<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-backgrounds-3/#background-color">
<style>
@keyframes bgcolor {

  0% { background: blue; }
  100% { background: none; }
}
.target {
  animation: bgcolor 50ms;
  opacity: 0.9;
  height: 0.4px;
  margin: 21.6px;
}
</style>
<div class="target"></div>