CSS 实现菜单折叠与展开的小三角

/* 菜单展开向下的箭头 */
.aside h4 .nav-open {
    display: inline-block;
    width: 0px;
    height: 0px;
    border-bottom: none;
    border-top: 6px solid rgba(255, 255, 255, .7);
    border-left: 6px solid transparent;
    border-right: 6px solid transparent;
    float: right;
    position: relative;
    top: 5px;
}

/* 菜单折叠向上的箭头 */
.aside h4 .nav-close {
    display: inline-block;
    width: 0px;
    height: 0px;
    border-bottom: 6px solid #fff;
    border-top: none;
    border-left: 6px solid transparent;
    border-right: 6px solid transparent;
    float: right;
    position: relative;
    top: 5px;
}