“Pase JS Variable a CSS Animation” Código de respuesta

Pase JS Variable a CSS Animation

document.querySelector('.p2').style.setProperty('--m','100%');
document.querySelector('.p2').style.setProperty('--w','300%');
.p1,.p2 {
  animation-duration: 3s;
  animation-name: slidein;
}

@keyframes slidein {
  from {
    margin-left: var(--m, 0%);
    width: var(--w, 100%);
  }
  to {
    margin-left: 0%;
    width: 100%;
  }
}
<p class="p1">
 This will not animate as the animation will use the default value set to the variable
</p>
<p class="p2">
  This will animate because we changed the CSS variable using JS
</p>