document.head = document.head || document.getElementsByTagName('head')[0];

function changeFavicon(src) {
 var link = document.createElement('link'),
     oldLink = document.getElementById('dynamic-favicon');
 link.id = 'dynamic-favicon';
 link.rel = 'shortcut icon';
 link.href = src;
 if (oldLink) {
  document.head.removeChild(oldLink);
 }
 document.head.appendChild(link);
}

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

async function loop() {
  while (true) {
      changeFavicon('/favicon.ico');
      await sleep(2000);
      changeFavicon('/favicon2.ico');
      await sleep(2000);
  }
}

loop();