This is an incorrect assumption. A webpage loads as follows in generic terms:
- Server content is received by the browser.
- Rendering begins starting with objects and then styles from a layout as in all at once and if you slowed it down you would see a pixelated progression as it took shape starting with the content areas and then the objects and then the styles for those objects.
- On Load events fire during this process as well
You can just wait on the general page load, but it's not guaranteed to be the final loaded element on the page. If you have access to the code or dev help I recommend reverse tracing each page like that to be sure you have the last loaded element.