I need a recommendation for a online website monitoring service (like https://www.uptrends.de, not self hosted) for javascript heavy websites with threejs (3d canvas). The monitoring should inform me (by email) if the website (a given url) is down or if it takes too long to fully load. Because most of the website is dynamic and relies on a 3d canvas, the criterion that the website "is not down" == "fully loaded" is that the javascript part of the website runned successfully. For this, the javascript adds a special class to the body tag of the website ("app-ready"). The website is "fully loaded" only after the body tag has a class "app-ready". Because the website relies on a 3d canvas, the monitoring service can not run it in headless mode, it would need a real browser.
Does such a online monitoring service exist?