web-platform-tests is a cross-browser testsuite. Writing tests as web-platform-tests helps ensure that browsers all implement the same behaviour for web-platform features.

Upstream Documentation

This documentation covers the integration of web-platform-tests into the Firefox tree. For documentation on writing tests, see web-platform-tests.org. In particular the following documents cover common test-writing needs:

Running Tests

Tests can be run using mach:

mach wpt

To run only certain tests, pass these as additional arguments to the command. For example to include all tests in the dom directory:

mach wpt testing/web-platform/tests/dom

Tests may also be passed by id; this is the path plus any query or fragment part of a URL and is suitable for copying directly from logs e.g. on treeherder:

mach wpt /web-nfc/idlharness.https.window.html

A single file can produce multiple tests, so passing test ids rather than paths is sometimes necessary to run exactly one test.

The testsuite contains a mix of various test types including Javascript (testharness) tests, reftests and wdspec tests. To limit the type of tests that get run, use --test-type=<type> e.g. --test-type=reftest for reftests.

Note that if only a single testharness test is run the browser will stay open by default (matching the behaviour of mochitest). To prevent this pass --no-pause-after-test to mach wpt.

When the source tree is configured for building android, tests will also be run on Android, by default using a local emulator.

Running Tests In Other Browsers

web-platform-tests is cross browser, and the runner is compatible with multiple browsers. Therefore it’s possible to check the behaviour of tests in other browsers. By default Chrome, Edge and Servo are supported. In order to run the tests in these browsers use the --product argument to wptrunner:

mach wpt --product chrome dom/historical.html

By default these browsers run without expectation metadata, but it can be added in the testing/web-platform/products/<product> directory. To run with the same metadata as for Firefox (so that differences are reported as unexpected results), pass --meta testing/web-platform/meta to the mach command.

Results from the upstream CI for many browser, including Chrome and Safari, are available on wpt.fyi. There is also a gecko dashboard which by default shows tests that are failing in Gecko but not in Chrome and Safari, organised by bug component, based on the wpt.fyi data.


Under testing/web-platform are the following directories:


An automatically-updated import of the web-platform-tests repository. Any changes to this directory are automatically converted into pull requests against the upstream repository, and merged if they pass CI.


Gecko-specific metadata including expected test results and configuration e.g. prefs that are set when running the test. This is explained in the following section.


Tests that will not be upstreamed and may make use of Mozilla-specific features. They can access the SpecialPowers APIs.


Metadata for the Mozilla-specific tests.


In order to separate out the shared testsuite from Firefox-specific metadata about the tests, all the metadata is stored in separate ini-like files in the meta/ sub-directory.

There is one metadata file per test file with associated gecko-specific data. The metadata file of a test has the same relative path as the test file and has the the suffix .ini e.g. for the test in testing/web-platform/tests/example/example.html, the corresponding metadata file is testing/web-platform/meta/example/example.html.ini.

The format of these files is similar to ini files, but with a couple of important differences; sections can be nested using indentation, and only : is permitted as a key-value separator. For example:

    [Subtest 1 name]
        key: value

    [Subtest 2 name]
        key: [list, value]

For cases where a single file generates multiple tests (e.g. variants or .any.js tests), the metadata file has one top-level section for each test, for example:

    [Subtest name]
        key: value

    [Subtest name]
        key: other-value

Values can be made conditional using a Python-like conditional syntax:

        if os == "linux": linux-value

The available variables for the conditions are those provided by mozinfo, plus some additional wpt-specific values.

For more information on manifest files, see the wptrunner documentation

Expectation Data

All tests that don’t pass in our CI have expectation data stored in the metadata file, under the key expected. For example the expectation file for a test with one failing subtest and one erroring subtest might look like:

    [Subtest name for failing test]
        expected: FAIL

    [Subtest name for erroring test]
        expected: ERROR

Expectations can be made configuration-specific using the conditional syntax:

        if os == "linux" and bits == 32: TIMEOUT
        if os == "win": ERROR

Tests that are intermittent may be marked with multiple statuses using a list of possibilities e.g. for a test that usually passes, but intermittently fails:

    [Subtest name for intermittent test]
        expected: [PASS, FAIL]

Auto-generating Expectation Data

After changing some code it may be necessary to update the expectation data for the relevant tests. This can of course be done manually, but tools are available to automate much of the process.

