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I am trying to run JavaScript only after the page has loaded, and according to here - https://developer.apple.com/documentation/safariservices/safari_app_extensions/injecting_a_script_into_a_webpage, I should use DOMContentLoaded. However, it does not seem to work. This is my content.js file: function runOnStart() {     document.addEventListener('DOMContentLoaded', function(e) {         document.body.style.background = "rgb(20, 20, 20)";         document.html.style.background = "rgb(20, 20, 20)";                var divElements = document.body.getElementsByTagName('div');         for(var i = 0; i < divElements.length; i++) {             let elem = divElements[i];             elem.style.background = "rgba(255, 255, 255, 0.05)";         }     }); } runOnStart(); If I take the code outside of the event listener, it runs fine, but a lot of the elements haven't loaded in yet so it doesn't work as it should. The function is definitely running, but the event listener simply doesn't work. I appreciate any help you can give!