<!doctype HTML>
<style>
p:first-letter {
  float: left;
  color: green;
}
</style>
<p id="replace">Lorem ipsum</p>
<script>
  document.body.offsetTop; // Force layout.
  var el = document.getElementById("replace");
  el.firstChild.data = el.firstChild.data.replace(/[aeiu]/g, "o");
</script>