The given code allow the user to enter username and password.It will show an error message if user leave the fields blank.

<script>
function validate(){
var a=document.form.username.value; 
if(a=="" ) {
alert("Enter a valid UserName");
return false; 
}
if(!isNaN(a) ) 
{
alert("Enter a valid UserName");
return false;
}
var b=document.form.password.value;
if(b=="") {
{

alert("Enter password"); 
return false; 
} 
}
}
</script>
<form name="form" method="post"  action="" onsubmit="return validate();">
  <table>
  <tr>
    <td>Username:</td>
    <td><input type="text" name="username"></td>
  </tr>
  <tr>
    <td>Password:</td>
    <td><input type="password" name="password"></td>
  </tr>
  <tr>
  <tr>
  <td></td>
  <td>
  <input type="submit" value="Submit" name="addproduct"/>
  </td>
  </tr>
</table> 
</form>