how to display iframe's div over another iframe

i have 2 iFrames namely iframe1 and iframe2 and my iframe1 contains a div.

In some instances, iframe1's div goes behind the开发者_如何学运维 iframe2, but i want that my iframe1's div display over iframe2.

I'm using z-index in css to display div over ifarme2 but no effect.

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Add a javascript function that changes z-Indices of the divs in question at whatever "instance" you want the behavior changed.