Show select input to change icon font-size for preview in Fontastic generated HTM

fontastic_sizer.js

/** Show select input to change icon font-size for preview in Fontastic generated HTML (http://app.fontastic.me) */
const xpath = "//h2[text()='CSS mapping']";
const mainHeader = document.evaluate(xpath, document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
const fontSizes = [16, 24, 32, 64, 128, 256];

const selectList = document.createElement('select');
selectList.id = "font-size-select";

fontSizes.forEach(size => {
  const option = document.createElement('option');
  option.value = `${size}px`;
  option.text = `${size}px`;
  selectList.appendChild(option);
});

selectList.addEventListener('change', (event) => {
  const fontSize = event.target.value;
  const icons = document.getElementsByClassName('icon');
  for (let icon of icons) {
    icon.style.width = fontSize;
    icon.style.height = fontSize;
    icon.style.fontSize = fontSize;
    icon.style.padding = 0;
}
});

mainHeader.parentNode.insertBefore(selectList, mainHeader.nextSibling);