<form action="" id="myform" oninput="num.value=parseInt(num1.value)+parseInt(num2.value)">
<input type="number" id="num1">+
<input type="number" id="num2">=
<output name="num" for="num1 num2"></output>
</form>
原文：http://www.cnblogs.com/houweidong/p/5932711.html