On the other hand, the world would really benefit from someone updating the libxml2 HTML parser to match the HTML(5) spec, since it has popular bindings for man

On the other hand, the world would really benefit from someone updating the libxml2 HTML parser to match the HTML(5) spec, since it has popular bindings for many other languages (e.g. lxml in python). The current implementation is broken in lots of (not so) edge cases. This can be a problem when there is the choice of being fast and wrong (using libxml2) or being slow and correct (using html5lib or one of the other high level implementations of the standardised algorithm).