JSFiddle - React, Tailwind, and code Playground

HTML

<a href='https://www.google.com' id='myLink'>click me and watch me go red before the page unloads</a>

CSS

#myLink {
    display:block;
    padding:20px;
    background:#CCC;
    transition:background 300ms;
}
#myLink.out {
    background:#CC0000;
}

JavaScript

window.onbeforeunload = function (e) {
    document.getElementById('myLink').className = 'out';
}