I think people forget the early 2000s. Many sites only worked on IE but Firefox was the better browser. Firefox users had an extension that would open certain links in IE or open the current page in IE via the context menu.

If a lightweight browser could be significantly faster and more secure, people would tolerate using two browsers again. Although Ladybird hasn't reached that bar.

YouTube certainly could use a small set of web standards, although YT regularly breaks on Firefox. It's a video player with links and forms.