Users going through the book on safari, at least, get a bunch of annoying errors about favicon missing on each page load:

[debug] ** (Phoenix.Router.NoRouteError) no route found for GET /apple-touch-icon-precomposed.png (TunezWeb.Router)
    (tunez 0.1.0) deps/phoenix/lib/phoenix/router.ex:465: TunezWeb.Router.call/2
    (tunez 0.1.0) lib/tunez_web/endpoint.ex:1: TunezWeb.Endpoint.plug_builder_call/2
    (tunez 0.1.0) deps/plug/lib/plug/debugger.ex:155: TunezWeb.Endpoint."call (overridable 3)"/2
    (tunez 0.1.0) lib/tunez_web/endpoint.ex:1: TunezWeb.Endpoint.call/2
    (phoenix 1.8.0-rc.3) lib/phoenix/endpoint/sync_code_reload_plug.ex:22: Phoenix.Endpoint.SyncCodeReloadPlug.do_call/4
    (bandit 1.7.0) lib/bandit/pipeline.ex:131: Bandit.Pipeline.call_plug!/2
    (bandit 1.7.0) lib/bandit/pipeline.ex:42: Bandit.Pipeline.run/5
    (bandit 1.7.0) lib/bandit/http1/handler.ex:13: Bandit.HTTP1.Handler.handle_data/3
    (bandit 1.7.0) lib/bandit/delegating_handler.ex:18: Bandit.DelegatingHandler.handle_data/3
    (bandit 1.7.0) lib/bandit/delegating_handler.ex:8: Bandit.DelegatingHandler.handle_continue/2
    (stdlib 6.2.2) gen_server.erl:2335: :gen_server.try_handle_continue/3
    (stdlib 6.2.2) gen_server.erl:2244: :gen_server.loop/7
    (stdlib 6.2.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

[debug] ** (Phoenix.Router.NoRouteError) no route found for GET /apple-touch-icon.png (TunezWeb.Router)
    (tunez 0.1.0) deps/phoenix/lib/phoenix/router.ex:465: TunezWeb.Router.call/2
    (tunez 0.1.0) lib/tunez_web/endpoint.ex:1: TunezWeb.Endpoint.plug_builder_call/2
    (tunez 0.1.0) deps/plug/lib/plug/debugger.ex:155: TunezWeb.Endpoint."call (overridable 3)"/2
    (tunez 0.1.0) lib/tunez_web/endpoint.ex:1: TunezWeb.Endpoint.call/2
    (phoenix 1.8.0-rc.3) lib/phoenix/endpoint/sync_code_reload_plug.ex:22: Phoenix.Endpoint.SyncCodeReloadPlug.do_call/4
    (bandit 1.7.0) lib/bandit/pipeline.ex:131: Bandit.Pipeline.call_plug!/2
    (bandit 1.7.0) lib/bandit/pipeline.ex:42: Bandit.Pipeline.run/5
    (bandit 1.7.0) lib/bandit/http1/handler.ex:13: Bandit.HTTP1.Handler.handle_data/3
    (bandit 1.7.0) lib/bandit/delegating_handler.ex:18: Bandit.DelegatingHandler.handle_data/3
    (bandit 1.7.0) lib/bandit/delegating_handler.ex:8: Bandit.DelegatingHandler.handle_continue/2
    (stdlib 6.2.2) gen_server.erl:2335: :gen_server.try_handle_continue/3
    (stdlib 6.2.2) gen_server.erl:2244: :gen_server.loop/7
    (stdlib 6.2.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

While obviously not your fault, and easily fixable by the reader who knows what is going on, it might be worth the effort to add the various favicons that browsers attempt to fetch into the GitHub repo. I’m sure other browsers do the same too.