animation-worklet-v.0001.js

registerAnimator('parallax', class {
  animate(inputs, outputs) {
    // read scroller vertical scroll offset.
    const scrollTop = inputs.scroller.scrollTop;

    // update parallax transform
    let t = outputs.parallax.styleMap.get('transform').m42;
    t =  -0.2 * scrollTop;
    outputs.parallax.styleMap.set('transform', new CSSTransformValue(... t));
  }
  
});

// Main thread.
const animator = new Animator('parallax', /* inputs dictionary */ {
  'scroller': new AnimatorScrollSource(elem1),
}, /* outputs dictionary */ {
  'parallax': new AnimatorStyle(elem2, ['transform'])
});

animator.run();
aniator.stop();