You can read and output compiled app.css file (or other target if you have one). This should work in development and in prod, also with Erlang releases. In your layout or template do:

  <style type="text/css">
    <%= {:safe, File.read!(Path.join(:code.priv_dir(:ui), "static/css/app.css"))} %>
  </style>

And it will output the compiled CSS in place, provided your compiled file ends up in priv/css/app.css and that your application name is :ui. Replace these with your own if needed

Is it read once during compilation or every time the template is invoked / rendered?

If you load the contents into a module attribute, it’s read from the filesystem only once during the compilation.

Oh, I misunderstood you. Ignore me.

I’d put it in a function instead of a module attribute though. This way you would be able to invoke it from other modules as well, not just within the module where you defined the module attribute.

defmodule Web.CSSView do
  app_css = File.read!(Path.join(:code.priv_dir(:your_app_name), "static/css/app.css"))
  def inline_css(:app) do
    {:safe, unquote(app_css)}
  end
end

and then in any template

<style type="text/css"><%= Web.CSSView.inline_css(:app) %></style>