[2] HTMLの直列化、XMLの直列化。

[1] Editorial: rewrite send()'s body/content-type processing (domenic著, 2018-04-24 00:09:11 +09:00) <https://github.com/whatwg/xhr/commit/f47bbab42dabe1f52e5e9f1ed1fa6df06a6eb310>