JSFiddle - React, Tailwind, and code Playground

HTML

<div class="oldclass">first</div>
<div class="oldclass">second</div>
<div>third</div>

CSS

.tagging {
    padding:30px;
    background-color:#ccf;
    width : 10px;
    height: 10px;
}
.oldclass {
    color:#f00;
}

JavaScript

var div = document.getElementsByTagName("div");
console.log(div[0].className);
div[0].className += " tagging";