Add trait-based virtual methods for commonly-overridden methods in derived DOM element types.
Integrate Element's after_set/remove behaviour into the virtual method hierarchy.
Add overridable tree binding/unbinding behaviour.
Make find_fragment_node a method of Page.
Initial spec-incompliant implementation of default click action for anchor elements.
Add documentation; gut the new document URL loading method and move it all into the new Window method.
Add test for default event prevention.