[1] Web Forms 2.0 (2009-01-05 20:07:15 +09:00 版) <http://www.whatwg.org/specs/web-forms/current-work/#labels>

[2] Remove <keygen> (domenic著, 2017-01-14 07:03:02 +09:00) <https://github.com/whatwg/html/commit/5baa38720f6e83c94a50018c4565808ad548d69c>

[3] Make the labels IDL attribute return null for <input type=hidden> (zcorpan著, 2017-02-16 02:42:29 +09:00) <https://github.com/whatwg/html/commit/596174367314ebc1b935d09e0ca9b66c9362977a>