Example HTML lint plug code

html_lint.ex

defmodule HtmlLintPlug do
  require Logger

  def init(opts), do: opts

  def call(conn, _opts) do
    Plug.Conn.register_before_send(conn, &lint/1)
  end

  defmodule Error do
    defexception message: "HTML linter returned errors"
  end

  defp lint(conn) do
    body_string = to_string(conn.resp_body)
    result = my_html_linting_function(body_string)
  
    case result do
      :ok ->
        conn
      {:error, error} ->
        case Mix.env() do
          :dev ->
            Logger.warn("#{inspect(error)}")
            conn
          :test ->
            raise Error, message: inspect(error)
        end
    end
  end
end