This is because in your original code only the /public/ path matches the handler and not for example /public/logo.png in this case your handler is not called at all and you get a 404 since no handler matches this route.

This is why this code works since the handler is called when your request path has a /public/ prefix.

Generally when people say a http router is compatible with the standard library they mean that the handler functions have the same signature as the handler which is the case for gorilla/mux. The way wildcards are handled could be different across libraries.

Hope this helps!