Throughout its history, HTML has always been a disaster when it comes to cross browser interoperability. Apps have always been desinged for the vogue browser, which has changed from Netscape to IE to webkit now.

The more you abstract, the more functionality you lose and the more generic your mobile apps will look. Facebook learned this lesson the hard way when they tried to make their mobile App in HTML5.

CSS, javascript, and DOM are just kudge piled upon kludge. HTML5 trying to shoehorn poorly thought out technologies created for hyperlinked static text documents into dynamic applications.

Until some disruptive technology completely replaces that massive HTML5 spec you speak of, the natives apps will always rule the mobile space and performance critical spaces.

I suspect you get portability to Firefox for Android for (nearly) free. That's a decent start at the very least (and an important proof-of-concept for other Android browsers).

The point is the goal is to make all of these API's standardised. Of course you will always have portability issues if you use the newest possible API's as someone will always be first out with an implementation. By the time FirefoxOS has any traction, it is likely a lot of these API's will have additional implementations and/or suitable "polyfills". That'd already put it in a better position out of the gate than Android or iOS in terms of portability.