As mentioned on the call, here's is what the DOM v3 spec says about the
Document.getElementById method[1]:
"Returns the Element that has an ID attribute with the given value. If
no such element exists, this returns null. If more than one element has
an ID attribute with that value, what is returned is undefined."
So, it's pretty clear they explicitly thought about the case of when ID
values were non-unique and then punted on it.
[1]
http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/core.html#ID-getElBId
1 message in org.oasis-open.lists.security-services[security-services] DOM and XML IDs