<!DOCTYPE html>
<body>
<style>

@keyframes anim {
    from { margin-left: 0 }
    to   { margin-left: 100px }
}

</style>
<script>

const target = document.body.appendChild(document.createElement("div"));
target.style.animation = "anim 1s";

const animation = target.getAnimations()[0];

const effect = animation.effect;
effect.target = null;
effect.getKeyframes();

</script>
</body>