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.
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.