chromium/content/test/data/accessibility/lines/lines.html

<p style="width: 220px; height: 100px; border: 1px solid #000;">
  <span id="9">9 </span><span id="0">0000000000</span> <span id="1">1111111111</span>
</p>
<button id="button">Grow</button>
<script>
  document.getElementById('button').addEventListener('click', function() {
    document.getElementById('9').firstChild.data = '9999999999 ';
    setTimeout(function() {
      document.getElementById('button').innerText = 'Done';
    }, 1000);
  });
</script>