圆角水平菜单问题 为什么下面加粗的代码中将背景图向上移30px就会在鼠标挨到的时候变颜色呢
*{margin:0; padding:0; font-size:14px;}
a{color:#333;text-decoration:none}
.nav{list-style:none; height:30px; border-bottom:10px solid #F60; margin-top:20px; padding-left:50px;}
.nav li{float:left}
.nav li a{display:block; height:30px;text-align:center; line-height:30px; width:120px; background:url(http://img1.sycdn.imooc.com//53846438000168f901200060.jpg); margin-left:1px;}
.nav li a.on, .nav li a:hover{ background:url(http://img1.sycdn.imooc.com//53846438000168f901200060.jpg) 0 30px; color:#fff;}