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Description
https://w3c.github.io/webdriver/#close-window calls HTML's https://html.spec.whatwg.org/#close-a-browsing-context, but as far as I'm aware that doesn't synchronously close the window. I think in most implementations by the point that it returns it always will have closed the window, but the spec doesn't seem to guarantee this?
(Note that for HTTP-only sessions before unload is never fired, thus I believe it should always eventually succeed.)