我按照视频写的代码，怎么会这样，这种要怎么弄？

.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) no-repeat ;
}
.nav_left, .nav_mid, .nav_right {
   height: 40px;
   float: left;
   display: inline;
}
.nav_mid_left ,.nav_mid_right {
   float: left;
}
.nav_mid li {
   float: left;
   list-style-type: none;
   width: 100px;
   font-family: "微软雅黑";
   text-align: center;
   height: 40px;
}
.nav_mid a:link, .nav_mid a:visited {
   font-size: 16px;
   color: #FFF;
   text-decoration: none;
}
.nav_mid a:hover, .nav_mid a:active {
   font-size: 16px;
   color: #FF0;
   text-decoration: none;
}
.nav_mid_left {
   width: 680px;
}
.nav_mid_right {
   width: 300px;
}
.nav_mid_left, .nav_mid_right {
   float: left;
   display: inline;
}