I think this is mostly answered in the README. This is probably the most relevant section:

> The structure validation is simplistic by necessity, as it defers to the type system: a few elements will have one or more required children, and any element which accepts children will have a restriction on the type of the children, usually a broad group as defined by the HTML spec. Many elements have restrictions on children of children, or require a particular ordering of optional elements, which isn't currently validated.

It's not complete or thorough at present.

Regarding your link: I'm impressed someone took the time to write a DTD for HTML5!