[1] [CITE@en[Compound Document by Reference Framework 1.0]]
( ([TIME[2010-08-17 16:50:04 +09:00]] 版))
<http://www.w3.org/TR/CDR/#parent-to-child-dom-access>

[2] [CITE@en-US-x-hixie[HTML Standard]]
( ([TIME[2012-12-05 05:38:42 +09:00]] 版))
<http://www.whatwg.org/specs/web-apps/current-work/#dom-iframe-contentdocument>

[3] [CITE@en[Web Applications 1.0 r8272     Change iframe, frame, and object.contentDocument to return null when cross-origin documents are involved.]]
( ([TIME[2013-11-13 06:50:00 +09:00]] 版))
<http://html5.org/tools/web-apps-tracker?from=8271&to=8272>

[4] [CITE@en[829872 – Consider returning null from contentDocument getters when the caller does not subsume the document]]
( ([TIME[2013-11-13 20:41:52 +09:00]] 版))
<https://bugzilla.mozilla.org/show_bug.cgi?id=829872>