<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
</head>
<body>
<div style="text-align: center;">
    输入售价后点击计算税费，自动显示在税费输入框！
</div>

<div class="item">
    <div >售价：</div>
    <div >
        <input type="text" id="price" value=""/>
    </div>
</div>

<div>
    <div >税费：</div>
    <div >
        <input id="tax" type="text" autocomplete="off" value=""/>
        <input type="button" id="calculate" value="计算税费" onclick="calculate()"/>
    </div>
</div>

<script>
    function calculate(){
        /*获取输入的售价*/
        var price = document.getElementById('price').value;
        caltax = price*0.5;/*计算税费*/
        var tax = document.getElementById('tax');/*获得tax*/
        tax.value=caltax;/*把计算好的税费赋值到tax的value*/
    }
</script>

</body>
</html>

效果图：在这里插入图片描述