JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper_menu">
<ul class="menu">
    <li>
        <a>Раздел1</a>
        <ul class="submenu">
            <li>
                <a>Пункт11</a>
            </li>
            <li>
                <a>Пункт12</a>
            </li>
            <li>
                <a>Пункт13</a>
            </li>
        </ul>
    </li>
    <li>
        <a>Раздел2</a>
        <ul class="submenu">
            <li>
                <a>Пункт21</a>
            </li>
            <li>
                <a>Пункт22</a>
            </li>
            <li>
                <a>Пункт23</a>
            </li>
        </ul>
    </li>
</ul>
</div>

CSS

html,body{
    padding:0px;
    margin:0px;
    height:100%;
    font-size:14px;
}

.wrapper_menu{
    position:relative;
    background:#999;
    text-align:center;
    min-width:500px;
}
.wrapper_menu:before{
    content:"";
    display:block;
    position:absolute;
    left:0px;
    right:0px;
    bottom:0px;
    height:2px;
    background:#fff;
    z-index:0;
}

ul.menu, ul.submenu{
    margin:0px;
    padding:0px;
    list-style: none;
}
ul.menu>li>a, ul.submenu>li>a{
    display:inline-block;
    padding:10px;
    position:relative;
    cursor:pointer;
}

ul.menu{
    display:inline-block;
    white-space: nowrap;
    margin:0px;
    padding:0px;
    vertical-align:top;
}
ul.menu>li{
    float:left;
    margin:0px 10px;
}
ul.menu>li>a:after{
    content:"";
    position:absolute;
    bottom:0px;
    left:50%;
    margin-left:-8px;
    border: 8px solid transparent;
    border-bottom: 8px solid #fff;
    opacity:0;
    transition: opacity 0.0s ease 0s;
}
ul.menu>li:hover>a:after{
    opacity:1;
    transition: opacity 0.2s ease 0.5s;    
}

ul.menu>li:hover>ul.submenu{
    opacity:1;
    max-height:1000px;
    transition: opacity 1.0s ease 0.5s, max-height 1.0s ease 0.5s;
}

ul.submenu{
    position:absolute;
    top:36px;
    left:0px;
    right:0px;
    background:#ddd;
    text-align:center;
    opacity:1;
    max-height:0px;
    transition: opacity 0.0s ease 0s;
    overflow:hidden;
}
ul.submenu>li{
    display:inline-block;
    margin:0px 10px;
    vertical-align:top;
}