为什么会这样？http://www.imooc.com/video/2496
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.top_content{width: 1000px;margin: 0 auto;position: relative;}
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.logo_right{float: right;line-height: 80px;height: 20px;color: #868686;margin-right: 10px;}
.logo_right img{vertical-align: middle;margin-right: 10px;}
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.list_div{width: 1000px;height:50px;margin: 0 auto;background-color: chocolate;}
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.list{float: left;clear: left;line-height: 50px;}
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.list li a{color: white;font-family: "微软雅黑";font-size: 18px;text-decoration: none;font-weight: bolder;}
.input{float: right;line-height: 50px;}
.input input{height: 26px;width: 200px;background: url(../images/search.jpg) no-repeat;}会有2px的空隙，不知道什么情况。如下图！
