The spec current has this:

iframe:fullscreen {
  border:none;
}

Until recently, Blink had this:

iframe:-webkit-full-screen {
    border: none !important;
    padding: 0 !important;
}

Removing them found two sites assuming that any border/padding would be ignored in fullscreen:
https://code.google.com/p/chromium/issues/detail?id=468292#c4 (https://play.google.com/store/movies?hl=en)
https://code.google.com/p/chromium/issues/detail?id=469133#c5 (http://html5-demos.appspot.com/static/fullscreen.html)

Because the border rule is not !important and there is no padding rule, any site that has some border or padding on iframes must take care to remove them in fullscreen.

No border and padding will likely be the right thing more often than not, so I would like for the spec to align with Blink.