Azul[1] is my solution for that, it is based on WebRender, so basically the same as XUL, but in a more modern way. I didn't get around to finish it in 2019, but

Azul[1] is my solution for that, it is based on WebRender, so basically the same as XUL, but in a more modern way. I didn't get around to finish it in 2019, but I will work on it this year, maybe I'll get it to be mature enough to post it here. > wide portability (at least Windoze, Linux, MacOS, iOS, Android, embedded: Azul is Windows-Linux-Mac only, don't underestimate the effort to properly port something to a new platform > "though a Vulkan-based renderer can be made to run pretty much anywhere": WebRender is OpenGL + using software rendering as a fallback > a permissive open source license: MPL-2.0 > a C interface/wrap to allow a wide programming language binding support: yes > and an easily extensible and themable set of basic widgets: also yes, but the CSS styling works a bit differently depending on whether you want convenience or speed Check the screenshots[2], I personally use it for developing GIS applications. [1] https://azul.rs/ https://azul.rs/ [2] https://github.com/fschutt/azul/releases/tag/1.0.0-alpha1 https://github.com/fschutt/azul/releases/tag/1.0.0-alpha1