Some ImageBitmapSource variants unhandled

The ImageBitmapSource union is defined as,

typedef (HTMLImageElement or
         SVGImageElement) HTMLOrSVGImageElement;

typedef (HTMLOrSVGImageElement or
         HTMLVideoElement or
         HTMLCanvasElement or
         ImageBitmap or
         OffscreenCanvas) CanvasImageSource;

typedef (CanvasImageSource or
         Blob or
         ImageData) ImageBitmapSource;

This specification does not have cases covering the Blob, OffscreenCanvas or SVGImageElement cases for this union. The implementation in Chromium handles OffscreenCanvas in the most obvious fashion but not the other two.

Handling of Blob inputs is particularly tricky because it requires first decoding the blob. For practical purposes developers should simply call createImageBitmap() with the Blob and pass the resulting ImageBitmap to the detect() method.

It may be best to simply redefine handling of this union in terms of createImageBitmap().

Start with the ImageBitmapSource union and the specification's existing cases, then compare them with the Chromium behavior mentioned in the issue. Consider how createImageBitmap() relates to Blob, OffscreenCanvas, and SVGImageElement inputs. Done means the specification explicitly covers the missing variants or clearly defines the intended handling.