[einpoklum]

How can such a library be both tiny and portable, when the C standard library has no graphics facilities? Don't you need to lay down a lot of basis for different platforms and graphics backends, to be portable? And if you do that, how can you be tiny?

[b3orn]

Almost all of these immediate mode UI libraries for C/C++ come without a rendering backend. Usually there are "example" implementations for libraries like SDL that you can use, or you implement your own backend. A lot of these libraries are popular for debug UIs in games and you probably need a custom backend for whatever graphics engine you use in your game.