From github user Bluebie
"I'm particularly interested in bringing good solid HTML5 support to it (using html5 syntax, not xml syntax, for compatibility and compliance reasons)"
unsopported html5 tags include: abbr, article, aside, audio, canvas, datalist, details, figure, footer, header, hgroup, mark, menu, meter, nav, output, progress, section, time, video. These would be a good start.
See my HTML5 branch and pull request!