I have followed all of the documentation for building a custom module, it all seems pretty straight forward. However, I seem to be missing a couple of small things.
No CSS rules placed in my-plugin/my-module/css/frontend.css (example file path) are not being found, or even loaded. However if they are placed in my-plugin/my-module/includes/frontend.php under a style tag they work fine.
What am I missing here?