diff -u b/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
--- b/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -161,10 +161,12 @@
   }
 
   /**
-   * Adds attributes or merges them on to array of existing attributes.
+   * Adds attribute or merges values on to array of existing attribute values.
    *
-   * @param string|array ...
-   *   Attributes to add to the class attribute array.
+   * @param string $attribute
+   *   Name of the Attribute.
+   * @param string|array $value
+   *   Value(s) to add or merge to the given attribute.
    *
    * @return $this
    */