Another recent complication is serving different favicons for browsers running a dark mode. For example, somehow nytimes.com inverts theirs on my browser and it actually looks great. Sites need to be aware that a dark icon over a transparent background will be invisible in dark mode.
I don't know if there's a good solution to this. It's unfeasible to require developers to provide two icons, particularly in places that are mothballed. Should browsers always render favicons against a light/white background?