@property --scale {
  syntax: "<number>";
  inherits: false;
  initial-value: 1;
}

.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;
  }
}