LayoutTests/animations/keyframe-em-unit.html

<style>
div {
  font-size: 2em;
  animation: div 3s infinite alternate;
}

@keyframes div {
  from {
    font-size: 3em;
  }

  to {
    font-size: 3em;
  }
}
</style>
<div>Hello world!</div>