JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>a</li>
    <li>b</li>
    <li>c</li>
    <li>d</li>
    <li>e</li>
</ul>

CSS

ul li {
    background-color: red;
    margin: 2px;
}
ul:hover li:not(:hover) {
    opacity: .5;
}