I don't see why this shouldn't be possible:

#element {
  animation: size 1s infinite alternate;
}
@keyframes size {
  from { inline-size: 100px }
  to { inline-size: 200px }
}

I expect it to animate just like if I used width. But the spec says

Animation type: discrete

On Chrome it's effectively discrete, on Firefox it doesn't animate at all. https://jsfiddle.net/45e6okrm/