old_contents: /**
   * Load .textContent by iframe from specified HTML elemen of specified (same origin)
   * web page.
   *
   * @param {string} iframeSrcURL
   *   The web page to be loaded. An iframe will be used to load it. After .textContent
   * has been extracted, the iframe will be released.
   *
   * @param {string} elementIdInsideIFrame
   *   The HTML element id. When the web page has been loaded, the .textContent of the
   * the HTML element with this id will be extracted.
   *
   * @return {Promise}
   *   Return a Promise. When the Promise resolve, the .textContent of the HTML element
   * will be returned.
   */
  function createPromise_GetTextContent_ByIFrame(iframeSrcURL, elementIdInsideIFrame) {
    return new Promise(function(resolve, reject) {
      let theIFrame = document.createElement("iframe");
      theIFrame.style.display = "none";
      theIFrame.src           = iframeSrcURL;
      theIFrame.sandbox       ="allow-same-origin";
      theIFrame.onload        = function() { resolve(theIFrame); }
      document.body.appendChild(theIFrame);         // Load the URL by iframe.
  
    }).then(function(theIFrame) {
      let theTextContent = theIFrame.contentDocument
        .getElementById(elementIdInsideIFrame).textContent;
      theIFrame.parentNode.removeChild(theIFrame);  // Release the iframe.
      return theTextContent;
    });
  }
  
new_contents: /**
   * Dynamically load .textContent by iframe from specified HTML elemen of specified
   * (same origin) web page.
   *
   * @param {string} iframeSrcURL
   *   The web page to be loaded. An iframe will be used to load it. After .textContent
   * has been extracted, the iframe will be released.
   *
   * @param {string} elementIdInsideIFrame
   *   The HTML element id. When the web page has been loaded, the .textContent of the
   * the HTML element with this id will be extracted.
   *
   * @return {Promise}
   *   Return a Promise. When the Promise resolve, the .textContent of the HTML element
   * will be returned.
   */
  function createPromise_GetTextContent_ByIFrame(iframeSrcURL, elementIdInsideIFrame) {
    return new Promise(function(resolve, reject) {
      let theIFrame = document.createElement("iframe");
      theIFrame.style.display = "none";
      theIFrame.src           = iframeSrcURL;
      theIFrame.sandbox       ="allow-same-origin";
      theIFrame.onload        = function() { resolve(theIFrame); }
      document.body.appendChild(theIFrame);         // Load the URL by iframe.
  
    }).then(function(theIFrame) {
      let theTextContent = theIFrame.contentDocument
        .getElementById(elementIdInsideIFrame).textContent;
      theIFrame.parentNode.removeChild(theIFrame);  // Release the iframe.
      return theTextContent;
    });
  }
  
subject: Improve comments.