关于一个页面即时计算的问题！

Question

<input id=i1 onpropertychange=c()>
<input id=i2 onpropertychange=c()>
<input id=i3>
<script>
function c(){
with(document.all){
i3.value=Number(i1.value)+Number(i2.value)
}
}
</script>

Answer 1

<input id=t1 onpropertychange=t1change() >
<input id=t2 onpropertychange=t2change()>
<input id=t3 >
<script>
function t1change(){
if (!t2.value) t3.value=t1.value
}function t2change(){
t3.value=t1.value*1+t2.value*1
}
</script>

Answer 2

<form name=form1>
<input name=txt1 onPropertyChange="jia()"
 onkeypress="return event.keyCode>=48&&event.keyCode<=57||event.keyCode==46"
 onpaste="return !clipboardData.getData('text').match(/\D/)"
 style="ime-mode:Disabled" ondragenter="return false"><br>
<input name=txt2 onPropertyChange="jia()"
 onkeypress="return event.keyCode>=48&&event.keyCode<=57||event.keyCode==46"
 onpaste="return !clipboardData.getData('text').match(/\D/)"
 style="ime-mode:Disabled" ondragenter="return false"><br>
<input name=txt3 readonly>
</form><script language=javascript>
function jia()
{
   var n1 = document.form1.txt1.value;
   var n2 = document.form1.txt2.value;
   document.form1.txt3.value = parseFloat(n1) + parseFloat(n2);
}
</script>