It should have as it is not dependent on the browser, the CSS is hiding the frame until loaded.
I think the problem here is that only page one is loaded, so your script will work and only reveal the iframe when it's loaded, but after this the button to "Change Iframe" loads a new iframe so as the browser already thinks the page as complete so your code no longer works ???

I see the Flash on previewing in Xara. I know the browser iXara uses for previews is IE but don't know what version. Perhaps Rob could enlighten us.

I also see the flash in ie11 & occasionally in FF but not Chrome or Opera.

It's a strange way to load iframes, don't know why it's being done this way.