animation-worklet-v.0002.js

registerAnimator('parallax', class {
  static inputs = {
    'scroller': ScrollSource;
  };
  
  static outputs = {
    'parallax': Style(['transform']) // This is bad.
  }
  
  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': elem1,
}, /* outputs dictionary */ {
  'parallax': elem2,
});

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