It's something of a UX question too.

One could design a page that visually loaded, then jumped to a redirect after 10 seconds on the page. But who would?

The primary approach is always event-based, because most pages do that sanely.

If not... the best approach I've found is looking for sentinel elements.

Essentially, something that only matches once the website is de facto loaded (regardless of events). Sometimes it's a "search results found" bit of text, sometimes a first element. But more or less, "How do I (a human) know when the page is ready?"