Passing JavaScript Function object to EventTarget.addEventListener()

[1] specifies that the second argument taken by addEventListener to be 
an EventListener instance. However, Mozilla accepts plain Function 
objects just fine.

Could anyone provide any pointers why this works (is this a DOM feature 
or an extension), and in which module this is implemented (dom, content, 
or xpconnect maybe)?

1. http://www.w3.org/TR/DOM-Level-2-Events/events.html
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Thanks in advance,
Sergey Yanovich