diff -u b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
--- b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
@@ -326,20 +326,31 @@
    * @see testDescape()
    */
   public function providerDescape() {
-    return array(
-      array('Drupal', 'Drupal'),
-      array('<script>', '&lt;script&gt;'),
-      array('&lt;script&gt;', '&amp;lt;script&amp;gt;'),
-      array('&#34;', '&amp;#34;'),
-      array('"', '&quot;'),
-      array('&quot;', '&amp;quot;'),
-      array("'", '&#039;'),
-      array('&#039;', '&amp;#039;'),
-      array('©', '©'),
-      array('→', '→'),
-      array('➼', '➼'),
-      array('€', '€'),
-    );
+    return [
+      ['Drupal', 'Drupal'],
+      ['<script>', '&lt;script&gt;'],
+      ['&lt;script&gt;', '&amp;lt;script&amp;gt;'],
+      ['&#34;', '&amp;#34;'],
+      ['"', '&quot;'],
+      ['&quot;', '&amp;quot;'],
+      ["'", '&#039;'],
+      ['&#039;', '&amp;#039;'],
+      ['©', '©'],
+      ['→', '→'],
+      ['➼', '➼'],
+      ['€', '€'],
+    ];
+  }
+
+  /**
+   * Ensure Html::descape() is the opposite of Html::escape().
+   *
+   * @covers ::descape
+   * @covers ::escape
+   */
+  public function testEscapeAndDescape() {
+    $string = "<em>kittén&eacute;</em>";
+    $this->assertEquals($string, Html::descape(Html::escape($string)));
   }
 
 }