Piet is the backend for the Druid UI toolkit. It is indeed cross-platform. The SVG backend looks like it would work without an OS, but I'm not sure the value of a GUI toolkit without an OS anyway :)
A GPU toolkit without an OS API could be useful for WASM rendering, and a rendering library without a GUI is useful for headless rendering (matplotlib graphing, video generation/compositing, meme generators, etc.).
https://github.com/linebender/piet#backends
https://github.com/linebender/druid/tree/master/druid-shell/...