When a website uses iframe ad tags, an iframe is created around the ad. For example, if www.mysite.com has an iframe ad tag that displays an ad from ad.doubleclick.net, the ad is restricted from accessing anything outside the enclosing iframe. This restriction prevents interstitial and expanding creatives from being served through iframe ad tags. However, there are cases, as with ads served by third parties, where publishers want to allow communication from the iframe environment to domains that they trust. - Google
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