JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://raw.github.com/jnbdz/elInside/master/Source/Element.inside.js"></script>
<div id="msg"><strong>Inside:</strong> <span></span></div>
<div id="out"></div>
<div id="in"></div>

CSS

strong {
    font-weight: bold;
}

#msg {
    margin: 15px;
}

#out {
    position:absolute;
    background-color: blue;
    width: 100px;
    height: 100px;
    margin: 100px 0 0 100px;
}

#in {
    position:absolute;
    background-color: red;
    width: 50px;
    height: 50px;
    margin: 125px 0 0 160px;
}

JavaScript

$$('#msg span')[0].set('text', document.id('in').inside(document.id('out')));