为什么我的显示是这样的呢？

.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

{

float:left;

height:40px;

}