HtmlUnknownElement Class
https://html.spec.whatwg.org/multipage/dom.html#htmlunknownelement
public sealed class HtmlUnknownElement : HtmlElement- Inheritance
HtmlElementHtmlUnknownElement
https://html.spec.whatwg.org/multipage/dom.html#htmlunknownelement
public sealed class HtmlUnknownElement : HtmlElementHtmlElement HtmlUnknownElement