An entire Mobile OS built using HTML, CSS and JS - by Mozilla

Looks like with Enyo, the JavaScript app framework, and leveraging this new Boot2Gecko, smartphone hardware will become the focus rather the phone OS.

Would this not dramatically increase data usage? ie data fees to carriers

Boy, the video's constant use of "the web" really annoyed me. I know that you have to cut down the jargon to make it presentable, and "HTML, CSS and JS" gets a bit confusing. What about "web technologies" or even just "a browser"?

Also: "You can run on less expensive hardware, because you've got less technology going on under the covers."

You can't really call a full HTML5 stack light-weight. Sure, you save some libraries, but that's mostly a space issue. I sincerely doubt that a JS-based "Angry Birds" would use fewer resources than a native app.

Having said that, I'm really looking forward to this. Hope there's a way to get it on older Android phones soon, my Nexus One is waiting for new life…

That would be awesome if it actually became a mainstream mobile OS.

Otoh, what about security here? Having JS from a web site dial the phone or taking pics? How will they make sure that lazy I-don't-read-popups-but-just-click-okay users will not get pwned?

Dialing the phone and taking a picture work no differently than they currently do on iOS/Android - your app simply calls the device's native functions and then the OS takes over the operation. And once the operation is complete/cancelled, focus can return to your app if you set the proper callbacks.

Potential security concerns would be akin to the Chrome-extension snoop and inject issue where a random app or web-page could potentially try to read/write the data of other apps if the security model isn't setup right. But these are problems fixable in the short-term. On the plus side, as the phone owner, I might be able to redesign the OS dialer by changing some CSS and finally have a phone pad that goes 7-8-9/4-5-6/1-2-3 like a num-pad instead of 1-2-3/4-5-6/7-8-9.