Interesting, this seems more resource efficient than even fltk, which is the lightest usable cross-platform GUI library. And it's pure C unlike fltk.
It's great to see such projects in an era of unabashed resource consumption.
The price for the 'lightest' general UI toolkit probably goes to microui:
https://github.com/rxi/microui
Just around 1100 lines of C code.
You need to bring your own renderer, but that's the same for Nuklear or Dear ImGui.
I wrote a WASM wrapper for the microui demo too: