Speculation from this article is it's Linux-based. With how MS is admittedly(and obviously) trying to merge Phone and Desktop OS's, and therefore create their own App Store ecosystem, it'd be no surprise if Valve is trying to migrate away from Windows. Since said App Store would be competition w/ Steam(and you possibly would have to use that App Store to even get Steam). But if it is Linux, and companies jump aboard that plan, it would be awesome. buhbyes last reason to use Windows. :x

As a preferably PC gamer, I hope it succeeds. :3