let target = document.querySelector('div');

let anim = target.animate(

  {

    borderColor: ['green', 'green']

  }, {

  timeline: new ScrollTimeline(),

  rangeStart: '200px',

  rangeEnd: '400px'

});

​

function raf(ts) {

  var isResolved = false;

  anim.ready.then(function() {

    isResolved = true;

  });

  let p = new Promise((r) => {r();});

  p.then(() => {

    target.innerHTML = `startTime: ${anim.startTime}\nplay state: ${anim.playState}\nready: ${isResolved}\ntimeline time: ${anim.timeline.currentTime}`

    requestAnimationFrame(raf);

  });

}

raf();