HTML Rendering Tests with Rails


The Problem

Picture the situation: 3pm on deploy day, after fully testing that your Rails application outputs the proper HTML, you run Capistrano and bring up the newest version of your beautiful website. An uneventful half-hour passes, after which you are told by your CEO that in Internet Explorer 7, a certain component of your web page is misaligned by 20px. While investigating further, you find your supposedly-vertical lists of checkboxes are being rendered in a diagonal line by Internet Explorer 6. Your deploy has transformed your once-perfect website into an unprofessional disaster.

You blow off dinner plans, fix bugs, validate all other pages on all major browsers, and finally, around 8pm, relax. At this point, the question is not so much, “why?” as it is, “how can we make sure this never ever happens again?”

Testing manually is simply too time-consuming (especially if you support many browsers). You want to go The Rails Way and automate your tests. How can you automatically render your HTML on various browsers and ensure the output looks right?


  • You are using Ruby on Rails.
  • You want to verify every pixel on every browser/platform in your tests.
  • You want your test to fail if any pixel on any browser/platform changes from what you verified.
  • Different browsers/platforms will very rarely produce exact, pixel-by-pixel matches, and the differences should be validated manually.
  • You do not mind if the HTML structure changes, so long as the rendered output does not change.
  • Tests should be easy to write and edit.


We need a test framework which can render any Rails partial or template using any browser/platform. Though we only intend to test our own templates, we are actually implicitly forced to test our models and the browsers we support, as shown in this diagram:

HTML Request Lifecycle


The trickiest part of implementation is finding a way to automatically gather web browser output. This is especially difficult because:

  • Web browsers were historically not designed to render off-screen, so “render a PNG of a web page” is generally a difficult task involving very different code on each browser.
  • Rendering is a very slow operation, compared to, say, parsing an HTML document’s DOM structure.
  • Some web browsers are mutually exclusive (for instance, Internet Explorer 7 and Internet Explorer 8) and cannot usually appear on the same computer.

A distributed testing environment is necessary, either through physical or virtual machines. It will look something like this:

HTML Rendering Distributed Computing Layout

This leaves plenty of programs for us to write:

  • The “Test Runner”
  • The “Render Server”
  • The “Thumbnailing Program” (which is different for each browser/platform)

Render Server, Thumbnailing Programs

I have decided to bundle (platform-specific) thumbnailing programs with the (cross-platform) render server. The project is hosted on GitHub: html_to_png_server, which includes compiled versions but currently only handles IE6/7 on Windows and Firefox 3 on GNOME. (New thumbnailing programs, while trivial to plug into the render server code, are difficult to implement.)

The server accepts a POST request with HTML data, and it returns a PNG response. The URL determines which thumbnailing program (i.e., browser) to use for rendering. What a wondrous black box!

Visit for details.

Test Runner

The test runner is written in Ruby and hosted on GitHub: html_render.

It contains a Rails-agnostic component, which is simply a client to the Render Server, and a Rails-specific testing component.

Visit for details.

Your Render Test Framework

The idea is for Rails rendering tests to be as simple as possible to write, while incorporating your own application’s specific needs. You, the test automation engineer, will have two tasks:

  • Application-specific (or, if you feel creative, sub-application-specific) HTML-preparation and render server configuration: your project-specific framework.
  • Actual tests which use your framework.

Your Project-Specific Framework

Here is a sample framework, in which we have decided the following:

  • Tests are stored in RAILS_ROOT/test/render/
  • Tests are invoked by running ruby test/render_test.rb
  • You will render on winxp-ie6.local, winxp-ie7.local, and ubuntu-ff3.local

Create the file test/render_test.rb with the following contents:

require File.dirname(__FILE__) + '/test_helper'

require 'html_render/render_test/rails'

class RenderTest < ActiveSupport::TestCase
  extend HTMLRender::RenderTest::Rails::TestCaseDefinitions

  PREFIX = File.join(File.dirname(__FILE__), 'render')
    :winxp_ie6 => 'http://winxp-ie6.local:20558/ie',
    :winxp_ie7 => 'http://winxp-ie7.local:20558/ie',
    :ubuntu_ff3 => 'http://linux-ff3.local:20558/ff3_linux'

  class ApplicationRenderTest < HTMLRender::RenderTest::Rails::RenderTest
    def css
      @css ||=, '..', 'public', 'stylesheets', 'application.css')) { |f| }

    def javascript
      '' # We're not ambitious enough to test our JavaScript just yet

  define_tests(PREFIX, :servers => SERVERS, :base_class => ApplicationRenderTest)

Your Tests

To test the partial /foos/_foo.html.erb (or .haml), create the file test/render/foos/_foo/basic/setup.rb with the following contents:

def locals
  { foo: }

(Yes, that is the entire test.)

