diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 75107e1..f6de366 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -336,15 +336,11 @@ protected function parseLibraryInfo($extension, $path) {
   protected function applyLibrariesOverride($libraries, $extension) {
     $active_theme = $this->themeManager->getActiveTheme();
     $libraries_overrides = $active_theme->getLibrariesOverride();
-    $valid_libraries = [];
     foreach ($libraries as $name => $library) {
       // Process libraries overrides.
       foreach ($libraries_overrides as $asset => $override) {
         // Active theme defines an override for this library.
         if ($asset === "$extension/$name") {
-          // Keep track of libraries-overrides that are valid.
-          $valid_libraries[] = $asset;
-
           // The active theme defines an override for the whole library. Use the
           // override key to specify that this library will be overridden when
           // it is called.
@@ -382,9 +378,6 @@ protected function applyLibrariesOverride($libraries, $extension) {
           $key_exists = NULL;
           $attributes = NestedArray::getValue($libraries[$name], $parents, $key_exists);
           if ($key_exists) {
-            // Keep track of libraries-overrides that are valid.
-            $valid_libraries[] = $asset;
-
             // Remove asset to be overridden, but keep its attributes.
             NestedArray::unsetValue($libraries[$name], $parents);
             if ($override) {
@@ -395,11 +388,6 @@ protected function applyLibrariesOverride($libraries, $extension) {
         }
       }
     }
-    // Remaining libraries-override definitions not in $valid_libraries are
-    // likely invalid. Throw an exception.
-    if ($invalid_libraries = array_diff(array_keys($libraries_overrides), $valid_libraries)) {
-      throw new InvalidLibrariesOverrideSpecificationException(sprintf('Overridden library asset(s) "%s" could not be found.', implode(', ', $invalid_libraries)));
-    }
 
     return $libraries;
   }
diff --git a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
index 9ef0fc6..7d7c7b9 100644
--- a/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryDiscoveryIntegrationTest.php
@@ -29,7 +29,7 @@ protected function setUp() {
   /**
    * Ensures that the element info can be altered by themes.
    */
-  public function ptestElementInfoByTheme() {
+  public function testElementInfoByTheme() {
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
     $theme_initializer = $this->container->get('theme.initialization');
 
@@ -46,7 +46,7 @@ public function ptestElementInfoByTheme() {
   /**
    * Tests that libraries-override are applied to library definitions.
    */
-  public function ptestLibrariesOverride() {
+  public function testLibrariesOverride() {
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
     $theme_initializer = $this->container->get('theme.initialization');
 
@@ -85,7 +85,7 @@ public function ptestLibrariesOverride() {
   /**
    * Tests libraries-override on drupalSettings.
    */
-  public function ptestLibrariesOverrideDrupalSettings() {
+  public function testLibrariesOverrideDrupalSettings() {
     $this->container->get('theme_handler')->install(['test_theme_libraries_override_with_drupal_settings']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -113,7 +113,7 @@ public function ptestLibrariesOverrideDrupalSettings() {
   /**
    * Tests libraries-override on malformed assets.
    */
-  public function ptestLibrariesOverrideMalformedAsset() {
+  public function testLibrariesOverrideMalformedAsset() {
     $this->container->get('theme_handler')->install(['test_theme_libraries_override_with_invalid_asset']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -141,7 +141,7 @@ public function ptestLibrariesOverrideMalformedAsset() {
   /**
    * Tests library assets with other ways for specifying paths.
    */
-  public function ptestLibrariesOverrideOtherAssetLibraryNames() {
+  public function testLibrariesOverrideOtherAssetLibraryNames() {
     $this->container->get('theme_handler')->install(['test_theme']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -175,7 +175,7 @@ public function ptestLibrariesOverrideOtherAssetLibraryNames() {
   /**
    * Tests that base theme libraries-override still apply in sub themes.
    */
-  public function ptestBaseThemeLibrariesOverrideInSubTheme() {
+  public function testBaseThemeLibrariesOverrideInSubTheme() {
     $this->container->get('theme_handler')->install(['test_subtheme']);
 
     /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
@@ -199,34 +199,6 @@ public function ptestBaseThemeLibrariesOverrideInSubTheme() {
   }
 
   /**
-   * Tests libraries-override on non-existent assets specifications on LHS.
-   */
-  public function testLibrariesOverrideNonExistentLhs() {
-    $this->container->get('theme_handler')->install(['test_theme_libraries_override_non_existing']);
-
-    /** @var \Drupal\Core\Theme\ThemeInitializationInterface $theme_initializer */
-    $theme_initializer = $this->container->get('theme.initialization');
-
-    /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
-    $theme_manager = $this->container->get('theme.manager');
-
-    /** @var \Drupal\Core\Render\ElementInfoManagerInterface $element_info */
-    $library_discovery = $this->container->get('library.discovery');
-
-    $theme_manager->setActiveTheme($theme_initializer->getActiveThemeByName('test_theme_libraries_override_non_existing'));
-
-    // Assert that improperly formed asset "specs" throw an exception.
-    try {
-      $library_discovery->getLibraryByName('core', 'drupal.dialog');
-      $this->fail('Throw Exception when specifying non-existent override');
-    }
-    catch (InvalidLibrariesOverrideSpecificationException $e) {
-      $expected_message = 'Overridden library asset(s) "core/drupal.dialogs, core/drupal.dropbutton/css/components/misc/dropbutton/dropbutton.css" could not be found.';
-      $this->assertEqual($e->getMessage(), $expected_message, 'Throw Exception when specifying non-existent override');
-    }
-  }
-
-  /**
    * Asserts that the given asset is in the specified library.
    *
    * @param mixed $library_item
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_non_existing/test_theme_libraries_override_non_existing.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_non_existing/test_theme_libraries_override_non_existing.info.yml
deleted file mode 100644
index a86d3bf..0000000
--- a/core/modules/system/tests/themes/test_theme_libraries_override_non_existing/test_theme_libraries_override_non_existing.info.yml
+++ /dev/null
@@ -1,14 +0,0 @@
-name: 'Test theme libraries-override'
-type: theme
-description: 'Theme with non-existent libraries-override asset spec.'
-version: VERSION
-base theme: classy
-core: 8.x
-libraries-override:
-  # A non-existent library asset specified for overriding. Should throw a
-  # \Drupal\Core\Asset\Exception\InvalidLibrariesOverrideSpecificationException.
-  # Left-hand side typos.
-  core/drupal.dialogs: false
-  core/drupal.dropbutton/css/components/misc/dropbutton/dropbutton.css: /themes/my_theme/css/dropbutton.css
-  # Right-hand side non-existing library.
-  core/drupal.collapse: test_theme/collapses