[dom] DOM should add hooks for sync events that happen during insert/remove operations (#57)

As discsussed at Custom Elements F2F:
* blur
* focus
* beforeunload
(other?)
...events require some synchronous processing during tree insertion and/or removal
(This may have been discussed during the part of the meeting where there was nothing scribed...?)

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/57