Untitled

<head>
<script>
function myFunction()
{
    var a=document.forms["login"]["uname"].value;
    var b=document.forms["login"]["pwd"].value;
    if(a=="" || b =="")
    {
        error="All fields must be entered.";
        document.getElementById("errorid").innerHTML=error;
    }
}
</script>
</head>
<body>
<form name="login" action="">
<b>Enter username:</b>
<input type="text" name="uname" /><br />
<b>Enter password:</b>
<input type="password" name="pwd" /><br />
<p id="errorid"></p>
<input onclick="myFunction()" type="submit" />
</form>
</body>

</html>