core/includes/common.inc                           | 12 ++-
 .../Core/Page/DefaultHtmlFragmentRenderer.php      | 10 +--
 .../Tests/CommentDefaultFormatterCacheTagsTest.php |  2 +-
 core/modules/system/src/Form/ThemeSettingsForm.php |  2 +-
 .../src/Tests/Cache/PageCacheTagsTestBase.php      |  2 +-
 core/tests/Drupal/Tests/Core/Cache/CacheTest.php   | 85 ++++++++++++++++++++++
 6 files changed, 97 insertions(+), 16 deletions(-)

diff --git a/core/includes/common.inc b/core/includes/common.inc
index 877f623..24ac99a 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -3115,16 +3115,14 @@ function drupal_render_cache_set(&$markup, array $elements) {
     $data['#post_render_cache'] = $elements['#post_render_cache'];
   }
 
-  // Tag every render cache item with the "rendered" cache tag. This allows us
-  // to invalidate the entire render cache, regardless of the cache bin.
-  $cache_tags = Cache::mergeTags($elements['#cache']['tags'] ?: [],  ['rendered']);
-
-  // Persist cache tags associated with this element.
-  $data['#cache']['tags'] = $cache_tags;
+  // Persist cache tags associated with this element. Also associate the
+  // "rendered" cache tag. This allows us to invalidate the entire render cache,
+  // regardless of the cache bin.
+  $data['#cache']['tags'] = Cache::mergeTags($elements['#cache']['tags'], ['rendered']);
 
   $bin = isset($elements['#cache']['bin']) ? $elements['#cache']['bin'] : 'render';
   $expire = isset($elements['#cache']['expire']) ? $elements['#cache']['expire'] : Cache::PERMANENT;
-  \Drupal::cache($bin)->set($cid, $data, $expire, $cache_tags);
+  \Drupal::cache($bin)->set($cid, $data, $expire, $data['#cache']['tags']);
 }
 
 /**
diff --git a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
index d7b5531..77bfc3c 100644
--- a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
+++ b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
@@ -69,12 +69,10 @@ public function render(HtmlFragmentInterface $fragment, $status_code = 200) {
     }
 
     if ($fragment instanceof CacheableInterface) {
-      // Collect cache tags for all the content in all the regions on the page.
-      $tags = $page_array['#cache']['tags'];
-      // Tag every render cache item with the "rendered" cache tag. This allows us
-      // to invalidate the entire render cache, regardless of the cache bin.
-      $tags = Cache::mergeTags($tags, ['rendered']);
-      $page->setCacheTags($tags);
+      // Persist cache tags associated with this page. Also associate the
+      // "rendered" cache tag. This allows us to invalidate the entire render
+      // cache, regardless of the cache bin.
+      $page->setCacheTags(Cache::mergeTags($page_array['#cache']['tags'], ['rendered']));
     }
 
     return $page;
diff --git a/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php b/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
index bc4466d..9c14685 100644
--- a/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
+++ b/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
@@ -111,7 +111,7 @@ public function testCacheTags() {
       'user:2',
     );
     sort($expected_cache_tags);
-    $this->assertEqual($build['#cache']['tags'], $expected_cache_tags); //, 'The test entity has the expected cache tags when it has comments.');
+    $this->assertEqual($build['#cache']['tags'], $expected_cache_tags, 'The test entity has the expected cache tags when it has comments.');
   }
 
 }
diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index 7859138..fb4df2a 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -426,7 +426,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
     // Invalidate either the theme-specific cache tag or the global theme
     // settings cache tag, depending on whose settings were actually changed.
     if (isset($values['theme'])) {
-      Cache::invalidateTags(array('theme:'));
+      Cache::invalidateTags(array('theme:' . $values['theme']));
     }
     else {
       Cache::invalidateTags(array('theme_global_settings'));
diff --git a/core/modules/system/src/Tests/Cache/PageCacheTagsTestBase.php b/core/modules/system/src/Tests/Cache/PageCacheTagsTestBase.php
index de97aab..f3a0d76 100644
--- a/core/modules/system/src/Tests/Cache/PageCacheTagsTestBase.php
+++ b/core/modules/system/src/Tests/Cache/PageCacheTagsTestBase.php
@@ -14,7 +14,7 @@
  * Provides helper methods for page cache tags tests.
  */
 abstract class PageCacheTagsTestBase extends WebTestBase {
-protected $dumpHeaders = TRUE;
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/tests/Drupal/Tests/Core/Cache/CacheTest.php b/core/tests/Drupal/Tests/Core/Cache/CacheTest.php
new file mode 100644
index 0000000..612899d
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Cache/CacheTest.php
@@ -0,0 +1,85 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Cache\CacheTest.
+ */
+
+namespace Drupal\Tests\Core\Cache;
+
+use Drupal\Core\Cache\Cache;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Cache\Cache
+ * @group Cache
+ */
+class CacheTest extends UnitTestCase {
+
+  /**
+   * Provides a list of cache tags arrays.
+   *
+   * @return array
+   */
+  public function validateTagsProvider() {
+    return [
+      [[], FALSE],
+      [['foo'], FALSE],
+      [['foo', 'bar'], FALSE],
+      [['foo', 'bar', 'llama:2001988', 'baz', 'llama:14031991'], FALSE],
+      [[FALSE], 'Cache tags must be strings, boolean given.'],
+      [[TRUE], 'Cache tags must be strings, boolean given.'],
+      [['foo', FALSE], 'Cache tags must be strings, boolean given.'],
+      [[NULL], 'Cache tags must be strings, NULL given.'],
+      [['foo', NULL], 'Cache tags must be strings, NULL given.'],
+      [[1337], 'Cache tags must be strings, integer given.'],
+      [['foo', 1337], 'Cache tags must be strings, integer given.'],
+      [[3.14], 'Cache tags must be strings, double given.'],
+      [['foo', 3.14], 'Cache tags must be strings, double given.'],
+      [[[]], 'Cache tags must be strings, array given.'],
+      [['foo', []], 'Cache tags must be strings, array given.'],
+      [['foo', ['bar']], 'Cache tags must be strings, array given.'],
+      [[new \stdClass()], 'Cache tags must be strings, object given.'],
+      [['foo', new \stdClass()], 'Cache tags must be strings, object given.'],
+    ];
+  }
+
+  /**
+   * @covers validateTags
+   *
+   * @dataProvider validateTagsProvider
+   */
+  public function testValidateTags(array $tags, $expected_exception_message) {
+    if ($expected_exception_message !== FALSE) {
+      $this->setExpectedException('LogicException', $expected_exception_message);
+    }
+    Cache::validateTags($tags);
+  }
+
+  /**
+   * Provides a list of pairs of cache tags arrays to be merged.
+   *
+   * @return array
+   */
+  public function mergeTagsProvider() {
+    return [
+      [[], [], []],
+      [['bar'], ['foo'], ['bar', 'foo']],
+      [['foo'], ['bar'], ['bar', 'foo']],
+      [['foo'], ['bar', 'foo'], ['bar', 'foo']],
+      [['foo'], ['foo', 'bar'], ['bar', 'foo']],
+      [['bar', 'foo'], ['foo', 'bar'], ['bar', 'foo']],
+      [['foo', 'bar'], ['foo', 'bar'], ['bar', 'foo']],
+    ];
+  }
+
+  /**
+   * @covers mergeTags
+   *
+   * @dataProvider mergeTagsProvider
+   */
+  public function testMergeTags(array $a, array $b, array $expected) {
+    $this->assertEquals($expected, Cache::mergeTags($a, $b));
+  }
+
+}