<div id="container">
  <h4>Content be here..</h4>
</div>
<div id="log"><p>Mutations:</p></div>

#log {
  position: absolute;
  top: 0;
  right: 0;
  width: 25vw;
  min-width: 100px;
  min-height: 100px;
  border: 1px solid #ccc;
  height: 25vh;
  background-color: rgba(200,200,200,0.5);
  padding: 1em;
  overflow: scroll;
}
#log > div {
  font-size: 0.8em;
}

const content = document.getElementById('container');
const log = document.getElementById('log');

const onMutation = events => {
  const div = document.createElement('div');
  div.innerText = events[0].type;
  log.appendChild(div);
};
const observer = new MutationObserver(onMutation);
observer.observe(content, {subtree: true, childList: true});

let pause = 500;
const delayedAdd = () => setTimeout(() => {
  const el = document.createElement('div');
  el.innerText = Math.random().toString(36).substring(2, 11);
  content.appendChild(el);
  delayedAdd();
  pause *= 1.5;
}, pause);

delayedAdd();