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The web-platform-tests Project IRC chat

The web-platform-tests Project is a W3C-coordinated attempt to build a cross-browser testsuite for the Web-platform stack. Writing tests in a way that allows them to be run in all browsers gives browser projects confidence that they are shipping software that is compatible with other implementations, and that later implementations will be compatible with their implementations. This in turn gives Web authors/developers confidence that they can actually rely on the Web platform to deliver on the promise of working across browsers and devices without needing extra layers of abstraction to paper over the gaps left by specification editors and implementors.

Setting Up the Repo

Clone or otherwise get

Note: because of the frequent creation and deletion of branches in this repo, it is recommended to "prune" stale branches when fetching updates, i.e. use git pull --prune (or git fetch -p && git merge).

Running the Tests

The tests are designed to be run from your local computer. The test environment requires Python 2.7+ (but not Python 3.x).

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 necessary content can be generated with ./wpt make-hosts-file; on Windows, you will need to precede the prior command with python or the path to the Python binary (python wpt make-hosts-file).

For example, on most UNIX-like systems, you can setup the hosts file with:

./wpt make-hosts-file | sudo tee -a /etc/hosts

And on Windows (this must be run in a PowerShell session with Administrator privileges):

python wpt make-hosts-file | Out-File $env:systemroot\System32\drivers\etc\hosts -Encoding ascii -Append

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

Running Tests Manually

The test server can be started using

./wpt serve

On Windows: You will need to precede the prior command with python or the path to the python binary.

python wpt serve

This will start HTTP servers on two ports and a websockets server on one port. By default the web servers start on ports 8000 and 8443 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, create a config.json file in the wpt root directory, and add port definitions of your choice e.g.:

  "ports": {
    "http": [1234, "auto"],

After your hosts file is configured, the servers will be locally accessible at:

https://web-platform.test:8443/ *

*See Trusting Root CA

Running Tests Automatically

Tests can be run automatically in a browser using the run command of the wpt script in the root of the checkout. This requires the hosts file setup documented above, but you must not have the test server already running when calling wpt run. The basic command line syntax is:

./wpt run product [tests]

On Windows: You will need to precede the prior command with python or the path to the python binary.

python wpt run product [tests]

where product is currently firefox or chrome and [tests] is a list of paths to tests. This will attempt to automatically locate a browser instance and install required dependencies. The command is very configurable; for example to specify a particular binary use wpt run --binary=path product. The full range of options can be see with wpt run --help and wpt run --wptrunner-help.

Not all dependencies can be automatically installed; in particular the certutil tool required to run https tests with Firefox must be installed using a system package manager or similar.

On Debian/Ubuntu certutil may be installed using:

sudo apt install libnss3-tools

And on macOS with homebrew using:

brew install nss

On other platforms, download the firefox archive and common.tests.tar.gz archive for your platform from Mozilla CI.

Then extract certutil[.exe] from the tests.tar.gz package and libnss3[.so|.dll|.dynlib] and put the former on your path and the latter on your library path.

Command Line Tools

The wpt command provides a frontend to a variety of tools for working with and running web-platform-tests. Some of the most useful commands are:

  • wpt serve - For starting the wpt http server
  • wpt run - For running tests in a browser
  • wpt lint - For running the lint against all tests
  • wpt manifest - For updating or generating a MANIFEST.json test manifest
  • wpt install - For installing the latest release of a browser or webdriver server on the local machine.


Some optional components of web-platform-tests (test components from third party software and pieces of the CSS build system) are included as submodules. To obtain these components run the following in the root of your checkout:

git submodule update --init --recursive

Prior to commit 39d07eb01fab607ab1ffd092051cded1bdd64d78 submodules were required for basic functionality. If you are working with an older checkout, the above command is required in all cases.

When moving between a commit prior to 39d07eb and one after it git may complain

$ git checkout master
error: The following untracked working tree files would be overwritten by checkout:

...followed by a long list of files. To avoid this error, remove the resources and tools directories before switching branches:

$ rm -r resources/ tools/
$ git checkout master
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'

When moving in the opposite direction, i.e. to a commit that does have submodules, you will need to git submodule update, as above. If git throws an error like:

fatal: No url found for submodule path 'resources/webidl2/test/widlproc' in .gitmodules
Failed to recurse into submodule path 'resources/webidl2'
fatal: No url found for submodule path 'tools/html5lib' in .gitmodules
Failed to recurse into submodule path 'resources'
Failed to recurse into submodule path 'tools'

...then remove the tools and resources directories, as above.

Windows Notes

On Windows wpt commands must be prefixed with python or the path to the python binary (if python is not in your %PATH%).

python wpt [command]

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 wpt commands.

Please make sure git and your text editor do not automatically convert line endings, as it will cause lint errors. For git, please set git config core.autocrlf false in your working tree.


