Given I don't like i.e. a favicon and want my system to have i.e. https://kai.dj/favicon.ico pointing to https://git.kai.dj/favicon.ico instead. Of cource I wouldn't like to reroute the whole domain (via /etc/hosts etc.).
Changing favicon is just an example – browser plugins may be a solution for the example usecase, but don't answer the general question.
How can I implement a system wide rule to reroute a URL pointing to a single file for that file only?