[cssom] Question about multiple key pair values

A small question : how will the "ideal" vNext CSS Object Model handle the case where a property is defined two or more time in the same rule? 

This happens often when it's needed to maintain compatibility with browsers which support a property but not all its possible values.

Dealing with this case seems not so easy at first glance and, while support for it seems needed for editing software, I'm wondering whether general WebPages would benefit from a too complete OM or if the memory overhead would be prohibitive. I'm also wondering if it wouldn't be better to have specific programs that need granular control over stylesheets build their own modules for it.

Please note that it doesn't mean we should not continue to work on CSS OM since the current model is broken in many ways and some proposals I've seen seemed interesting.

François