.nav{
    display:flex;   
    list-style:none; 
    position:relative;
    z-index:999;}
.nav>li{position:relative;width:280px;text-align:center;}      /* 为二级菜单定位做准备 */
.nav>li>a{
    display:block;
    padding:14px 26px;
    color:#fff;
    text-decoration:none;
    font-size:18px;
    font-weight:bold;
    transition:background .3s;}
.nav>li:hover>a{background:#3c8854;}


.nav ul{                         /* 所有下拉 ul */
    position:absolute;
    top:100%;                    /* 紧贴父级 li 底部 */
    left:0;
    min-width:280px;
    background:#fff;
    border:1px solid #e0e0e0;
    box-shadow:0 4px 8px rgba(0,0,0,.15);
    list-style:none;
    display:none;}
.nav>li:hover ul{display:block;} /* 悬停时显示 */
.nav ul li a{
    display:block;
    padding:10px 20px;
    color:#333;
    font-size:16px;
    text-decoration:none;
    transition:background .2s;}
.nav ul li a:hover{background:#3c8854;color:#fff;}