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.
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:
Server features - e.g. multiple origins, substitutions, server-side Python scripts.
Tests can be run using
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
testharness) tests, reftests and wdspec tests. To limit
the type of tests that get run, use
--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
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
directory. To run with the same metadata as for Firefox (so that
differences are reported as unexpected results), pass
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.
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
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
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
corresponding metadata file is
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:
[filename.html] [Subtest 1 name] key: value [Subtest 2 name] key: [list, value]
For cases where a single file generates multiple tests (e.g. variants
.any.js tests), the metadata file has one top-level section for
each test, for example:
[test.any.html] [Subtest name] key: value [test.any.worker.html] [Subtest name] key: other-value
Values can be made conditional using a Python-like conditional syntax:
[filename.html] key: if os == "linux": linux-value default-value
For more information on manifest files, see the wptrunner documentation
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:
[filename.html] [Subtest name for failing test] expected: FAIL [Subtest name for erroring test] expected: ERROR
Expectations can be made configuration-specific using the conditional syntax:
[filename.html] expected: if os == "linux" and bits == 32: TIMEOUT if os == "win": ERROR FAIL
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:
[filename.html] [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
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).
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:
[filename.html] disabled: 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.
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:
[filename.html] 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.
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
[filename.html] prefs: [dom.serviceWorkers.enabled:true, dom.serviceWorkers.exemptFromPerDomainMax:true, dom.caches.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
[filename.html] leaks: if os == "linux": https://bugzilla.mozilla.org/show_bug.cgi?id=1234567
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
@Reset (usually used with prefs), or to force a value
to true or false using
@False. For example to enable
the tests in
meta/feature/unsupported/subfeature-supported one might
create an ini file
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:
[filename.html] expected: 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:
[filename.html] expected: if nightly_build: PASS FAIL
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.
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/testsonce 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 matrix.