By default pre-generated certificates for the web-platform.test domain are provided in tools/certs. If you wish to generate new certificates for any reason it's possible to use OpenSSL when starting the server, or starting a test run, by providing the --ssl-type=openssl argument to the wpt serve or wpt run commands.

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"}}

On Windows using OpenSSL 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 create a file called config.json. Then add the OpenSSL configuration below, ensuring 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):

  "ssl": {
    "type": "openssl",
    "encrypt_after_connect": false,
    "openssl": {
      "openssl_binary": "openssl",
      "base_path: "_certs",
      "force_regenerate": false,
      "base_conf_path": "C:\\OpenSSL-Win32\\bin\\openssl.cfg"

Trusting Root CA

To prevent browser SSL warnings when running HTTPS tests locally, the web-platform-tests Root CA file cacert.pem in tools/certs must be added as a trusted certificate in your OS/browser.

NOTE: The CA should not be installed in any browser profile used outside of tests, since it may be used to generate fake certificates. For browsers that use the OS certificate store, tests should therefore not be run manually outside a dedicated OS instance (e.g. a VM). To avoid this problem when running tests in Chrome or Firefox use wpt run, which disables certificate checks and therefore doesn't require the root CA to be trusted.


The master branch is automatically synced to

Pull requests are automatically mirrored except those that modify sensitive resources (such as .py). The latter require someone with merge access to comment with "LGTM" or "w3c-test:mirror" to indicate the pull request has been checked.

Finding Things

Each top-level directory matches the shortname used by a standard, with some exceptions. (Typically the shortname is from the standard's corresponding GitHub repository.)

For some of the specifications, the tree under the top-level directory represents the sections of the respective documents, using the section IDs for directory names, with a maximum of three levels deep.

So if you're looking for tests in HTML for "The History interface", they will be under html/browsers/history/the-history-interface/.

Various resources that tests depend on are in common, images, and fonts.


In the vast majority of cases the only upstream branch that you should need to care about is master. If you see other branches in the repository, you can generally safely ignore them.


Save the Web, Write Some Tests!

Absolutely everyone is welcome (and even encouraged) to contribute to test development, so long as you fulfill the contribution requirements detailed in the Contributing Guidelines. No test is too small or too simple, especially if it corresponds to something for which you've noted an interoperability bug in a browser.

The way to contribute is just as usual:

  • Fork this repository (and make sure you're still relatively in sync with it if you forked a while ago).
  • Create a branch for your changes: git checkout -b topic.
  • Make your changes.
  • Run the lint script described below.
  • Commit locally and push that to your repo.
  • Send in a pull request based on the above.

Issues with web-platform-tests

If you spot an issue with a test and are not comfortable providing a pull request per above to fix it, please file a new issue. Thank you!

Lint tool

We have a lint tool for catching common mistakes in test files. You can run it manually by starting the lint executable from the root of your local web-platform-tests working directory like this:

./wpt lint

The lint tool is also run automatically for every submitted pull request, and reviewers will not merge branches with tests that have lint errors, so you must fix any errors the lint tool reports.

In the unusual case of error reports for things essential to a certain test or that for other exceptional reasons shouldn't prevent a merge of a test, update and commit the lint.whitelist file in the web-platform-tests root directory to suppress the error reports.

For more details, see the lint-tool documentation.

Adding command-line scripts ("tools" subdirs)

Sometimes you may want to add a script to the repository that's meant to be used from the command line, not from a browser (e.g., a script for generating test files). If you want to ensure (e.g., for security reasons) that such scripts won't be handled by the HTTP server, but will instead only be usable from the command line, then place them in either:

  • the tools subdir at the root of the repository, or

  • the tools subdir at the root of any top-level directory in the repository which contains the tests the script is meant to be used with

Any files in those tools directories won't be handled by the HTTP server; instead the server will return a 404 if a user navigates to the URL for a file within them.

If you want to add a script for use with a particular set of tests but there isn't yet any tools subdir at the root of a top-level directory in the repository containing those tests, you can create a tools subdir at the root of that top-level directory and place your scripts there.

For example, if you wanted to add a script for use with tests in the notifications directory, create the notifications/tools subdir and put your script there.

Test Review

We can sometimes take a little while to go through pull requests because we have to go through all the tests and ensure that they match the specification correctly. But we look at all of them, and take everything that we can.

META.yml files are used only to indicate who should be notified of pull requests. If you are interested in receiving notifications of proposed changes to tests in a given directory, feel free to add yourself to the META.yml file. Anyone with expertise in the specification under test can approve a pull request. In particular, if a test change has already been adequately reviewed "upstream" in another repository, it can be pushed here without any further review by supplying a link to the upstream review.

Search filters to find things to review:

Getting Involved

If you wish to contribute actively, you're very welcome to join the mailing list (low traffic) by signing up to our mailing list. The mailing list is archived.

Join us on irc #testing (, port 6665). The channel is archived.