html怎么实现进度条

<progress id='progress1' value="0" max="100">

</progress>

<button onclick="start_run(100)">下载</button>

<script>

function start_run(n)

{

    if(n==0){alert("下载完成")}

    var progress1=document.getElementById("progress1")

    n=n-1

    cur_task=100-n

    progress1.value=cur_task

    setTimeout("start_run("+n+")",100)

}

</script>