[4] 次の場面で使われます。
[3] Percent encode fragments too (annevk著, ) <https://github.com/whatwg/url/commit/373dbedbbf0596f723ce8a195923da98b698aeb0>
[5] Editorial: use specific names for encode sets (annevk著, ) <https://github.com/whatwg/url/commit/1d69a774bab856651e0d792e825a976056aeaf59>