A good explanation of HTML5’s sectioning content and outline algorithm.
A brave attempt to explain the new outline algorithm in HTML (although it inaccurately states that no browsers have support for it—Firefox shipped with it a while back).
You can safely skip the comments: most of them are discouraging, ignorant, and frankly, just plain stupid.
Mike takes on the very tricky task of explaining the new outline algorithm—definitely one of the hardest features of HTML5 to explain, in my opinion.
A very handy tool to help you check the outline algorithm in HTML5.