JSFiddle - React, Tailwind, and code Playground

HTML

<div id = "red">
    <div id = "blue"></div>
</div>

CSS

#red{
    background: red;
    width: 200px;
    height:200px;
}
#blue{
    width: 20px;
    height:20px;
    background:blue;
}

JavaScript

// Run initially on page load
$(document).ready(function() {
    resize();
});
// Run when resizing the window
$(window).resize(function() { resize() });

// Handle the resize
function resize() {
    $('#red').width($(window).width());
    $('#red').height($(window).height());
    $('#blue').width($(window).width() * 0.1);
    $('#blue').height($(window).height() * 0.1);
}