// https://html.spec.whatwg.org/multipage/canvas.html#offscreenrenderingcontext
typedef (OffscreenCanvasRenderingContext2D or 
         WebGLRenderingContext or 
//FIXME: ImageBitmapRenderingContext or
         WebGL2RenderingContext 
//FIXME: GPUCanvasContext
         ) OffscreenRenderingContext;