为什么当我将#main div 向右浮动时，右边框与标题 div 的右边框不对齐？
* {
margin: 0;
padding: 0;
}
html, body {
height: 100%;
}
#wrapper {
width: 960px;
height: 100%;
margin: 0 auto;
}
#header {
width: 960px;
height: 70px;
border: 1px solid black;
margin-bottom: 20px;
margin-top: 20px;
}
#leftcol {
width: 250px;
height: 500px;
border: 1px solid black;
float: left;
margin-right: 20px;
}
#main {
width: 686px;
height: 500px;
border: 1px solid black;
float:right;
}
HTML