You are somewhat right in that HTML and CSS heavily inspired what kiUi became. But there are two major concerns that are not adressed by your solution :

- Embedding a whole browser in your app just to display a few widgets is not my definition of "lightweight"

- Often coding UI in native code (C++) makes a lot more sense, integrate with exiting code in a much cleaner way

Game developpers would much less tend to build their own libraries from scratch, if there was at least one that satisfied all of these 'basic' requirements : lightweight, skinnable, auto-layout