JSFiddle - React, Tailwind, and code Playground

HTML

<div id="loginForm">
    <form>
        <label>Label 1</label>
        <label>Label 2</label>
    </form>
</div>

<button id="insert">insert moar</button>

CSS

#loginForm label {
    color: #FF0000
}

#loginForm label:nth-child(2n+1) {
    color: #8a8a8a;
}

JavaScript

document.getElementById('insert').addEventListener('click', function( event ) {
    document.querySelector('#loginForm form').insertAdjacentHTML( 'beforeend', '<label>New Guy!</label>' );
}, false);