Implement restrictions on the children of Document nodes.
Introduce HTMLOutputElement::new.
Introduce HTMLOptGroupElement::new.
Introduce HTMLOptionElement::new.
Introduce HTMLOListElement::new.
Introduce HTMLObjectElement::new.
Introduce HTMLModElement::new and use it for del/ins, not the non-existent mod.
Introduce HTMLMeterElement::new.
Introduce HTMLMetaElement::new.
Introduce HTMLMainElement::new.
Introduce HTMLMapElement::new.
Introduce HTMLLIElement::new.
Introduce HTMLLinkElement::new.
Introduce HTMLLegendElement::new.
Introduce HTMLLabelElement::new.
Introduce HTMLInputElement::new.
Introduce HTMLHtmlElement::new.
Introduce HTMLHeadElement::new.
Introduce HTMLHRElement::new.
Introduce HTMLFrameSetElement::new.