Making it move back and forth

HTML

<!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Animations/Using_CSS_animations -->

<p>The Caterpillar and Alice looked at each other for some time in silence:
at last the Caterpillar took the hookah out of its mouth, and addressed
her in a languid, sleepy voice.</p>

CSS

p {
  animation-duration: 3s;
  animation-name: slidein;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}
@keyframes slidein {
  from {
    margin-left: 50%;
    width: 50%; 
  }

  to {
    margin-left: 0%;
    width: 50%;
  }
}