Safari fires an unload event when an iframe element is removed from the DOM; other browsers don't.
Created attachment 84524 [details]
It looks like Opera and IE do not fire unload, but WebKit and Gecko do. Noone appears to fire beforeunload (unless my test is borked). I'd be OK with making WebKit match IE if we dont' believe there to be a compatibility concern.
This bug may block the 53031 as the current work in progress patch there hits a crash on editing/style/iframe-onload-crash-mac.html