老师帮忙检查一下作业，怎么没有效果

来源：1-26 编程练习

汤姆克鲁琛

2020-04-19 00:53:18

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>setInterval()</title>
        <style type="text/css">
            div{
                width:300px;
                height:300px;
                border:1px solid #333;
            }
        </style>
    </head>
    <body>
        <input type="button" value="开始">
        <br><br>
        <div></div>
        <script type="text/javascript">
        var div=document.getElementsByTagName("div")[0],
            btn=document.getElementsByTagName("input")[0];
        btn.onclick=function(){
            setInterval(function(){
                div.innerHTML+="hello &nbsp;";
            },1000)
        }
        </script>
    </body>
</html>

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