html中背景图按比例缩放全屏显示，核心代码如下div：
<body style="height: 100%; margin: 0;" onload="createTreep()">
<div id="Layer1"
style="position: absolute; width: 100%; height: 100%; z-index: -1">
<img src="<%=basePath %>images/main.jpg" height="100%" width="100%" />
</div>
<table>
</table>
</body>
</html>