.img1，.img2，.img3 {border：1px solid black;过渡：1秒; position：absolute;}。img1 {left：20％; height：300px;宽度：300px; z-index：1; background-image：url（http://cdn.pcwallart.com/images/balloons-photography-vintage-wallpaper-1.jpg）;} .img2，.img3 {right：20％;宽度：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;}  

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 opacity?

can someone help?

html

 <div class="img1">

     </div>
 <div class="img2">

 </div>

CSS

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

.img1{
    left: 25%;
    height: 500px;
    width: 500px;
    z-index: 1;
    background-image: url(http://cdn.pcwallart.com/images/balloons-photography-vintage-wallpaper-1.jpg);
   }

.img2{
  right: 25%;
  width: 500px;
  height: 500px;
  bottom: 0;
  z-index: 2;
  background-image:  url(https://i.pinimg.com/736x/c1/7b/15/c17b150e93c4e9c50d963b076484bee7--apple-wallpaper-iphone-wallpaper.jpg);
}

.img1:hover{
       z-index: 999;
}

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;
}

<div class="img1"></div>
<div class="img2"></div>
<div class="img3"></div>