JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="list">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
</ul>

CSS

.row-break {
    color: red;
}

JavaScript

var lists = document.getElementsByClassName('list');
for (var i = 0, l = lists.length; i < l; i++) {
    var items = lists[i].getElementsByTagName('li');
    for (var ii = 0, ll = items.length; ii < ll; ii++) {
        if ((ii + 1) % 2 == 0) {
            items[ii].classList.add('row-break');
        }
    }
}