How can I find out what UI framework the Amazon Cloud Player is using. At least for the PC, is it just a shell that is running HTML5/JavaScript ? If so is it using a standard UI framework? The fluidity and responsiveness of the UI seems remarkable to me, compared to any other PC based, iTunes like, music players.
The Amazon Cloud Player uses a lot of Open Source components. Just open the player and open Help > About Amazon music > Legal notice
In my case the following link was opened http://www.amazon.de/gp/help/customer/display.html?ie=UTF8&nodeId=201415470&pop-up=1 The page gives a good overview of what is used.
Among the components are jQuery and Twitter Bootstrap. Probably Twitter bootstrap is the UI framework you are looking for.