How to play CSS3 transitions in a loop?

The following style is just an example of how to set transitions in CSS3.
Is there a pure CSS trick to make this play in loop?

div {
    width:100px;
    height:100px;
    background:red;
    transition:width 0.1s;
    -webkit-transition:width 0.1s; /* Safari and Chrome */
    -moz-transition:width 0.1s; /* Firefox 4 */
    -o-transition:width 0.1s; /* Opera */
    transition:width 0.1s; /* Opera */
}

div:hover {
    width:300px;
}

CSS transitions only animate from one set of styles to another; what you're looking for is CSS animations.

You need to define the animation keyframes and apply it to the element:

@keyframes changewidth {
  from {
    width: 100px;
  }
 
  to {
    width: 300px;
  }
}

div {
  animation-duration: 0.1s;
  animation-name: changewidth;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}

Check out the link above to figure out how to customize it to your liking, and you'll have to add browser prefixes.

If you want to take advantage of the 60FPS smoothness that the "transform" property offers, you can combine the two:

@keyframes changewidth {
  from {
    transform: scaleX(1);
  }

  to {
    transform: scaleX(2);
  }
}

div {
  animation-duration: 0.1s;
  animation-name: changewidth;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}

More explanation on why transform offers smoother transitions here: https://medium.com/outsystems-experts/how-to-achieve-60-fps-animations-with-css3-db7b98610108