Nuklear: A small ANSI C GUI toolkit
Shame that you specify pixels in the function calls themselves.  If it were really abstracted, it would have an arbitrary 'size unit' which the glue code could translate on the backend.In a world where one user has a 1366x768 screen and her neighbor has a 4k display, we need to move past interface code specifying pixel sizes up front, both on the web and off.