As a dev on Firefox OS I had been trying to disuade us from promoting / people from using the 'Building Blocks' ui library, it makes sense for the OS itself to use it but for 3rd parties "I hope this becomes a bygone of separate apps per platform"

I lost that, but I think it will come around as the platform matures, I will always just be building web apps

Your problem is that browsers are a very restrictive sandbox (so you can't extend stuff yourself, you have to wait for the browser vendors to implement it) and the standards process is glacially slow. People seem to want to use the web as a platform for general purpose software. The current standards are not good enough. So inevitably people look to browser specific features because they want to make stuff that actually works now rather than waiting possibly years for features to be standardised and implemented in every browser. It seems very unrealistic to expect people to cripple their software just to stay 'standards compliant'. History seems to indicate that people will use the available tools to solve their problems. A few purists will shout from the sidelines while everybody else gets on with doing real work.

To put it another way: targeting one browser gives you a competitive advantage because that is where cutting edge and interesting features are, and that isn't going to change any time soon unless there is a big change to the standards process. Hence "Made for Chrome", "Best viewed on Chrome" and soon "Made for FirefoxOS".