parallel selector matching
Introduce HTML{Audio,Video}Element::new.
Introduce HTMLTable{Data,Header}CellElement::new.
Introduce HTMLHeadingElement::new.
Introduce HTMLIFrameElement::new.
Introduce HTMLImageElement::new.
Introduce HTMLUListElement::new.
Introduce HTMLTrackElement::new.
Introduce HTMLTableRowElement::new.
Introduce HTMLTitleElement::new.
Introduce HTMLTimeElement::new.
Introduce HTMLTextAreaElement::new.
Introduce HTMLTemplateElement::new.
Introduce HTMLTableSectionElement::new.
Implement basic overflow computation. Closes #1148.

This adds just 4 ms out of ~120 ms on the rainbow page.
Introduce HTMLTableColElement::new.
Introduce HTMLTableCaptionElement::new.
Introduce HTMLTableElement::new.
Introduce HTMLStyleElement::new.
Introduce HTMLSpanElement::new.