What's the norm if you need a desktop and mobile version of a webpage, to have one separate code base for each one(can be on the same repo) or all the code together with the appropriate screen size detections?
I think the question is who is your target customer. If you are building a website that focus on mobile user, it's no need to design and test for desktop.
In this case Mobi.css is a choice. You can show a QRCode to guide people to mobile.