Untitled

<html>
<head>

 <title> Addition</title>

<script type="text/javascript">

    document.write("Please enter values to be added into the fields and click Add button1");


function sum()
{
     var num1 = document.formOne;
     value1 = Number(num1.input1.value);

     var num2 = document.formOne;
     value2 = Number(num2.input2.value);

  if (value1=="")
    {
       document.write("Valid value cannot be empty!");

    }
  else if(value2=="")
    {
        document.write("Valid value cannot be empty!");

    }
    else
    {
       var add = value1 + value2;
       document.write ("The sum is " + add );
    }

 return true;
}

</script>
</head>

<body>

<form id="forma" name="formOne" runat="server" onsubmit="sum()">
    <p><input id="Textone" name="input1" type="text" size="8"></p>
    <p><input id="Texttwo" name="input2" type="text" size="8"></p>
    <p><input type="submit" value="Add"> </p>
</body>
</html>

function sum()
{
  var num1 = document.formOne;
  value1 = Number(num1.input1.value);

  var num2 = document.formOne;
  value2 = Number(num2.input2.value);

  if (value1=="")
  {
    document.write("Valid value cannot be empty!");

  }
  else if(value2=="")
  {
    document.write("Valid value cannot be empty!");

  }
  else
  {
    var add = value1 + value2;
    document.write ("The sum is " + add );
  }


  return false; // Changed to "false"
}

<form id="forma" name="formOne" runat="server" onsubmit="return sum();">