How to transition z-index?

Question

https://jsfiddle.net/vaf6nv36/1/

Can the balloons image slowly transition over the apple image?

I think that I need more transition parameters, or I should use

Answer 1

Although theoretically you can transition z-index, it wouldn't make much sense, i.e. would not result in the crossfade effect which you obviously are looking for: z-index values are integers, which - when you change them in the smallest possible steps (integers, no commas) - results in states either before the other one OR behind the other one - no transitional "half states" in between. If you want to do a kind of continuous crossfade between two elements, you should use a transition on opacity.

In your particular case, since your DIVs are not directly above each other, but only overlap each other, you can solve that by having a second DIV identical to img2 (I called its class .img3), but with z-index: 0 and this CSS rule:

.img1:hover + .img2 {
  opacity: 0;
}

This will fade out img2, but still show img3, which however is behind img1, creating the impression of a transition between img1 and img2.

https://jsfiddle.net/2a2epLfv/1/

.img1,
.img2,
.img3 {
  border: 1px solid black;
  transition: 1s;
  position: absolute;
}

.img1 {
  left: 20%;
  height: 300px;
  width: 300px;
  z-index: 1;
  background-image: url(http://cdn.pcwallart.com/images/balloons-photography-vintage-wallpaper-1.jpg);
}

.img2,
.img3 {
  right: 20%;
  width: 300px;
  height: 300px;
  top: 100px;
  background-image: url(https://i.pinimg.com/736x/c1/7b/15/c17b150e93c4e9c50d963b076484bee7--apple-wallpaper-iphone-wallpaper.jpg);
}

.img2 {
  z-index: 2;
}

.img3 {
  z-index: 0;
}

.img1:hover+.img2 {
  opacity: 0;
}