[3] 要素は、カスタム要素状態を持ちます。
カスタム要素状態は、 undefined, failed,
uncustomized, custom
のいずれかです。 >>2
[5] 要素のカスタム要素状態は、要素の作成や要素の格上げや
HTMLElement コンストラクターや養子化で設定されます。
[4] 要素のカスタム要素状態が uncustomized か custom
のとき、要素は定義済みであるといいます >>2。
[1] Add custom elements to HTML · whatwg/html@6e7eaa4 ( 版) https://github.com/whatwg/html/commit/6e7eaa4bd2912965fd83766f99f984f249531f3a
[6] Don't set custom element state redundantly ( (domenic著, )) https://github.com/whatwg/dom/commit/d077da16b7fc307fc51820dbe541f6cf0b088115
[7] Un-customify adopted elements ( (domenic著, )) https://github.com/whatwg/dom/commit/e417f9f4ca5aa16f11b8e04378f8c3425554b0fd
[8] Set the stage for preventing double custom element constructor calls (domenic著, ) https://github.com/whatwg/dom/commit/539e45214f682797f4fa022815a08b6f044e1240
[9] [selectors-4] :defined · Issue #2258 · w3c/csswg-drafts () https://github.com/w3c/csswg-drafts/issues/2258
[10] Which standard defines :defined · Issue #665 · w3c/webcomponents () https://github.com/w3c/webcomponents/issues/665
[11] Remove the data-dfn-type attribute for the :defined pseudo-class by isaacdurazo · Pull Request #4433 · whatwg/html · GitHub () https://github.com/whatwg/html/pull/4433