[1] Safari Dev Center: Safari HTML Reference: Supported Attributes ( 版) <http://developer.apple.com/safari/library/documentation/AppleApplications/Reference/SafariHTMLRef/Articles/Attributes.html#//apple_ref/doc/uid/TP40008058-keytype>
[2] Remove <keygen> (domenic著, ) <https://github.com/whatwg/html/commit/5baa38720f6e83c94a50018c4565808ad548d69c>