It's buggy. For instance, I can see an anchor inside a span, but the span is closed before the anchor is closed. Also, and admittedly nitpicking a little here (don't hate me 4Chan! I come in peace...) but they are still using CDATA in their script tags. If you are using HTML5, I don't think you still need to support legacy browsers :-)
HTML5 is supposed to degrade gracefully. Nothing about it should instantly break old browsers, so there's nothing really inconsistent about using CDATA with HTML5.
Like I say: I'm nitpicking. To be honest, I'm not a fan of inline javascript. But I'm not really 4chan's core audience - they may have a reason why they have so much inline js.
