<div id=container>existing content<br></div>
<script>
for (let i = 0; i < 100; ++i) {
  let span = document.createElement("span");
  let text = document.createTextNode("some text");
  let br = document.createElement("br");

  span.appendChild(text);
  container.appendChild(span);
  container.appendChild(br);
  document.body.offsetHeight;
}
</script>