[clusmore]

I'd love to see you try serving the exact same file for both but the trick is that you need to return different Content-Type headers depending on what is requested. When the browser requests /favicon from a navigation event it will use Accept: text/html etc, you return the file with Content-Type: text/html and inside the response you have a <link rel="favicon" href="/favicon" type="img/png"> literally the same resource but the browser will now likely fetch with Accept: image/... and you could return the same file with Content-Type: image/png and the same resource will get used for both. Unless the browser caches the response, I feel like this would work.

If you don't control the headers of your webserver (eg GitHub Pages) I would settle for a symlink favicon.png that just links back to favicon.html which I think would trick the server into returning different Content-Types.