>you have to assume it's a small mobile device and arrange items on a single axis.

Is there really no other way to detect if it's desktop? It seems like screen aspect ratio would be easy.

Mobile sites look terrible on desktop browsers if they work at all. The one I encounter all the time is Firefox's add-on site. Just because I don't have css-grid it forces mobile which makes things worse. I know people like myself are a tiny fraction of a tiny fraction of users but we exist.