MicroUI

MicroUI emits a list of draw commands and demands input back; it draws nothing and allocates nothing beyond a fixed memory region. That's how it stays portable enough to compile to WebAssembly and run in a browser.

One longtime user called it his go-to for toy projects but flagged it as effectively abandonware, with a known draw-call bug that does a misaligned pointer access — harmless until you run it under Zig, which panics on exactly that.