I'm glad it mentioned it (though buried/saved at the very bottom): line width! I'd rather have too narrow than too wide, but good design should have a pleasing font size and line width (and all the other factors) on very different device sizes these days. Seems like many sites often get it right on mobile, but then on a desktop browser suddenly things are proportioned all weird or don't work with whatever browser size. And that's to say nothing about a sudden change of layout, ads, ...