Currently the GPUPresentationContext interface is only exposed to Window contexts, certainly it should also be exposed to DedicatedWorkers (according to what was said in #403).

[Exposed=(Window,, DedicatedWorker)]

and related, while in multiple places it's said [[canvas]] can be an OffscreenCanvas, its definition states

[[canvas]] of type HTMLCanvasElement.

OffscreenCanvas should be added, though I'm not sure of the nomenclature here.