As alluded in the comments on that bug, people sometimes actually use animated gifs but almost nobody does the animation via CSS. That said I can't even remember the last time I saw an animated favicon.

I made this for a catering service

http://img.go-here.nl/favicon.gif

Hilarious and cute at the same time.

wonderful

JS modifying favicon would be a bug.

AFAIU Faviocon was meant to differentiate browser bookmarks and tabs to make it easier to tell which is what.

In March 1999, Microsoft released Internet Explorer 5, which supported favicons for the first time.[4] Originally, the favicon was a file called favicon.ico placed in the root directory of a website. It was used in Internet Explorer's favorites (bookmarks) and next to the URL in the address bar if the page was bookmarked.

https://en.wikipedia.org/wiki/Favicon#History

The fundamental problem is that tabs are already a misfeature of sorts.

https://old.reddit.com/r/dredmorbius/comments/256lxu/tabbed_...

Depending on the browser’s favicon parsing, you can do some odd things with it by adding a new <icon> element. I was looking at this the other day and was able to get a basic example of blinking it together (the “What about the favicon?” section from https://alexanderell.is/posts/attention-javascript/).

I swear the more I see of such things the more Gemini appeals.

Principle of minimum privilege and execution.