JSFiddle - React, Tailwind, and code Playground

HTML

<fieldset id="pay" class="input-fieldset"></fieldset>
<fieldset id="address" class="input-fieldset"></fieldset>

JavaScript

// Get desired elements
var element = document.getElementsByClassName('input-fieldset');

// Iterate through the retrieved elements and add the necessary class names.
for(var i = 0; i < element.length; i++)
{
    element[i].classList.add('input-fieldset-awesome');
    console.log(element[i].className);
}