I don't think this a correct interpretation of the spec. Yes the property is marked `readonly` so doing something like `res.header = headers` is invalid. But the returned object is not marked `[Frozen]` so modifying it is still supposed to possible.
Bug 1684634 Comment 5 Edit History
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I don't think this a correct interpretation of the spec. Yes the property is marked `readonly` so doing something like `res.header = headers` is invalid. But the returned object is not marked `[Frozen]` so modifying it is still supposed to be possible.