.nav_mid_left li 设置了line-height：40px后为什么显示的时候多出两像素
.nav_mid_left li {
float: left;
list-style-type: none;
width: 100px;
text-align: center;
line-height: 40px;
}
.nav_mid_left a:link,.nav_mid_left a:visited{
text-decoration: none;
color: white;
font-size: 16px;
font-family: '微软雅黑';
}
