The attached test case renders incorrectly on Mac. The parent content is rotated using rotate3d, giving it a compositing layer. The content of the iframe has '-webkit-transform: rotateX(0)' which is a nop, but which also gives it a compositing layer. You see two copies of the iframe content, one correctly rotated with the parent content and one unrotated on top of it.
Created attachment 53848 [details] iframe content for test case (put in same dir as test case)
Created attachment 53849 [details] Test case
This works fine now.