[2] HTMLの直列化、XMLの直列化。
[1] Editorial: rewrite send()'s body/content-type processing (domenic著, 4/23/2018, 3:09:11 PM) <https://github.com/whatwg/xhr/commit/f47bbab42dabe1f52e5e9f1ed1fa6df06a6eb310>
Google search: 文書の直列化