1.<output>     Js计算结果

<form
oninput="res.value = num1.valueAsNumber*num2.valueAsNumber">

    <input
type="number" id="num1"> X            <!-- 没有闭合</input> -->

    <input
type="number" id="num2"> =            <!-- 没有闭合</input> -->

    <output
for ="num1 num2" name="res">         <!-- 没有闭合</output> -->

    <button>提交</button>

</form>

2. 输入验证