Coisa que não sei

<html>
<head>
<script>
function disp_prompt()
  {
  var fname=prompt("Please enter your name:","Your name")
  document.getElementById("msg").innerHTML="Greetings " + fname
  }
</script>
</head>
<body>

<input type="button" onclick="disp_prompt()" value="Display a prompt box">
<br><br>

<div id="msg"></div>

</body>
</html>