Why doesn't animation work in IE and FF?

Question

css

Animation - background change. Works in chrome and opera, not in IE and FF.
codepen.io/anon/pen/xkvby

Answer 1

Because it shouldn't work - the background-image description clearly says Animatable: no . And there is simply no such property in the Animatable properties list.
I understand that no one needs specifications now, but it is with them that you need to start looking for a problem.
Make several layers (for 3 images, the element itself and 2 pseudo-elements are enough), give them a background and control the opacity for them.

Answer 2

At first - an example and does not work in fokse.
Secondly - background-size - the values ​​are written in a row, not separated by commas. Perhaps this is the problem.