diff --git a/core/lib/Drupal/Core/Asset/Exception/IncompleteLibraryDefinitionException.php b/core/lib/Drupal/Core/Asset/Exception/IncompleteLibraryDefinitionException.php
index 46a9d9e..2ef4c98 100644
--- a/core/lib/Drupal/Core/Asset/Exception/IncompleteLibraryDefinitionException.php
+++ b/core/lib/Drupal/Core/Asset/Exception/IncompleteLibraryDefinitionException.php
@@ -7,6 +7,9 @@
 
 namespace Drupal\Core\Asset\Exception;
 
-class IncompleteLibraryDefinitionException {
+/**
+ * Defines a custom exception if a library has no js/css/setting specified.
+ */
+class IncompleteLibraryDefinitionException extends \RuntimeException {
 
 }
diff --git a/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryFileException.php b/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryFileException.php
index e22f361..33cf830 100644
--- a/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryFileException.php
+++ b/core/lib/Drupal/Core/Asset/Exception/InvalidLibraryFileException.php
@@ -7,6 +7,9 @@
 
 namespace Drupal\Core\Asset\Exception;
 
-class InvalidLibraryFileException {
+/**
+ * Defines an exception if the library file could not be parsed.
+ */
+class InvalidLibraryFileException extends \RunTimeException {
 
 }
diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index d53f630..b68f348 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Core\Asset;
 
+use Drupal\Core\Asset\Exception\IncompleteLibraryDefinitionException;
+use Drupal\Core\Asset\Exception\InvalidLibraryFileException;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -72,6 +74,12 @@ public function getLibraryByName($extension, $name) {
     return isset($this->libraries[$extension][$name]) ? $this->libraries[$extension][$name] : FALSE;
   }
 
+  /**
+   * Ensures that the libraries property is filled.
+   *
+   * @param string $extension
+   *   The name of the extension that registered a library.
+   */
   protected function ensureLibraryInformation($extension) {
     $this->retrieveFromCache($extension);
     if (!isset($this->libraries[$extension])) {
@@ -84,6 +92,12 @@ protected function ensureLibraryInformation($extension) {
     }
   }
 
+  /**
+   * Fills up the libraries property from cache, if available.
+   *
+   * @param string $extension
+   *   The name of the extension that registered a library.
+   */
   protected function retrieveFromCache($extension) {
     if (!isset($this->libraries[$extension])) {
       if ($cache = $this->cache->get('library:info:' . $extension)) {
@@ -92,6 +106,15 @@ protected function retrieveFromCache($extension) {
     }
   }
 
+  /**
+   * Sets the library information into a cache entry.
+   *
+   * @param string $extension
+   *   The name of the extension that registered a library.
+   *
+   * @param array $information
+   *   All library definitions of the passed extension.
+   */
   protected function setCache($extension, array $information) {
     $this->cache->set('library:info:' . $extension, $information, Cache::PERMANENT, array(
       'extension' => array(TRUE, $extension),
@@ -99,6 +122,20 @@ protected function setCache($extension, array $information) {
     ));
   }
 
+  /**
+   * Parses and builds up all the libraries information of an extension.
+   *
+   * @param string $extension
+   *   The name of the extension that registered a library.
+   *
+   * @return array
+   *   All library definitions of the passed extension.
+   *
+   * @throws \Drupal\Core\Asset\Exception\IncompleteLibraryDefinitionException
+   *   Thrown when a library has no js/css/setting.
+   * @throws \UnexpectedValueException
+   *   Thrown when a js file defines a positive weight.
+   */
   protected function buildLibrariesByExtension($extension) {
     $this->libraries[$extension] = array();
     if ($extension === 'core') {
@@ -123,7 +160,7 @@ protected function buildLibrariesByExtension($extension) {
 
     foreach ($this->libraries[$extension] as $id => &$library) {
       if (!isset($library['js']) && !isset($library['css']) && !isset($library['settings'])) {
-        throw new \RuntimeException(sprintf("Incomplete library definition for '%s' in %s", $id, $library_file));
+        throw new IncompleteLibraryDefinitionException(sprintf("Incomplete library definition for '%s' in %s", $id, $library_file));
       }
       $library += array('dependencies' => array(), 'js' => array(), 'css' => array());
 
@@ -244,18 +281,24 @@ protected function drupalGetPath($type, $name) {
   }
 
   /**
-   * @param $source
-   * @return bool
+   * Wraps file_valid_uri().
    */
   protected function fileValidUri($source) {
     return file_valid_uri($source);
   }
 
   /**
-   * @param $extension
-   * @param $library_file
-   * @return mixed
-   * @throws \RuntimeException
+   * Parses a given library file and allows module to alter it.
+   *
+   * This method sets the parsed information onto the library property.
+   *
+   * @param string $extension
+   *   The name of the extension that registered a library.
+   * @param string $library_file
+   *   The relative filename to the DRUPAL_ROOT of the wanted library file.
+   *
+   * @throws \Drupal\Core\Asset\Exception\InvalidLibraryFileException
+   *   Thrown when a parser exception got thrown.
    */
   protected function parseLibraryInfo($extension, $library_file) {
     $parser = new Parser();
@@ -264,7 +307,7 @@ protected function parseLibraryInfo($extension, $library_file) {
     }
     catch (ParseException $e) {
       // Rethrow a more helpful exception, since ParseException lacks context.
-      throw new \RuntimeException(sprintf('Invalid library definition in %s: %s', $library_file, $e->getMessage()), 0, $e);
+      throw new InvalidLibraryFileException(sprintf('Invalid library definition in %s: %s', $library_file, $e->getMessage()), 0, $e);
     }
     // Allow modules to alter the module's registered libraries.
     $this->moduleHandler->alter('library_info', $this->libraries[$extension], $extension);
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
index cc67cfc..3dca871 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryTest.php
@@ -169,7 +169,7 @@ public function testGetLibraryWithMissingLibraryFile() {
   /**
    * Tests that an exception is thrown when a libraries file couldn't be parsed.
    *
-   * @expectedException \RuntimeException
+   * @expectedException \Drupal\Core\Asset\Exception\InvalidLibraryFileException
    */
   public function testInvalidLibrariesFile() {
     $this->moduleHandler->expects($this->atLeastOnce())
@@ -187,7 +187,7 @@ public function testInvalidLibrariesFile() {
   /**
    * Tests that an exception is thrown when no CSS/JS/setting is specified.
    *
-   * @expectedException \RuntimeException
+   * @expectedException \Drupal\Core\Asset\Exception\IncompleteLibraryDefinitionException
    * @expectedExceptionMessage Incomplete library definition for 'example' in core/tests/Drupal/Tests/Core/Asset/library_test_files/example_module_missing_information.libraries.yml
    */
   public function testGetLibraryWithMissingInformation() {
@@ -383,6 +383,9 @@ public function testExternalCache() {
 
 }
 
+/**
+ * Wraps the tested class to mock the external dependencies.
+ */
 class TestLibraryDiscovery extends LibraryDiscovery {
 
   protected $paths;
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/invalid_file.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/invalid_file.libraries.yml
index cb20993..b3cebbb 100644
--- a/core/tests/Drupal/Tests/Core/Asset/library_test_files/invalid_file.libraries.yml
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/invalid_file.libraries.yml
@@ -1,2 +1,2 @@
-example:
+	example:
   -=--##;'~ test