[1] offsetParentプロパティ (11/20/1999, 8:57:32 PM 版) <http://homepage1.nifty.com/kodayan/dhtm/css/position20.htm>
[2] [cssom-view] Update offsetParent spec for Shadow DOM (TakayoshiKochi著, 8/11/2016, 8:48:51 AM) <https://github.com/w3c/csswg-drafts/commit/9386def5f11fe47ffbaa8cda2ddd9c204a6a07cf>
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