Improve HTML support
HTML support is given by libxml2 wrappers currently.
HTML is not an easy task due to there are many relaxed situations not compared with XML, because not always is a valid XML document, but one usable by browsers.
DOM4 have lot of features usable on HTML, so adding better support maybe is a natural step while implenting them.
Some DOM4 features, can used by other projects like GSVG to react on user interaction or tree changes, using an uniform API.