[CSS] css使用first-child 和last-child 实现三条杠效果

<div class="menuico"><span></span><span></span><span></span></div>

        .menuico span {
            display: block;
            width: 21px;
            height: 2px;
            background-color: #666;
        }
        .menuico span:first-child {
            margin-top: -6px;
        }
        .menuico span:last-child {
            margin-top: 6px;
        }