JSFiddle - React, Tailwind, and code Playground

HTML

<div class='yellow-block'>
    Lihat background asli pada CSS di atas, berwarna kuning (yellow), tapi berubah menjadi hijau dengan JavaScript.
</div>

CSS

.yellow-block {
    margin:20px;
    background:yellow;
    padding:20px;
    color:white;
    font:normal 16px Arial;
}

JavaScript

var elems = document.getElementsByClassName('yellow-block');
for(var i = 0; i < elems.length; i++) {
    elems[i].style.background = 'green';
}