web-platform-tests is a W3C-coordinated effort to build a cross-browser testsuite for the majority of the web platform; it excludes only CSS (whose testsuite lives in csswg-test), ECMAScript (whose testsuite lives in test262), and WebGL (whose testsuite lives in WebGL).

That said, csswg-test follows a superset of the policies of web-platform-tests, so the documentation provided here should similarly apply to it. Where extra policies apply, they are provided in separate documents and linked from the appropriate section.


If you get stuck or want clarification about anything, feel free to ask on either the mailing list or IRC (webclient, join channel #testing); IRC is generally busiest during the European working day but frequently has people on it at all times and should probably be the general first port of call for any help.

Testsuite Design

The vast majority of the testsuite is formed of HTML pages, which can be loaded in a browser and either programmatically provide a result or provide a set of steps to run the test and obtain the result.

The tests are, in general, short, cross-platform, and self-contained, and should be easy to run in any browser.

Test Layout

Each top level directory in the repository corresponds to tests for a single specification. For W3C specs, these directories are typically named after the shortname of the spec (i.e. the name used for snapshot publications under /TR/); for WHATWG specs, they are typically named after the subdomain of the spec (i.e. trimming .spec.whatwg.org from the URL); for other specs, something deemed sensible is used. In any case, there are occasional exceptions for historic reasons.

Within the specification-specific directory there are two common ways of laying out tests: the first is a flat structure which is sometimes adopted for very short specifications; the alternative is a nested structure with each subdirectory corresponding to the id of a heading in the specification. The latter provides some implicit metadata about the part of a specification being tested according to its location in the filesystem, and is preferred for larger specifications.

Test Types

The testsuite has a few types of tests, outlined below:

  • testharness.js tests, which are run through a JS harness and report their result back with JS.

  • Reftests, which render two (or more) web pages and combine them with equality assertions about their rendering (e.g., A.html and B.html must render identically), run either by the user switching between tabs/windows and trying to observe differences or through automated scripts.

  • Visual tests which display a page where the result is determined either by a human looking at it or by comparing it with a saved screenshot for that user agent on that platform.

  • Manual tests, which rely on a human to run them and determine their result.

  • WebDriver tests, which are used for testing the WebDriver protocol itself.


GitHub is used both for issue tracking and test submissions; we provide a limited introduction to both git and GitHub.

Pull Requests are automatically labeled based on the directory the files they change are in; there are also comments added automatically to notify a number of people: this list of people comes from OWNERS files in those same directories and their parents (i.e., they work recursively: a/OWNERS will get notified for a/foo.html and a/b/bar.html).

If you want to be notified about changes to tests in a directory, feel free to add yourself to the OWNERS file: there’s no requirement to own anything as a result!

Local Setup

The tests are designed to be run from your local computer. The test environment requires Python 2.7+ (but not Python 3.x). You will also need a copy of OpenSSL.

On Windows, be sure to add the Python directory (c:\python2x, by default) to your %Path% Environment Variable, and read the Windows Notes section below.

To get the tests running, you need to set up the test domains in your hosts file. The following entries are required:   web-platform.test   www.web-platform.test   www1.web-platform.test   www2.web-platform.test   xn--n8j6ds53lwwkrqhv28a.web-platform.test   xn--lve-6lad.web-platform.test     nonexistent-origin.web-platform.test

If you are behind a proxy, you also need to make sure the domains above are excluded from your proxy lookups.

Because web-platform-tests uses git submodules, you must ensure that these are up to date. In the root of your checkout, run:

git submodule update --init --recursive

The test environment can then be started using


This will start HTTP servers on two ports and a websockets server on one port. By default one web server starts on port 8000 and the other ports are randomly-chosen free ports. Tests must be loaded from the first HTTP server in the output. To change the ports, copy the config.default.json file to config.json and edit the new file, replacing the part that reads:

"http": [8000, "auto"]

to some port of your choice e.g.

"http": [1234, "auto"]

If you installed OpenSSL in such a way that running openssl at a command line doesn’t work, you also need to adjust the path to the OpenSSL binary. This can be done by adding a section to config.json like:

"ssl": {"openssl": {"binary": "/path/to/openssl"}}

Windows Notes

Running wptserve with SSL enabled on Windows typically requires installing an OpenSSL distribution. Shining Light provide a convenient installer that is known to work, but requires a little extra setup, i.e.:

Run the installer for Win32_OpenSSL_v1.1.0b (30MB). During installation, change the default location for where to Copy OpenSSL Dlls from the System directory to the /bin directory.

After installation, ensure that the path to OpenSSL (typically, this will be C:\OpenSSL-Win32\bin) is in your %Path% Environment Variable. If you forget to do this part, you will most likely see a ‘File Not Found’ error when you start wptserve.

Finally, set the path value in the server configuration file to the default OpenSSL configuration file location. To do this, copy config.default.json in the web-platform-tests root to config.json. Then edit the JSON so that the key ssl/openssl/base_conf_path has a value that is the path to the OpenSSL config file (typically this will be C:\\OpenSSL-Win32\\bin\\openssl.cfg).

Alternatively, you may also use Bash on Ubuntu on Windows in the Windows 10 Anniversary Update build, then access your windows partition from there to launch wptserve.