选择器里面什么意思，为什么设背景是只有这部分才有渐变？求大牛解答

.nav li::before,
.nav li::after{
 content:"";/*在li之间插入空内容*/
 position:absolute;/*相对于li标签*/
 /*定位分割线的位置大小*/
 top:14px;
 height: 25px;
 width: 1px;
}
.nav li::after{
			  right: 0px;
			  background: -moz-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
			  background: -webkit-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
			  background: -o-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
			  background: -ms-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
			  background: linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255,.2) 50%, rgba(255,255,255,0));
			}