Here, we name the test basic: we may want to create several tests of the same partial, and for those we would replace “basic” with something else. The path, foos/_foo, implicitly specifies that we mean to test app/views/foos/_foo.html.erb (or .haml, as the case may be). Tests may define locals, assigns, css, javascript, or any helper methods deemed necessary. Behind the scenes, the contents of setup.rb will be appended to a subclass of ApplicationRenderTest. Any methods, up to and including html (the only method the testing framework actually uses) may be overridden.

Running Your Test Suite

First, download html_to_png_server (specifically, html_to_png_server.jar) to each rendering machine and start the server on each machine:

java -jar html_to_png_server.jar

Next, on the test runner computer (which holds your Rails project code), install the html_render gem:

sudo rubygems install adamh-html_render --source

Now you can run the test suite from your Rails project directory:

ruby test/render_test.rb

(This command could just as easily be made into a Rake task, of course, and included as part of rake test.)

All your render tests will fail: the expected images have not been defined yet! But each render has actually taken place, and the results are now viewable.

Making Tests Pass

Look in test/render/foos/_foo/basic. Where before you only had setup.rb, you will now have a run/ subdirectory with html.html, winxp_ie6.png, winxp_ie7.png, and ubuntu_ff3.png.

Here is the manual validation part: make sure each of those images looks the way you want it. If it does not, you will need to fix the partial (or, if you made a mistake settings up the framework, you will need to fix the stylesheets you are supplying in ApplicationRenderTest).

Assuming the images look correct, you can set the expected value. Choose the best-looking image and copy it to canonical.png (in the same directory as setup.rb). Now, should you add any new servers to your testing framework, their output will be compared pixel-by-pixel with the image in canonical.png (and the test will fail if the pixels do not match). Typically, canonical.png will be chosen from a Firefox server. (Not that I’m opinionated against Internet Explorer or anything.)

Now, if you run your test again, you will probably find that it still fails. While your ubuntu_ff3.png matches canonical.png as expected, the Internet Explorer images are not identical to canonical.png and so they trigger the failure. This failure is expected and healthy. To mark the images as valid, create a valid/ subdirectory in the same directory as setup.rb and copy winxp_ie6.png and winxp_ie7.png into it.

In the end, your single test’s folder will look like this:

  • test/render/foos/_foo/basic/setup.rb
  • test/render/foos/_foo/basic/run/html.html
  • test/render/foos/_foo/basic/run/ubuntu_ff3.png
  • test/render/foos/_foo/basic/run/winxp_ie6.png
  • test/render/foos/_foo/basic/run/winxp_ie7.png
  • test/render/foos/_foo/basic/canonical.png
  • test/render/foos/_foo/basic/valid/winxp_ie6.png
  • test/render/foos/_foo/basic/valid/winxp_ie7.png

…and your test will pass. (Until you change the partial, that is: try editing it and watch the test fail!)

Test Policy Considerations

When adding your tests to version control, ignore the run/ subdirectories: the test only really comprises setup.rb, canonical.png, and the images within valid/. (The run/ directory can be considered a temporary directory, though it is useful enough that the html_render test framework leaves it behind.)

You may also choose to never set a canonical.png. I chose to include it because it seems idealogically appropriate and it will help when developing a nice user interface for the test runner.


You now have a test framework which validates, with pixel-perfection, the way your partial/template is output from major web browsers. It can work at any detail level, from a single input box to your whole home page. You can also write tests of your HTML-producing helpers, independently of any actual templates (by overriding html in the test’s setup.rb).

The system is somewhat fragile: all render servers must be running html_to_png_server.

The system is somewhat slow: your render test suite will take approximately as long to run as it takes your slowest render server to render each partial.

But despite the system’s faults, you now confidently catch those irksome 20px Internet Explorer hasLayout issues before your boss does.

Looking Forward

This system is currently prototypical. Here, I discuss some problems I intend to solve.

The test suite is difficult to manage, with its masses of images in several directories. A graphical front-end to this large directory structure is a huge priority and should make the process of fixing tests much easier.

More challenging, the process of writing a test is not as seamless as it could be. I envision a point-and-click interface running on top of the actual Rails application, though of course this is easier said than done.