https://en.m.wikipedia.org/wiki/Carbon
will open in mobile format. But getting the user agent is trivial, and so wiki should be able to easily redirect to this on a non-mobile device:
https://en.wikipedia.org/wiki/Carbon
But they don't, and since they're one of the most-visited sites in the world, I assume there's some really good reason why they don't bother. Anybody know why that might be? Is there some sort of esoteric security thing that sites have to worry about? Do they just not want to have to do the work of serving one page and then immediately switching to the other? Or is it something else entirely?
And BTW have you observed what kind of user agents browsers are spouting? Firefox claims to be Opera, Opera is Chrome etc. But umm this kind of offtopic, when it comes to a desktop/laptop/tablet/phablet/phone distinction.
in the case you have illustrated, a user can cut or paste the ".m." into the URL at will