Recent interest in the HTML favicon prompted me to move its specification to the
publication file.
This feels like a publisher activity, marking HTML output in consistent ways.
But the actual icon files go in the author's source, so maybe this is a bit of a
contradiction. Anyway, changes will allow for more (future) flexibility in
implementing these.
To migrate, remove the empty #favicon element from your #docinfo, and adjust
your publication file to look like:
<html favicon="simple">
....
Until then you will get warnings, and the old way will continue to be supported.
So there is no great urgency (yet).
More, including a very brief explanation of what the heck a favicon is:
Subsection 29.1.3: HTML Favicon Configuration
https://pretextbook.org/doc/guide/html/html-publisher-options.html#online-favicon-configuration
Rob