导航栏中的“首页”为什么没有显示在左侧

*{
	margin: 0px;
	padding: 0px;	
	font-size: 12px;
}
body
{
	background-color:#F5F5F5;
}
.top
{
	width: 100%;
	height: 27px;
	background: url(../images/top_bg.jpg) repeat-x;
}
.top_content
{
	width: 1000px;
	margin:0 auto;
}
.top_content li
{ 
	list-style-image: url(../images/li_bg.gif) ;
    float: right;
	width:70px;
	line-height: 27px;
}
.top_content a:link,.top_content a:visited
{
	color: #8E8E8E;
	text-decoration: none;
}
.top_content a:hover,.top_content a:active
{
	color: #900;
	text-decoration: none;
}
.wrap
{
	width: 1000px;
	margin: 0 auto;
}
.logo
{
	height: 80px;
	background-color: #FFF;
}
.logo_left
{
	width: 200px;
	float: left;
}
.logo_right
{
	width: 300px;
	height: 28px;
	float: right;
	margin-top: 26px;
	color: #8E8E8E;
}
.logo_right img
{
	vertical-align: middle;
	margin-right: 10px;
	
}
.tel
{
	
	font-family:"微软雅黑";
	font-size: 18px;
	color: #CC0000;
	
	
}
.nav
{
	height: 40px;
}
.nav_left
{
	width: 10px;
	background: url(../images/nav_left.jpg) no-repeat;
}
.nav_mid
{
	width: 980px;
	background: url(../images/nav_bg.jpg) repeat-x;
}
.nav_right
{
	width: 10px;
	background: url(../images/nav_right.jpg) repeat-x;
}
.nav_left,.nav_mid,.nav_right
{
	height: 40px;
	float: left;
}
.nav_mid_left,.nav_mid_right
{
	
	float: left;
}
.nav_mid_left
{
	width: 680px;
}
.nav_mid_right
{
	width: 300px;
}
.nav_mid_left li
{
	float: left;
	list-style: none;
	width: 100px;
	text-align: center;
	line-height: 40px;
}
.nav_mid_left a:link,.nav_mid_left a:visited
{
	text-decoration: none;
	color: #FFF;
	font-size: 16px;
	font-family: "微软雅黑";
}.nav_mid_left a:hover,.nav_mid_left a:active
{
	text-decoration: none;
	color: #FF0;
	font-size: 16px;
	font-family: "微软雅黑";
}
.search_text
{
	width: 190px;
	height: 25px;
	margin-top: 7px;
	background: url(../images/search.jpg) no-repeat right center;
	background-color: #fff;
	padding-right: 25px;
	border: none;
}

各位大神请看一下这是怎么回事，在线等

所写的代码与与视频教程相同，但“首页”显示位置不在左侧，这是为什么？