f..ad841a9 100644
--- a/core/modules/system/templates/container--main-content.html.twig
+++ b/core/modules/system/templates/container--main-content.html.twig
@@ -9,4 +9,4 @@
  * @ingroup themeable
  */
 #}
-<div {{ attributes }}>{{ title_suffix }}{{ children }}</div>
+<div{{ attributes }}>{{ title_suffix }}{{ children }}</div>