-                                                                               ->isRequired()
-                                                                               ->treatNullLike(array())
-                                                                               ->scalarPrototype()->end()
-                                                                               ->defaultValue($defaults['filter']['css'])
+                                                                               #XXX: undocumented, see Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php +513
+                                                                               ->addDefaultChildrenIfNoneSet()
+                                                                               ->arrayPrototype()
+                                                                                       ->children()
+                                                                                               ->scalarNode('class')
+                                                                                                       ->isRequired()
+                                                                                                       ->defaultValue($defaults['filters']['css']['class'])
+                                                                                               ->end()
+                                                                                               ->arrayNode('args')
+                                                                                                       ->isRequired()
+                                                                                                       ->treatNullLike(array())
+                                                                                                       ->defaultValue($defaults['filters']['css']['args'])
+                                                                                                       ->scalarPrototype()->end()
+                                                                                               ->end()
+                                                                                       ->end()
+                                                                               ->end()