[tenken]

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.