JSFiddle - React, Tailwind, and code Playground

HTML

<div class="asxdf">asdfasdfx/asdfasdf</div>
<div class="asxdf">asdfasdfxasdf/xxasdf</div>

CSS

.asxdf { 
    background-color: #dfd 
}

.red { 
    background-color: #f88;
}

JavaScript

$('body').html($('body').html()
    .replace(/x(?=[^>]*<)/gi, '<span class="red">x</span>'));