<script type="text/javascript">	

function chkFormular () {	
  var strFehler="";
  if (document.forms[0].username.value == "") {
    strFehler +="<span>Kein Username angegeben!</span>";
  }
  //.........
  
  if (strFehler != "") {
	alert(strFehler)
	return false;
  }
  return true;  
}

</script>
<form name="registration" method="post" action="/test/checkRegistration.htm" accept-charset="UTF-8" enctype="text/plain;charset=UTF-8" onSubmit="return chkFormular()">