Calculate the scroller percentage in html page.

calculate_scroll_percentage.js

// you can paste this code into chrome console, then check the result.
const pageHeight = (function () {
  return Math.max(
    document.body.scrollHeight,
    document.documentElement.scrollHeight,
    document.body.offsetHeight,
    document.documentElement.offsetHeight,
    document.body.clientHeight,
    document.documentElement.clientHeight
  );
})();

document.onscroll = (e) => {
  const top = e.target.scrollingElement.scrollTop,
        h = e.target.scrollingElement.clientHeight;
  console.log( Math.round( top / (pageHeight-h) * 100) );
};