isIframeBlank(ifr) {
    const bl = 'about:blank',
      src = ifr.getAttribute('src').trim(),
      href = ifr.contentWindow.location.href;
    return href === bl && src !== bl && src;
  }
Checks if an iframe is empty (if about:blank is the shown page)
@param {HTMLElement} ifr - The iframe DOM element
@return {boolean}
@access protected