Re: [whatwg/dom] Standardize certain Microsoft event extensions (#334)

`returnValue` and beforeunload is kind of messy. See some recent fixes for it in https://github.com/whatwg/html/pull/2353. /cc @cdumez, who is landing patches to align WebKit with the spec/Gecko in that regard.

My understanding is that the HTML spec is currently based on a world where returnValue does not exist on Event, and does exist on BeforeUnloadEvent. Gecko, and WebKit as of @cdumez's recent changes, implement that part of the HTML spec percectly; Chrome does so imperfectly, see https://bugs.chromium.org/p/chromium/issues/detail?id=692203. (I'm not sure about Edge because I cannot run tests on beforeunload there due to their longstanding issues where the outer frame has issues reaching into other iframes; see e.g. their inability to pass https://github.com/w3c/web-platform-tests/pull/4800 tests.)

I think it's possible we could try to aim for a world where all the returnValue weirdness lives in the base Event class, and BeforeUnloadEvent perhaps even disappears entirely. But we should outline concretely what that looks like, and get as many vendors as possible on board with such a plan, before causing more churn.

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