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https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custom/index.html#aw-snap "Well this stinks. I was planning to just process a declaration here, but transferring callbacks may run script, which ain't being nice to Mr. Parser."
The answer is obviously another microtask.
https://dvcs.w3.org/hg/webcomponents/rev/879790e093f8