This UI library can be re-targeted to Raylib, Canvas, HTML from its markup ... Good luck doing that with just CSS.
Put another way, this library is a step into writing an agnostic UI layer for your application which it just invokes from its business logic. If you can decouple your UI from your application code ala MVC or MVP architecture, as a developer you can find the right tool for the components that compose the sum of your project.
Why are all these tools written in python? I'm not complaining, sometimes I'd like to see such a tool in Php or Bash or something where say my project doesn't need to include yet another Language dependency ... Doesn't SQLite cli for example ship as a bash or win32 CMD file...?
Put another way, this library is a step into writing an agnostic UI layer for your application which it just invokes from its business logic. If you can decouple your UI from your application code ala MVC or MVP architecture, as a developer you can find the right tool for the components that compose the sum of your project.