JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li class="item">item</li>
    <li class="item">item</li>
    <li class="item">item</li>
</ul>

CSS

.item {
    opacity: 0;
    transition: opacity .8s;
}
.mostrar {
    opacity: 1 !important;
}

JavaScript

(function () {
    var speed = 1000;
    var items = document.querySelectorAll('ul li');
    var index = 0;

    function iterar() {
        setTimeout(iterar, speed);
        items[index].classList.toggle('mostrar');
        index++;
        if (index == items.length) index = 0;
    }
    iterar();
})();