HTML+CSS+JavaScript

<script>
function mFunc ()
{
var tx1 = document.getElementById ('txt1');
var tx2 = document.getElementById ('txt2');
var summa=(tx1.value + tx2.value );
document.data.tx3.value=summa.toLocaleString();
}
</script>
<form name=data>
<pre>
первое <input name="txt1" SIZE="4" MAXLENGTH="2" onkeyup="mFunc ()">
второе <input name="txt2" SIZE="4" MAXLENGTH="6" onkeyup="mFunc ()">
сумма <input name="txt3" SIZE="4" MAXLENGTH="2" >

</pre>
</form>

var tx1 = parseFloat(document.data.txt1.value);  var tx2 = parseFloat(document.data.txt2.value);  document.data.tx3.value = tx1 + tx2;