I've never used the onfilterchange event before so I don't know if there are any quirks to it. The 'el' var in that context is the actual original element, it's not a clone or anything. So the way you're hooking up the listener seems like it should work.

You might see if your changes to the filter trigger the onpropertychange event which is already hooked up. My understanding is that any change to a property of element.style, which includes filter, should trigger that. If that's the case then you should probably use that to avoid the cost of the extra event listener.