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

[2] Remove the valueAsDate branch from valueAsNumber (annevk著, 2016-07-06 15:28:28 +09:00) <https://github.com/whatwg/html/commit/4ac4e10c8d5cc7fad67190365e4f823b64ad085d>