Firefox OS has something like this. On Firefox Marketplace, you can submit a mobile website that serves a small manifest.json file and it will be indistinguishable from a native app. I've been praising this innovative approach so many times on HN, but I have to do it again---it elegantly solves a lot of the problems we're having with native apps, and it all mostly works and integrates fine with the OS (the idea could use some polishing but is good as a proof-of-concept). It's a shame that the OS itself never really took off.