These will help us write node-type-aware template functions.
We now show a tooltip for the hovered node's enclosing HTML element's title attribute, if one is present. This patch also adds HTMLHeadingElement. The tags h1-h6 will now create the right kind of objects.
HtmlView now calls Node::enclosing_link_element() to find the nearest ancestor <a> element. This patch also adds HTMLElement and HTMLAnchorElement.