First one must run the tests that have changed status, and save the raw log output to a file:

mach wpt /url/of/test.html --log-wptreport wptreport.json

Then the wpt-update command may be run using this log data to update the expectation files:

mach wpt-update wptreport.json

CI runs also produce wptreport.json files that can be downloaded as artifacts. When tests are run across multiple platforms, and all the wptreport files are processed together, the tooling will set the appropriate conditions for any platform-specific results:

mach wpt-update logs/*.json

For complete runs the --full flag will cause metadata to be removed when a) the test was updated and b) there is a condition that didn’t match any of the configuration in the input files.

When tests are run more than once --update-intermittent flag will cause conflicting results to be marked as intermittents (otherwise the data is not updated in the case of conflicts).

Disabling Tests

Tests are disabled using the same manifest files used to set expectation values. For example, if a test is unstable on Windows, it can be disabled using an ini file with the contents:

        if os == "win": https://bugzilla.mozilla.org/show_bug.cgi?id=1234567

For intermittents it’s generally preferable to give the test multiple expectations rather than disable it.

Fuzzy Reftests

Reftests where the test doesn’t exactly match the reference can be marked as fuzzy. If the difference is inherent to the test, it should be encoded in a meta element, but where it’s a Gecko-specific difference it can be added to the metadata file, using the same syntax:

    fuzzy: maxDifference=10-15;totalPixels=200-300

In this case we expect between 200 and 300 pixels, inclusive, to be different, and the maximum difference in any RGB colour channel to be between 10 and 15.

Enabling Prefs

Some tests require specific prefs to be enabled before running. These prefs can be set in the expectation data using a prefs key with a comma-separated list of pref.name:value items:

    prefs: [dom.serviceWorkers.enabled:true,

Disabling Leak Checks

When a test is imported that leaks, it may be necessary to temporarily disable leak checking for that test in order to allow the import to proceed. This works in basically the same way as disabling a test, but with the key leaks:

        if os == "linux": https://bugzilla.mozilla.org/show_bug.cgi?id=1234567

Per-Directory Metadata

Occasionally it is useful to set metadata for an entire directory of tests e.g. to disable then all, or to enable prefs for every test. In that case it is possible to create a __dir__.ini file in the metadata directory corresponding to the tests for which you want to set this metadata e.g. to disable all the tests in tests/feature/unsupported/, one might create meta/feature/unsupported/__dir__.ini with the contents:

disabled: Feature is unsupported

Settings set in this way are inherited into sub-directories. It is possible to unset a value that has been set in a parent using the special token @Reset (usually used with prefs), or to force a value to true or false using @True and @False. For example to enable the tests in meta/feature/unsupported/subfeature-supported one might create an ini file meta/feature/unsupported/subfeature-supported/__dir__.ini like:

disabled: @False

Setting Metadata for Release Branches

Run info properties can be used to set metadata for release branches that differs from nightly (e.g. for when a feature depends on prefs that are only set on nightly), for example:

    if release_or_beta: FAIL

Note that in general the automatic metadata update will work better if the nonstandard configuration is used explicitly in the conditional, and placed at the top of the set of conditions, i.e. the following would cause problems later:

    if nightly_build: PASS

This is because on import the automatic metadata updates are run against the results of nightly builds, and we remove any existing conditions that match all the input runs to avoid building up stale configuration options.

Test Manifest

web-platform-tests use a large auto-generated JSON file as their manifest. This stores data about the type of tests, their references, if any, and their timeout, gathered by inspecting the filenames and the contents of the test files. It is not necessary to manually add new tests to the manifest; it is automatically kept up to date when running mach wpt.

Synchronization with Upstream

Tests are automatically synchronized with upstream using the wpt-sync bot. This performs the following tasks:

  • Creates upstream PRs for changes in testing/web-platform/tests once they land on autoland, and automatically merges them after they reach mozilla-central.

  • Runs merged upstream PRs through gecko CI to generate updated expectation metadata.

  • Updates the copy of web-platform-tests in the gecko tree with changes from upstream, and the expectation metadata required to make CI jobs pass.

The nature of a two-way sync means that occasional merge conflicts and other problems. If something isn’t in sync with upstream in the way you expect, please ask on #interop on matritx.


Sometimes, it’s preferable to run the WPT’s web server on its own, and point different browsers to the test files.

./mach wpt-serve

can be used for this, after a short setup:

On Unix, one can run:

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

from the root of the WPT checkout, present at testing/web-platform/tests/.

On Windows, from an administrator mozilla-build shell, one can run:

./wpt make-hosts-file >> /c/Windows/System32/drivers/etc/host

from the WPT checkout.

Most of the time, browsing to http://localhost:8000 will allow running the test, although some tests have special requirements, such as running on a specific domain or running using https.