在每个 元素的内容之前 插入新内容，适合做下拉的子菜单

ul.submenu:before {
    content: "";
    display: block;
    position: absolute;
    z-index:1;
    left: 23px;
    top: 0;
    bottom: 19px;
    border-left: 1px solid #e2e2e2;
}
ul li a:before {
    content: "";
    display: inline-block;
    position: absolute;
    width: 4px;
    height: 4px;
    left: 21px;
    top: 17px;
    background-color: #fff;
    border: 1px solid #e2e2e2;
    z-index: 2;
}