Try this code
<html>
<head>
<script type="text/javascript">
function validate() {
if(myform.fname.value.length==0)
{
document.getElementById('errfn').innerHTML="this is invalid name";
}
}
</script>
</head>
<body>
<form name="myform">
First_Name
<input type=text id=fname name=fname onblur="validate()"> </input><div id="errfn"> </div>
<br> <br>
Last_Name
<input type=text id=lname name=lname onblur="validate()"> </input>
<br>
<input type=button value=check>
</form>
</body>
</html>