[1] offsetParentプロパティ (1999-11-21 05:57:32 +09:00 版) <http://homepage1.nifty.com/kodayan/dhtm/css/position20.htm>
[2] [cssom-view] Update offsetParent spec for Shadow DOM (TakayoshiKochi著, 2016-08-11 17:48:51 +09:00) <https://github.com/w3c/csswg-drafts/commit/9386def5f11fe47ffbaa8cda2ddd9c204a6a07cf>