JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>Default item</li>
    <li>Default item</li>
</ul>
<button>Append</button>

JavaScript

$('button').live('click', function(){
    $('ul').append('<li>Added item</li>'); //This action is done by an external script.
});

$('button').live('click', markButtons);

markButtons();

function markButtons() {
    $('ul li:not(.marked)')
        .addClass("marked")
        .append('<b> x</b>');
}