JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li class="marcacao" onMouseOver="aumenta('html')">HTML5 e CSS3
        <div id="html" class="nivel">100</div>
    </li>
</ul>

CSS

div {
    border:1px solid black;
}

JavaScript

var valorInicial = 0;

function aumenta(id) {
    var elemento = document.getElementById(id);
    var valorElemento = elemento.innerHTML;
    setInterval(function () {
        if (valorInicial <= valorElemento) {
            elemento.innerHTML = valorInicial;
        }
        valorInicial++;
    }, 20)
}