Favicons can be PNG files
Apparently all major browsers support using regular PNG files as favicon, so it is possible to happily ditch the .ico
file extension and format in favor of “pretty normal” PNG files.
Where I read it: https://sympli.io/blog/2017/02/15/heres-everything-you-need-to-know-about-favicons-in-2017/
More info on Wikipedia: https://en.wikipedia.org/wiki/Favicon