https://w3c.github.io/epub-specs/epub33/rs/#sec-xhtml-custom-attributes says: To facilitate interoperability of custom attributes across Reading Systems, vendors are strongly encouraged to document any extensions they implement in [AttributeExtensions].

That links to https://idpf.github.io/epub-extensions/attributes/, dated October 2016, which says No custom attributes have been defined at this time.

I think we should either document all known custom attributes (on behalf of the vendors), or should remove this recommendation. In the current form it's a frustrating experience and looks embarrassing.