Allow ids on structural elements.
Allow ids on structural elements.

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