JSFiddle - React, Tailwind, and code Playground

HTML

<div id=t>
    <hr><hr><hr><hr><hr><hr><hr><hr><hr><hr><hr><hr>
</div>

CSS

hr {
    display: block; float: left;
    width: 50px; height: 50px;
    border: solid 2px #aaa; margin: 10px;
}

#t > :nth-child(n+3):nth-last-child(n+3) {
    background-color: #ddd;
}