getContext('worker') returns a CanvasTransferableRenderingContext and sets /inhibited canvas/ mode
/inhibited canvas/ mode (disables the toBlob* and toDataURL* methods, height/width resetting)

interface CanvasTransferableRenderingContext {
  attribute unsigned long height;
  attribute unsigned long width;

  DOMString toDataURL(optional DOMString type, any... arguments);
  DOMString toDataURLHD(optional DOMString type, any... arguments);
  void toBlob(FileCallback? _callback, optional DOMString type, any... arguments);
  void toBlobHD(FileCallback? _callback, optional DOMString type, any... arguments);

  object? getContext(DOMString contextId, any... arguments);
  boolean supportsContext(DOMString contextId, any... arguments);
};
CanvasTransferableRenderingContext implements Transferable;

CanvasTransferableRenderingContext's "neutering" behaviour is to make all the attribute
and methods start throwing on getting/setting and invocation respectively.
   
CanvasTransferableRenderingContext.getContext, when it is invoked, neuters
the CanvasTransferableRenderingContext.    

XHR gets a new responseType 'image' which, in the main thread, returns an
HTMLImageElement, and in workers, returns an opaque Image object        

In the worker thread:
* CanvasRenderingContext2D.canvas returns an Image object representing the canvas     
* Anything that takes an HTMLImageElement is extended to also take an Image
* CanvasRenderingContext2D.draw*FocusRing() methods can't be invoked
* scrollPathIntoView() has no effect
* anything derived from computed CSS values uses defaults