91.练习题：计时器，到3分钟停止
DOM结构：就是原来那些元素，原来叫标签，现在交DOM结构，因为它可以被DOM所操作
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>frame</title>
<style type = text/css>
input{
border:1px solid black;
font-size:20px;
text-align:right;
font-weight:bold;
}
</style>
</head>
<body>
minutes : <input type="text" value="0">
seconds : <input type="text" value="0">
<script type="text/javascript">
var minutesNode = document.getElementsByTagName('input')[0];
var secondsNode = document.getElementsByTagName('input')[1];
var seconds = 0;
var minutes = 0;
var timer = setInterval(function (){
seconds++;
if(seconds == 60){
seconds = 0;
minutes++;
}
secondsNode.value = seconds;
minutesNode.value = minutes;
if(minutes == 3){
clearInterval(timer);
}
},1000)
</script>
</body>
</html>
