I tried using my answer today and it needed improvement because CSS was not being loaded.

I have a hacky way of making the CSS work (which is better than no way).

First, use this slightly better open_browser_wsl function.

      def open_browser_wsl(view) do
        Phoenix.LiveViewTest.open_browser(view, fn html_unix_path ->
          drive_dir = "/c"
          {wsl_path, 0} = System.cmd("wslpath", ["-aw", html_unix_path])
          cmd_args = [drive_dir, "start", String.trim_trailing(wsl_path), drive_dir, "zsh"]
          opts = [cd: "/mnt" <> drive_dir]
          System.cmd("cmd.exe", cmd_args, opts)
        end)
      end

It is better for 2 reasons:

1. It changes directory so there is no more annoying warning about not being a Windows path.
2. It makes the drive letter clearly customizable, in case that is necessary.

I usually just slap that function into the block of code in conn_case.ex that starts with:

  using do
    quote do

With the function inside the using block, any test that has use MyAppWeb.ConnCase will be able to call open_browser_wsl(view).

Now for the CSS. Configure the test Endpoint so that static URLs can be resolved. In config/text.exs, add this:

static_url_path =
  try do
    {windows_static_path, 0} = System.cmd("wslpath", ["-aw", "priv/static"])

    windows_static_path
    |> String.trim()
    |> String.trim_leading("\\")
    |> String.replace("\\", "/")
    |> then(&Kernel.<>("file://", &1))
  rescue
      # revert to default path if command not found, else re-raise the error
      e in ErlangError -> if e.original == :enoent, do: "/", else: reraise(e, __STACKTRACE__)
  end

and then finally tell the Endpoint (still in the test config file) to use this static path:

config :my_app, RangerWeb.Endpoint,
  # ...
  static_url: [host: "localhost", path: static_url_path]

The try/rescue block is to make sure people who don’t use WSL can still run tests.

It’s hacky, for sure, but now CSS will be loaded.

@ricksonoliveira Please mark this answer as the solution. It is better than my previous answer.