Also ran across these somewhat contradictory statements:

Visual rendering information for the currently-playing EPUB Content Document element MAY be expressed in the CSS Style Sheet using author-defined classes. These author-defined class names SHOULD be declared in the Package Document metadata using the metadata properties active-class and playback-active-class.

https://www.w3.org/publishing/epub/epub-mediaoverlays.html#sec-docs-assoc-style

The first sentence is clear that you don't have to supply the classes if you don't want, but the "should" in the second sentence is unclear. It could be read as contradicting the first (you should specify them), being non-committal about where to declare them (should be in the package document, but could be elsewhere?), or non-committal about the names (you should use these property names, but could use others?).

I expect, of course, it's none of the above and this could be reworded as a "must":

"When used, author-defined class names MUST be defined in the Package Document using the active-class and playback-active class properties."