JSFiddle - React, Tailwind, and code Playground

JavaScript

(function ($) {
    $(function () {
        var popup = window.open('/test');
            // JSFiddle 404 page
        
        $(popup.document).ready(function () {
            // Should fire when the DOM of the 404 page has loaded...
            
            $('h2', popup.document).css('color', '#FF0000');
                // Change the color of the header to red.
            
            console.log($('h2', popup.document).length);
                // Should log 1
                // Logs 0, though, because this function fires immediately, before the DOM loads.
        });
        
        setTimeout($.proxy(function () {
            // This will definitely fire after the DOM of the 404 page is loaded.
            
            var popup = this;
            $('h2', popup.document).css('text-decoration', 'underline');
                // This works, because it waited long enough.
                // But I don't want to guess how long it will take the DOM to load....
        }, popup), 5000);
            // After 5 seconds...
    });
})(jQuery);