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: https://floooh.github.io/sokol-html5/sgl-microui-sapp.html
https://floooh.github.io/sokol-html5/
Interestingly the imgui and nuclear both work on mobile while the microui demo doesn't appear to handle any input. Imgui works surprisingly well (although everything is a bit small for touch).