Background-image in keyframe does not display in Firefox or Internet Explorer

As per the specs, background-image is not an animatable or a transitionable property. But it does not seem to say anything about what or how the handling should be when it is used as part of transition or animation. Because of this, each browser seem to be handling it differently. While Chrome (Webkit) is displaying the background image, Firefox and IE seem to do nothing.

The below quote found in an article at oli.jp provides some interesting information:

While CSS Backgrounds and Borders Module Level 3 Editor’s Draft says “Animatable: no” for background-image at the time of writing, support for crossfading images in CSS appeared in Chrome 19 Canary. Until widespread support arrives this can be faked via image sprites and background-position or opacity. To animate gradients they must be the same type.

On the face of it, it looks like Firefox and IE are handling it correctly while Chrome is not. But, it is not so simple. Firefox seems to contradict itself when it comes to how it handles transition on background image as opposed to animation. While transitioning background-image, it shows up the second image immediately (hover the first div in the snippet) whereas while animating, the second image doesn’t get displayed at all (hover the second div in the snippet).

So, conclusion is that it is better to not set background-image inside keyframes. Instead, we have to use background-position or opacity like specified @ oli.jp.

div {
  background-image: url(https://placehold.it/100x100);
  height: 100px;
  width: 100px;
  margin: 10px;
  border: 1px solid;
}
div:nth-of-type(1) {
  transition: background-image 1s ease;
}
div:nth-of-type(1):hover {
  background-image: url(https://placehold.it/100/123456/ffffff);
}
div:nth-of-type(2):hover {
  animation: show-img 1s ease forwards;
}
@keyframes show-img {
  to {
    background-image: url(https://placehold.it/100/123456/ffffff);
  }
}

<div></div>
<div></div>

If you have multiple images that should be shown at different percentages within the keyframe then it would be a better idea to add all those images on the element at start and animate their position like in the below snippet. This works the same way in Firefox, Chrome and IE.

div {
  background-image: url(https://placehold.it/100x100), url(https://placehold.it/100/123456/ffffff);
  background-size: 100% 100%;
  background-repeat: no-repeat;
  background-position: 0px 0px, 100px 0px;
  height: 100px;
  width: 100px;
  margin: 10px;
  border: 1px solid;
}
div:hover {
  animation: show-img 1s steps(1) forwards;
}
@keyframes show-img {
  to {
    background-position: -100px 0px, 0px 0px;
  }
}

<div></div>

Or, like in the below snippet. Basically each image is the same size as the container as background-size is set as 100% 100% but only one image is shown at any given time because of them being the same size as container. Between 0% to 50% the first image is shown because it is at 0px,0px (left-top) whereas the second image is at 100px,0px (outside the right border). At 50.1%, the first image is at -100px,0px (outside left border) and second image is at 0px,0px and so it is visible.

div {
  background-image: url(https://picsum.photos/id/0/367/267), url(https://picsum.photos/id/1/367/267);
  background-size: 100% 100%; /* each image will be 100px x 100px */
  background-repeat: no-repeat;
  background-position: 0px 0px, 100px 0px;
  height: 100px;
  width: 100px;
  margin: 10px;
  border: 1px solid;
  animation: show-img 5s ease forwards;
}
@keyframes show-img {
  0%, 50%{
    background-position: 0px 0px, 100px 0px; /* initially 1st image will be shown as it it as 0px 0px */
  }
  50.1%, 100% {
    background-position: -100px 0px, 0px 0px; /* at 50.1% 2nd image will be shown as it it as 0px 0px */
  }
}

<div></div>