JSFiddle - React, Tailwind, and code Playground

CSS

@media (max-width: 500px) {
    body:after {
        content: 'foo';
        display: none;
    
        }
body{background:#eee;}
    }

JavaScript

var size = window.getComputedStyle(document.body,':after').getPropertyValue('content');
if (size == 'foo') {
    alert("bar!");
}