They're standardized now: https://html.spec.whatwg.org/multipage/scripting.html#custom-elements https://html.spec.whatwg.org/multipage/scripting.html#custom...

They're standardized now: https://html.spec.whatwg.org/multipage/scripting.html#custom-elements https://html.spec.whatwg.org/multipage/scripting.html#custom... https://dom.spec.whatwg.org/#shadow-trees https://dom.spec.whatwg.org/#shadow-trees