The best way to do it would be to not redirect at all but instead simply serve the different layout at the same URL using the same criteria they already use to

The best way to do it would be to not redirect at all but instead simply serve the different layout at the same URL using the same criteria they already use to redirect. (Or, of course, they could use something like CSS media queries for a responsive design with a single set of code!) There's no reason for them to have a different URL for the mobile views (it should be a cookie or session state if the user explicitly overrides, otherwise automatic) and there IS a reason that Wikipedia is basically the only major site still in existence that doesn't serve up their mobile-designed view on the same URL. Yes, `m.` was a huge trend back in the days of "mobile" meaning "a crappy J2ME browser", but it's not anymore and for damn good reason!