animation-worklet-v.0003.js

// animation worklet
registerAnimator('parallaxAnim', class {
  static inputs = {
    'scroller': {
      'scrollTop'
    },
    'parallax': {
      'style': ['transform']
    }
  }
  
  static outputs = {
    'parallax': {
      'style': ['transform']
    }
  }
  
  animate(inputs, outputs) {
    const scrollTop = inputs.scroller[0].scrollTop;
    let t = inputs.parallax[0].styleMap.get('transform').m42;
    t =  -0.2 * scrollTop;

    outputs.parallax[0].styleMap.set('transform', new CSSTransformValue(... t));
  }
});

// css
.scroller {
  animator: parallaxAnim scroller;
}

.parallaxEl {
  animator: parallaxAnim parallax; 
}