JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
    <div class="main">
        This div has an aspect-ratio of 16:9...
    </div>
</div>

CSS

html, body {
 height: 100%;
 width: 100%;    
}
.wrapper {
 width: 50%;
 display: inline-block;
 position: relative;
}
.wrapper:after {
    padding-top: 56.25%; /*16:9 ratio*/
    display: block;
    content: '';
}
.main {
    position: absolute;
    top: 0; bottom: 0; right: 0; left: 0; /*fill parent*/
    background-color: blue;
    color: white;
}