History of beforeunload

Added specifications table, related 'Event' tag and 'NeedsCompatTable' tag

Removed "See also" link to self.

de-macro

Chrome behavior changed more than a year ago

minor typo: another alternative would be to change it to "example below"

It doesn't make any sense to use `e || window.event` within `element.addEventListener()` since all browsers that support `addEventListener` do not require `window.event` workaround.

IE[9] will display a prompt with an empty string if `e.returnValue` is assigned an empty string, but Chrome [43] treats empty strings as void and will not prompt.