This is not a regression from bug 1360715, this is more of the node adoption behavior rather than of the instanceof behavior. `el2 instanceof HTMLElement` is true on Gecko too before `el.appendChild(e2)`, but the adoption somehow magically changes the `el2`'s prototype, which is very surprising.
Bug 1821790 Comment 3 Edit History
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This is not a regression from bug 1360715, this is more of the node adoption behavior rather than of the instanceof behavior. `el2 instanceof HTMLElement` is true on Gecko too before `el.appendChild(e2)`, but the adoption somehow magically changes `el2`'s prototype, which is very surprising.