function mutationObserverCallback(mutations) {
  /* Grab the first mutation */
  var mutationRecord = mutations[0];
  /* If a child node was added,
     hide the msg after 2s  */
  if (mutationRecord.addedNodes[0] !== undefined)
    setTimeout(hideMsg, 2000);
}
function hideMsg() {
  msg.textContent = '';
  msg.style.background = 'none';
}