求问画框为什么浏览器没有显示？
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>canvas clock</title>
<style type="text/css">
div{
text-align: center;
margin-top: 250px;
}
#clock {
border: 1px solid #ccc;
}
</style>
</head>
<body>
<div>
<canvas id="clock" height="200px" width="200px"></canvas>
</div>
<script type="text/javascript" src="clock.js"></script>
</body>
</html>
