JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="menu">
    <li class="menu__item"><a href="https://goo.gl/9B3DHX">Задачка на прокачку</a>
        <ul class="menu menu-inner">
            <li class="menu__item"><a href="http://jedicss.com/">Jedicss.com</a></li>
            <li class="menu__item"><a href="https://goo.gl/r2Xcwn">Решение</a></li>
        </ul>
    </li>
</ul>

CSS

.menu{
    margin: 0 auto;
    padding: 0;
    width: 180px;
    list-style: none;
    border:2px solid red;
}

.menu__item a{
    display: block;
    padding: 8px 16px;
    background-color: purple;
    color: #fff;
    text-decoration: none;
}

.menu__item a:hover{
    background-color: blue;
}

.menu__item:hover .menu-inner {
  display: block;
}

.menu-inner{
    border: 4px solid blue;
    margin-top: 10px;
    display: none;
}

JavaScript

/* никакого js */