Puoi fare cosi....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
function rectesto(tagformatta) { 
 document.getElementById("descr").focus;
 var a     = document.selection.createRange();
 if (a.text == '') {
  testo = '<'+tagformatta+'>'+'</'+tagformatta +'>'; 
  document.getElementById("descr").value += testo;
 } else {
  testo = '<'+tagformatta+'>'+a.text+'</'+tagformatta +'>' 
  a.text = testo
 }
} 
//-->
</script>


<textarea id="descr" cols="50" rows="10" onclick="this.focus"></textarea>

<input id="btn" type="button" value="Bold" onclick="rectesto('B')">
</body>
</html>