> Nothing beats that in terms of performance and small size
Native controls do. For example, Steve Gibson wrote a DNS benchmarking utility[1] and the executable is 159k. No matter what you do, an HTML & CSS is still being rendered by a slow interpreter.
BTW, when I clicked on that link for HTML GMail, Google made me click through a warning trying to scare me off. I'm not sure why they would care if I use the HTML version.
In the end, I don't think the speed of CSS + HTML controls matters much. It's limiting and not a great choice for an operating system UI toolkit.