yo do realizes that some CSS will not work on Microsoft browsers, right?
That's a bald-faced lie. On the rare occasion tt may work differently but it still works.
I assume, from your description that you meant this (just to illustrate a little more clearly):
And your question is how to get the child to have a higher z-index than the parent?
The answer is the child, by default, will always have a higher z-index (and therefore be painted on top of) the parent (and yes, this is the situation in all browers). Perhaps you meant something else?