markNarration
Marks the element that href names as the one being read aloud, as EPUB Reading Systems 3.3, 9.2.3 asks (#525). The element gets the book's media:active-class, and the root element of its chapter gets the media:playback-active-class while playing. One element of the book holds the mark at a time, so a new call takes it off the last one. Null takes it off. A class that the document gave the element itself stays.
Only a chapter whose style rules use one of the two classes is laid out again, and chapterChanges moves. A chapter whose tree a script session holds is not marked.
Return
whether the style rules of the chapter use the active class, so the element shows the book's own style for it.