JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
		<div class="child progress"></div>
		<div class="child shrinker timelapse"></div>
</div>

CSS

#container {
  width: 800px;
  height: 60px;
  margin: 0 auto;
  position: relative;
  top: 50px;
  transform: translateY(-50%);
  border-radius: 35px;
  overflow: hidden;
}

.child {
  width: 100%;
  height: 100%;
}

.progress {
  color: white;
  text-align: center;
  line-height: 75px;
  font-size: 35px;
  font-family: "Segoe UI";
  animation-direction: reverse;
  background: #e5405e;
  /* Old browsers */
  background: -moz-linear-gradient(left, #e5405e 0%, #ffdb3a 25%, #3fffa2 50%, #3fffa2 50%, #1a9be0 73%, #ba68ed 100%);
  /* FF3.6-15 */
  background: -webkit-linear-gradient(left, #e5405e 0%, #ffdb3a 25%, #3fffa2 50%, #3fffa2 50%, #1a9be0 73%, #ba68ed 100%);
  /* Chrome10-25,Safari5.1-6 */
  background: linear-gradient(to right, #e5405e 0%, #ffdb3a 25%, #3fffa2 50%, #3fffa2 50%, #1a9be0 73%, #ba68ed 100%);
}

.shrinker {
  background-color: black;
  position: absolute;
  top: 0;
  right: 0;
  width: 100%;
}

.timelapse {
  animation-name: timelapse;
  animation-fill-mode: forwards;
  animation-duration: 5s;
  animation-timing-function: cubic-bezier(.86, .05, .4, .96);
}

@keyframes timelapse {
  0% {
    width: 100%;
  }

  100% {
    width: 0%;
  }
}