@Hermanverschooten - Oh! Interesting hint - yes, indeed!
I just checked it and took a color class that I for sure never had before in this project - and yes… that’s also not working without mix phx.digest and restart …
… and that might also be the reason why I constantly think I suck in css 
Can you check this in your config files?
in config/config.exs there should be a block like this:
config :tailwind,
version: "3.3.5",
default: [
args: ~w(
--config=tailwind.config.js
--input=css/app.css
--output=../priv/static/assets/app.css
),
cd: Path.expand("../assets", __DIR__)
]
and config/dev/exs
config :your_app, YourAppWeb.Endpoint,
# Binding to loopback ipv4 address prevents access from other machines.
# Change to `ip: {0, 0, 0, 0}` to allow access from other machines.
http: [
ip: {0, 0, 0, 0},
port: 4000,
http_options: [
log_protocol_errors: false
]
],
check_origin: false,
code_reloader: true,
debug_errors: true,
secret_key_base: "...secret...hidden",
watchers: [
# Start the esbuild watcher by calling Esbuild.install_and_run(:default, args)
esbuild: {Esbuild, :install_and_run, [:default, ~w(--sourcemap=inline --watch)]},
tailwind: {Tailwind, :install_and_run, [:default, ~w(--watch)]}
]
Especially the :watchers part ,that’s what lets tailwind do its work.
Yes, pretty much like that:
# --------------------------
# TAILWIND CONFIG
# --------------------------
config :tailwind,
version: "3.3.2",
default: [
args: ~w(
--config=tailwind.config.js
--input=css/app.css
--output=../priv/static/assets/app.css
),
cd: Path.expand("../assets", __DIR__)
]
and…
config :my_app, MyAppWeb.Endpoint,
# Binding to loopback ipv4 address prevents access from other machines.
# Change to `ip: {0, 0, 0, 0}` to allow access from other machines.
http: [ip: {127, 0, 0, 1}, port: 4000],
check_origin: false,
code_reloader: true,
debug_errors: true,
secret_key_base: "...secret...",
watchers: [
esbuild: {Esbuild, :install_and_run, [:default, ~w(--sourcemap=inline --watch)]},
tailwind: {Tailwind, :install_and_run, [:default, ~w(--watch)]}
]
Its watching - when saving files, its immediately logging…
Done in 74ms.
Okay that seems to be doing what it should.
Could you show me the line in your HTML header where you link to your app.css?
These are the relevant lines…
...
<link phx-track-static rel="stylesheet" href={~p"/assets/app.css"} />
<script defer phx-track-static type="text/javascript" src={~p"/assets/app.js"}>
</script>
...
That also seems correct.
could you stop your app and do mix phx.digest.clean --all, then restart your app.
Do you have your assets now? Or are they missing?
@Hermanverschooten - Oh! WOW 
- that actually did it… there was then obviously sth. off…
Now changes are applied immediately correctly on file saving !!!
Thanks!!! 
… did not know that there is a --all parameter 
I think Phoenix uses the fingerprinted files if they are there.
You normally never do a phx.digest in development, that is pure for production.
I assume this would work for static heroicons, but if I wanted to pull the heroicon names from a database or another source, mix tailwind default will not land what I need in app.css. I.e., dynamically setting the icon is not an option. Any advice on that, please?