Use mix format for arbitrary HTML files

did you try renaming the filename to test.heex, running the formatter and then renaming it back to test.html ?

Formatters follow this contract mix format — Mix v1.20.2

I think you can write a custom formatter that mostly delegates to the Phoenix one (untested):

defmodule HTMLFormatter do
  @behaviour Mix.Tasks.Format

  @impl true
  def features(_options) do
    [sigils: [:H], extensions: [".heex", ".html"]]
  end

  @impl true
  defdelegate format(source, options), to: Phoenix.LiveView.HTMLFormatter
end

edit: oh, if you’d rather not need to have a Mix project, then it might be pretty tricky.

Maybe a single .exs file and Mix.install?