Quirks quirks aside there are other ways to tame old markup...

If a site won't update itself you can... use a user stylesheet or extension to fix things like font sizes and colors without waiting for the maintainer...

BUT for scripts that rely on CSS behaviors there is a simple check... test document.compatMode and bail when it's not what you expect... sometimes adding a wrapper element and extracting the contents with a Range keeps the page intact...

ALSO adding semantic elements and ARIA roles goes a long way for accessibility... it costs little and helps screen readers navigate...

Would love to see more community hacks that improve usability without rewriting the whole thing...