Apparently, there’s no declarative way to offer an HTML fallback message when all of the audio or video sources in a media object are unusable. Instead, it has to be done with JavaScript.

This parameter identifies the JavaScript file to load for this support. It must define a global function named docbook_object_fallback that takes the source node as a paramter.

To disable fallback, set this parameter to the empty sequence or the empty string.

Place this script in the head of the document and do not defer loading it.