歴史
[1] Add about:html-kind for use in MP4 and DASH
(annevk著, )
https://github.com/whatwg/html/commit/56ea222aa35502dc2e8d7a83eb007e7f43559af7
[3] Use [Exposed=Window] even more
(annevk著, )
https://github.com/whatwg/html/commit/f2a32c045aa7764533cda58b4452fe2bbc6652a0