I've decided to release the first version of my GUI toolkit Azul for Windows. It is in an alpha state (not entirely finished), but it may be already useful for some people looking for a GUI library for Rust, Python or C. The C++ bindings still need some work.
There are still a lot of things to do (fixing the C++ header, adding more examples, fixing the TextInput widget, etc.), auto-tab / keyboard navigation, packaging, etc. However, the API itself is very stable now, I haven't removed almost any functions from the API since February. You can see an overview of the implemented CSS keys on the CSS styling page. They don't work exactly like in HTML, but the goal is to get it to be close enough so that the application can then run on wasm32 on the web (using native DOM nodes instead of using WebGL).
Overall I'm very happy with the API, if you want you can read the Application Architecture post about "why not Qt / GTK / IMGUI?". I think that reactive UI is a step forward for UI development in general.
For my own purposes (creating a cartography editor), I'm almost finished (just need some functions to NumberInput and a ProgressBar), but those widgets are not too hard to add. There was a post a few years ago that Azul would be unmaintained - that's not true, I just couldn't work at the library anymore at the time. In the past year I've taken up the work again and this is the result.
Anyway, have fun!