The correct term would be non-responsive. I think if the second column be moved to the bottom, it would be hard to read on mobile.

My iOS/Safari on a 13 mini detects bad CSS immediately.

You can easily tell some sites are unresponsive or Chrome-only.

Have to agree. With as much reading that's done on small screens these days, I find it discouraging that a project about good web typography looks broken on my phone.

I see adapting parameters like size, measure, and line height to different screen sizes (and the related reading distances) as one of the main challenges of web typography.

Not as sophisticated of a project but I've tinkered with a stylesheet to address some of these issues here: https://pglevy.github.io/typeset.css/