CSS 积累
1. button按钮设置背景图片，边角无空白

<input type="button" id="submitButton" onclick="login()" style="background-image:url('images/dl.gif');background-color:buttonface;background-repeat:no-repeat;background-position:center;width:57px;height:20px;border:none;"/>

<input type="sumbit" id="submitButton" onclick="login()" value="" style="background-image:url('images/dl.gif');background-color:buttonface;background-repeat:no-repeat;background-position:center;width:57px;height:20px;border:none;"/>