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An Elixir client for the Nu HTML Checker (v.Nu).

v.Nu is a document validity checker used by the W3C. It offers validating HTML, CSS, and SVG documents.

This library brings that functionality to Elixir by using the Checker's JSON API. It offers ExUnit assertions for validating dynamic content in tests, Mix tasks for validating static content, and general purpose functions to fulfill other needs.


While it is possible to use this library with the service run by W3C at, I would recommend running your own instance. You will eliminate a lot of network latency if it runs on the same machine as your code, and you will not hit any rate limits that might exist for

The source of the Checker can be found in the repository validator/validator. Follow their instructions on how to download it and run it as a web server.

The easiest option is to use the Docker image, like this:

docker run -it --rm -p 8888:8888

Check if the server is running:

$ curl localhost:8888 -I
HTTP/1.1 200 OK


Make sure to read about the prerequisites first.

Add Vnu as a dependency to your project's mix.exs. To use the built-in, Hackney-based HTTP client adapter, add :hackney too:

defp deps do
    {:vnu, "~> 1.1", only: [:dev, :test], runtime: false},
    {:hackney, "~> 1.17"}

And run:

$ mix deps.get


Available on


ExUnit assertions

If you are building an application that generates HTML, CSS, or SVG files, you might want to use those validations in your tests.

  • Vnu.Assertions.assert_valid_html/2
  • Vnu.Assertions.assert_valid_css/2
  • Vnu.Assertions.assert_valid_svg/2

Phoenix controller test example

defmodule PhoenixAppWeb.PageControllerTest do
  use PhoenixAppWeb.ConnCase
  import Vnu.Assertions

    test "GET /", %{conn: conn} do
      vnu_opts = %{server_url: "http://localhost:8888", fail_on_warnings: true}
      conn = get(conn, "/")
      html_response =
        |> get("/")
        |> html_response(200)
        |> assert_valid_html(vnu_opts)
      assert html_response =~ "Welcome to Phoenix!"

See examples/1_phoenix_app/test/phoenix_app_web/controllers/page_controller_test.exs for more.

Mix task

If you have static HTML, CSS, or SVG files in your project, you might want to validate them with those mix tasks:

  • mix vnu.validate.html
  • mix vnu.validate.css
  • mix vnu.validate.svg


$ mix vnu.validate.css --server-url localhost:8888 assets/**/*.css

General purpose

If you need HTML, CSS, or SVG validation for something else, try one of those functions:

  • Vnu.validate_html/2
  • Vnu.validate_css/2
  • Vnu.validate_svg/2
iex> {:ok, result} = Vnu.validate_html("<!DOCTYPE html><html><head></head></html>",
  server_url: "http://localhost:8888")
   messages: [
       extract: "tml><head></head></html",
       first_column: 28,
       first_line: 1,
       hilite_length: 7,
       hilite_start: 10,
       last_column: 34,
       last_line: 1,
       message: "Element “head” is missing a required instance of child element “title”.",
       offset: nil,
       sub_type: nil,
       type: :error
       extract: "TYPE html><html><head>",
       first_column: 16,
       first_line: 1,
       hilite_length: 6,
       hilite_start: 10,
       last_column: 21,
       last_line: 1,
       message: "Consider adding a “lang” attribute to the “html” start tag to declare the language of this document.",
       offset: nil,
       sub_type: :warning,
       type: :info

iex> Vnu.valid?(result)


Make sure to read about the prerequisites first.

After cloning the repository, run mix deps.get and you should be ready for development.

To ensure code consistency, run mix format, mix credo, and mix dialyzer.

Running tests

All test that expect to talk with the server accept the server's URL as an VNU_SERVER_URL environment variable or fallback to the default http://localhost:8888.

$ VNU_SERVER_URL=http://localhost:4000/ mix test 

If you're adding a new test, make sure it will do that do.



If you noticed a problem with the library or its documentation, or have an idea for a feature, open an issue.

If you have an idea on how to act upon the problem or idea, feel free to open a pull request instead.

Pull requests

If you noticed a problem with the library or its documentation and know how to fix it, or have an idea for a feature, or want to fix a typo, open a pull request.

If you are not sure of your changes or need help finishing them, open a pull request anyway. I'll try to help!


Vnu is released under the MIT License. See the LICENSE file for further details.