<div></div>

div {
  width: 300px;
  height: 50px;
  position: fixed;
  inset: 0;
  margin: auto;
  animation: animate 10s linear infinite alternate;
  outline: 1px solid;
}

@keyframes animate {
  0% { outline-width: 1px; }
  100% { outline-width: 50px; }
}