[1] Web Forms 2.0 ( 版) <http://www.whatwg.org/specs/web-forms/current-work/#labels>
[2] Remove <keygen> (domenic著, ) <https://github.com/whatwg/html/commit/5baa38720f6e83c94a50018c4565808ad548d69c>
[3] Make the labels IDL attribute return null for <input type=hidden> (zcorpan著, ) <https://github.com/whatwg/html/commit/596174367314ebc1b935d09e0ca9b66c9362977a>