How do online text editors work

div.theScreen {
  width:320px;
  height:75px;
  border:1px solid #CCCCCC;
  background-color:#f1f1f1;
}

<div class="theScreen"></div>
<div><textarea class="typePad"></textarea></div>

$(document).ready(function(){
  $(".typePad").keyup(function(){
    $(".theScreen").html($(".typePad").val());
  });
});