diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index b4c716d508..91f702ffac 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -129,13 +129,17 @@ public function buildByExtension($extension) {
         //   properly resolve dependencies for all (css) libraries per category,
         //   and only once prior to rendering out an HTML page.
         if ($type == 'css' && !empty($library[$type])) {
+          assert('\Drupal\Core\Asset\LibraryDiscoveryParser::validateCssLibrary($library[$type]) < 2', 'CSS files should be specified as key/value pairs, where the values are configuration options. See https://www.drupal.org/node/2274843.');
+          assert('\Drupal\Core\Asset\LibraryDiscoveryParser::validateCssLibrary($library[$type]) === 0', 'CSS must be nested under a category. See https://www.drupal.org/node/2274843.');
           foreach ($library[$type] as $category => $files) {
+            $category_weight = 'CSS_' . strtoupper($category);
+            assert('defined($category_weight)', 'Invalid CSS category: ' . $category . '. See https://www.drupal.org/node/2274843.');
             foreach ($files as $source => $options) {
               if (!isset($options['weight'])) {
                 $options['weight'] = 0;
               }
               // Apply the corresponding weight defined by CSS_* constants.
-              $options['weight'] += constant('CSS_' . strtoupper($category));
+              $options['weight'] += constant($category_weight);
               $library[$type][$source] = $options;
             }
             unset($library[$type][$category]);
@@ -460,4 +464,34 @@ protected function resolveThemeAssetPath($theme_path, $overriding_asset) {
     return $overriding_asset;
   }
 
+  /**
+   * Validates CSS library structure.
+   *
+   * @param array $library
+   *   The library definition array.
+   *
+   * @return int
+   *   Returns based on validity:
+   *     - 0 if the library definition is valid
+   *     - 1 if the library definition has improper nesting
+   *     - 2 if the library definition specifies files as an array
+   */
+  public static function validateCssLibrary($library) {
+    $categories = [];
+    // Verify options first and return early if invalid.
+    foreach ($library as $category => $files) {
+      if (!is_array($files)) {
+        return 2;
+      }
+      $categories[] = $category;
+      foreach ($files as $source => $options) {
+        if (!is_array($options)) {
+          return 1;
+        }
+      }
+    }
+
+    return 0;
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index f65d93233c..cbb06668c0 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -502,6 +502,47 @@ public function testLibraryWithLicenses() {
     $this->assertEquals($library['license'], $expected_license);
   }
 
+  /**
+   * Verifies assertions catch invalid CSS declarations.
+   *
+   * @dataProvider providerTestCssAssert
+   */
+
+  /**
+   * Verify an assertion fails if CSS declarations have non-existent categories.
+   *
+   * @param string $extension
+   *   The css extension to build.
+   * @param string $exception_message
+   *   The expected exception message.
+   *
+   * @dataProvider providerTestCssAssert
+   */
+  public function testCssAssert($extension, $exception_message) {
+    $this->moduleHandler->expects($this->atLeastOnce())
+      ->method('moduleExists')
+      ->with($extension)
+      ->will($this->returnValue(TRUE));
+
+    $path = __DIR__ . '/library_test_files';
+    $path = substr($path, strlen($this->root) + 1);
+    $this->libraryDiscoveryParser->setPaths('module', $extension, $path);
+
+    $this->setExpectedException(\AssertionError::class, $exception_message);
+    $this->libraryDiscoveryParser->buildByExtension($extension);
+  }
+
+  /**
+   * Data provider for testing bad CSS declarations.
+   */
+  public function providerTestCssAssert() {
+    return [
+      'css_bad_category' => ['css_bad_category', 'See https://www.drupal.org/node/2274843.'],
+      'Improper CSS nesting' => ['css_bad_nesting', 'CSS must be nested under a category. See https://www.drupal.org/node/2274843.'],
+      'Improper CSS nesting array' => ['css_bad_nesting_array', 'CSS files should be specified as key/value pairs, where the values are configuration options. See https://www.drupal.org/node/2274843.'],
+    ];
+  }
+
 }
 
 /**
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
new file mode 100644
index 0000000000..803efc3908
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
@@ -0,0 +1,5 @@
+bad_category:
+  css:
+    # Non-existent category.
+    bad_category:
+      css/styles.css: { minified: true }
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting.libraries.yml
new file mode 100644
index 0000000000..44c9c0e228
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting.libraries.yml
@@ -0,0 +1,4 @@
+bad_nesting:
+  css:
+    # No nesting here will break.
+    css/styles.css: { minified: true }
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting_array.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting_array.libraries.yml
new file mode 100644
index 0000000000..3c59bc7479
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting_array.libraries.yml
@@ -0,0 +1,4 @@
+bad_nesting_array:
+  css:
+    # Specified as an array will break.
+    - css/styles.css
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
index 9a8b4ddf28..169d2f4bbe 100644
--- a/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
@@ -1,6 +1,7 @@
 example:
   css:
-    css/example.js: {}
+    theme:
+      css/example.css: {}
   dependencies:
     - external/example_external
     - example_module/example