diff --git a/includes/form.inc b/includes/form.inc
index a337b03..ae0b3d3 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -129,7 +129,11 @@
  *   $form_state['clicked_button']. It is the element that caused submission,
  *   which may or may not be a button (in the case of Ajax forms.) This is
  *   often used to distinguish between various buttons in a submit handler,
- *   and is also used in Ajax handlers.
+ *   and is also used in Ajax handlers. Note that the value of the triggering
+ *   element after a user pressed the enter key is browser specific, as the HTML
+ *   does not specify clear enough which element should be selected as the
+ *   triggering element.
+ *   @link http://dev.w3.org/html5/spec/association-of-controls-and-forms.html#implicit-submission @endlink
  * - 'cache': The typical form workflow involves two page requests. During the
  *   first page request, a form is built and returned for the user to fill in.
  *   Then the user fills the form in and submits it, triggering a second page