Accessory drive belts are nothing new, and neither are V-ribbed belts. What is new - the past couple of years new - are "Stretchy Belts".
As you can see in the above photo, which is of the 2.3 litre "MZR" four cylinder in a 2006 Mazda5, there is no tensioner for the outermost belt.
Normally, if you didn't have a dedicated tensioner, you'd have some provision for using one of the driven components as a source of adjustment.
Perhaps not entirely obvious in this photo is the fact that the one and only component driven by this belt (the upper pulley is the crankshaft) is the air conditioning compressor, and it's bolted firmly to the oil pan. No adjustment is possible.
So what gives? Well, as the name implies, the belt is stretchy - stretchy enough that it can be coaxed over the pulleys to get it into place, but not so stretchy that it fails to transfer power to the compressor, which can require several horsepower to function.
Actually, you might be surprised at just how strong a Stretchy Belt is. This particular Mazda5 is on engine number 2 - engine number 1 suffered catastrophic internal damage when the A/C compressor seized, and - instead of slipping or breaking - the Stretchy Belt hung on tight enough to spin the keyway-less crank pulley and the crankshaft's timing gear on the end of the crankshaft, resulting in the smashing of pistons into valves. Stretchy Belt 1; Engine 0.
Mazda isn't alone in using Stretchy Belts; Ford, Chrysler, and General Motors also use this technology, and it's not hard to imagine other quickly following suit once they realize that they can eliminate the expense, space, and potential warranty replacement of a tensioner by using one.
The downside? It's a bit more expensive than a conventional belt of equal size, it's not possible to reliably determine wear or health by just looking at it (there's a little plastic gauge available), and it requires special tools and/or creativity to remove or reinstall - your mechanic will curse the first one or two they have to change.