I wonder how the Ladybird [0] project handles Quirks mode. Since it's more of a hobby project, it might not be fun to get pages designed for IE6 running well in Ladybird. I'm fine with that, especially as time goes on and less and less of the well-trafficked web is using these old undefined behaviors. Perhaps now would be a good time to start working on projects to build an HTML5 compatibility layer around the old stuff, kind of like the Ruffle [1] project.
(Last updated January 2020. Can think of no reason why anyone in the postpandemic world needs to know about this. Although, if I must read about Quirks mode, I do at least enjoy reading about it in a font with such a beautifully elegant capital Q swash.)
[0] https://ladybird.dev/ [1] https://ruffle.rs/