JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li><div></div></li>
    <br/>
    <li><div></div></li>
</ul>

CSS

div {
    background-color: rgb(40,80,120);
    height: 100px;
    width: 200px;
}
ul li {
    display: inline-block;
}

ul li div {
    opacity: 1;
    -webkit-transition: opacity .5s;
    -moz-transition: opacity .5s;
    transition: opacity .5s;
}

ul:hover li div {
    opacity: .5;
}

ul:hover li div:hover {
    opacity: 1;
}