Interesting fact: Firefox (or Gecko to be accurate, because there was no single “Firefox” browser back then - there was Netscape Navigator and Mozilla Application Suite) had such rendering mode, but it was quickly abandoned.[1]

1. https://hsivonen.fi/doctype/: “In the summer of 2000 before Netscape 6 was released, Gecko actually had parser modes that enforced HTML syntax rules and one of these modes was called the “Strict DTD”. These modes were incompatible with existing Web content and were abandoned.”

I can’t imagine why I would want to save the interface on a YouTube page

Archiving a community post, for example.