Spec updates for new header content model.
Spec updates for new header content model.

oreillymedia/HTMLBook,oreillymedia/HTMLBook,oreillymedia/HTMLBook,gitenberg-dev/HTMLBook,wastemobile/HTMLBook,gitenberg-dev/HTMLBook,wastemobile/HTMLBook,macmillanpublishers/HTMLBook,macmillanpublishers/HTMLBook,gitenberg-dev/HTMLBook,mushroomgithub/HTMLBook,mushroomgithub/HTMLBook,mushroomgithub/HTMLBook,macmillanpublishers/HTMLBook