Value returned for missing DOMStringList properties

Hi,

DOMStringList is currently returning null when calling item() with 
properties that don't exist, and the empty string when getting a missing 
indexed property. Here's an example with a document that has less than 
100 style sheet sets:

document.styleSheetSets.item(100)
	-> null
document.styleSheetSets[100]
	-> ""

Apparently, the behavior when getting missing indexed properties isn't 
specified in the DOM Level 3 Core specification yet [1].
Do you think it would be better to return undefined for the 
document.styleSheetSets[100] case?
That would be more consistent with other array like dom properties:

document.body.childNodes.item(100)
	-> null
document.body.childNodes[100]
	-> undefined

(I'm asking because I'm currently extending nsStringArraySH for the 
DOMTokenList implementation so I'm getting the DOMStringList behavior, 
while the spec currently requires something more like what 
element.childNodes does [2]).

I don't think other browsers are implementing DOMStringList, so I 
couldn't compare what they are doing in that case.

Sylvain


[1] http://www.w3.org/TR/DOM-Level-3-Core/core.html#DOMStringList
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=501257#c6