Contradictory epub:type usage statements #2434

In the Structural semantics section we say:

As the [html] head element contains metadata for the document, structural semantics expressed on this element or any descendant of it have no meaning.

But then if you dig down into the epub:type attribute declaration it says:

Global attribute. It MAY be used in [html] and [svg] with the following restrictions:

For [html] content: it MUST be used on palpable content.

We added this restriction to match the requirement that reading systems ignore semantics in the header, but I noticed in epubcheck that this has never been enforced. (There's also a second contradiction in calling it a global attribute and then spelling out the only places it's allowed.)

I'm slightly concerned that we may introduce backward incompatibilities for content with this change if I request that epubcheck change its behaviour, although there aren't any semantics in the structure vocabulary that make any sense in the HTML head. But, assuming we keep the restrictions, we need to harmonize what we're saying across these sections so they don't contradict themselves.