<div class="box"></div>
.box {
  --scale: .5;
  width: 100px;
  height: 100px;
  background-color: purple;
  transform: scale(var(--scale));
  animation: pulse 1s alternate infinite;
}

@keyframes pulse {
  100% {
    --scale: 1;
    background-color: yellow;
  }
}

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