docs/testing/web_platform_tests.md

# web-platform-tests

Interoperability between browsers is
[critical](https://www.chromium.org/blink/platform-predictability) to Chromium's
mission of improving the web. We believe that leveraging and contributing to a
shared test suite is one of the most important tools in achieving
interoperability between browsers. The [web-platform-tests
repository](https://github.com/web-platform-tests/wpt) is the primary shared
test suite where all browser engines are collaborating.

Chromium has a 2-way import/export process with the upstream web-platform-tests
repository, where tests are imported into
[web_tests/external/wpt](../../third_party/blink/web_tests/external/wpt)
and any changes to the imported tests are also exported to web-platform-tests.

See https://web-platform-tests.org/ for general documentation on
web-platform-tests, including tips for writing and reviewing tests.

[TOC]

## Importing tests

Chromium has a [mirror](https://chromium.googlesource.com/external/w3c/web-platform-tests/)
of the GitHub repo and periodically imports a subset of the tests to
run as part of the regular Blink web test testing process.

The goals of this process are to be able to run web-platform-tests unmodified
locally just as easily as we can run the Blink tests, and ensure that we are
tracking tip-of-tree in the web-platform-tests repository as closely as
possible, and running as many of the tests as possible.

### Automatic import process

There is an automatic process for updating the Chromium copy of
web-platform-tests. The import is done by the builder [wpt-importer
builder][wpt-importer].

The easiest way to check the status of recent imports is to look at:

-   Recent logs on LUCI for [wpt-importer builder][wpt-importer]
-   Recent CLs created by [WPT
    Autoroller](https://chromium-review.googlesource.com/q/owner:wpt-autoroller%2540chops-service-accounts.iam.gserviceaccount.com).

The import jobs will generally be green if either there was nothing to do,
or a CL was successfully submitted.

For maintainers:

-   The source lives in
    [third_party/blink/tools/wpt_import.py](../../third_party/blink/tools/wpt_import.py).
-   If the importer starts misbehaving, it can be disabled by landing a
    [CL to skip the update step](https://crrev.com/c/1961906/).

### Failures caused by automatic imports.

If there are new test failures that start after an auto-import,
there are several possible causes, including:

 1. New baselines for flaky tests were added (http://crbug.com/701234).
 2. Modified tests should have new results for non-Release builds but they weren't added (http://crbug.com/725160).
 3. New baselines were added for tests with non-deterministic test results (http://crbug.com/705125).

Because these tests are imported from the Web Platform tests, it is better
to have them in the repository (and marked failing) than not, so prefer to
[add test expectations](web_test_expectations.md) rather than reverting.
However, if a huge number of tests are failing, please revert the CL so we
can fix it manually.

[wpt-importer]: https://ci.chromium.org/p/infra/builders/luci.infra.cron/wpt-importer

### Automatic export process

If you upload a CL with any changes in
[third_party/blink/web_tests/external/wpt](../../third_party/blink/web_tests/external/wpt),
once you add reviewers the exporter will create a provisional pull request with
those changes in the [upstream WPT GitHub repository](https://github.com/web-platform-tests/wpt/).
The exporter runs on [wpt-exporter builder][wpt-exporter].

Once you're ready to land your CL, please follow the link posted by the bot and
check the status of the required checks of the GitHub PR. If it's green, go
ahead landing your CL and the exporter will automatically merge the PR.

If GitHub status is red on the PR, please try to resolve the failures before
merging. If you run into any issues, or if you have a CL with WPT changes that
the exporter did not pick up, please reach out to ecosystem-infra@chromium.org.

Additional things to note:

-   CLs that change over 1000 files will not be exported.
-   All PRs use the
    [`chromium-export`](https://github.com/web-platform-tests/wpt/pulls?utf8=%E2%9C%93&q=is%3Apr%20label%3Achromium-export) label.
-   All PRs for CLs that haven't yet been landed in Chromium also use the
    [`do not merge yet`](https://github.com/web-platform-tests/wpt/pulls?q=is%3Apr+is%3Aopen+label%3A%22do+not+merge+yet%22) label.
-   The exporter cannot create upstream PRs for in-flight CLs with binary files
    (e.g. webm files). An export PR will still be made after the CL lands.

For maintainers:

-   The source lives in
    [third_party/blink/tools/wpt_export.py](../../third_party/blink/tools/wpt_export.py).
-   If the exporter starts misbehaving (for example, creating the same PR over
    and over again), put it in "dry run" mode by landing [this CL](https://crrev.com/c/462381/).

[wpt-exporter]: https://ci.chromium.org/p/infra/builders/luci.infra.cron/wpt-exporter

### GitHub credentials

When manually running the `wpt-import` and `wpt-export` scripts, several
requests are made to GitHub to query the status of pull requests, look for
existing exported commits etc. GitHub has a [fairly
low](https://developer.github.com/v3/#rate-limiting) request limit for
unauthenticated requests, so it is recommended that you let `wpt-export` and
`wpt-import` use your GitHub credentials when sending requests:

 1. Generate a new [personal access token](https://github.com/settings/tokens)
 1. Set up your credentials by either:
     * Setting the `GH_USER` environment variable to your GitHub user name
       and the `GH_TOKEN` environment variable to the access token you have
       just created **or**
     * Creating a JSON file with two keys: `GH_USER`, your GitHub user name,
       and `GH_TOKEN`, the access token you have just generated. After that,
       pass `--credentials-json <path-to-json>` to `wpt-export` and
       `wpt-import`.

### Manual import

To pull the latest versions of the tests that are currently being imported, you
can also directly invoke the
[wpt-import](../../third_party/blink/tools/wpt_import.py) script.

That script will pull the latest version of the tests from our mirrors of the
upstream repositories. If any new versions of tests are found, they will be
committed locally to your local repository. You may then upload the changes.

Remember your import might fail due to GitHub's limit for unauthenticated
requests, so consider [passing your GitHub credentials](#GitHub-credentials) to
the script.

### Skipped tests

We control which tests are imported via a file called
[W3CImportExpectations](../../third_party/blink/web_tests/W3CImportExpectations),
which has a list of directories to skip while importing.

In addition to the directories and tests explicitly skipped there, tests may
also be skipped for a couple other reasons, e.g. if the file path is too long
for Windows. To check what files are skipped in import, check the recent logs
for [wpt-importer builder][wpt-importer].

### Enabling import for a new directory

If you wish to add more tests (by un-skipping some of the directories currently
skipped in `W3CImportExpectations`), you can modify that file locally and commit
it, and on the next auto-import, the new tests should be imported.

If you want to import immediately (in order to try the tests out locally, etc)
you can also run `wpt-import`, but this is not required.

Remember your import might fail due to GitHub's limit for unauthenticated
requests, so consider [passing your GitHub credentials](#GitHub-credentials) to
the script.

## Writing tests

To contribute changes to web-platform-tests, just commit your changes directly
to [web_tests/external/wpt](../../third_party/blink/web_tests/external/wpt)
and the changes will be automatically upstreamed within 24 hours.

Changes involving adding, removing or modifying tests can all be upstreamed.
Any changes outside of
[external/wpt](../../third_party/blink/web_tests/external/wpt) will not be
upstreamed, and any changes `*-expected.txt`, `OWNERS`, and `MANIFEST.json`,
will also not be upstreamed.

Running the web tests will automatically regenerate MANIFEST.json to pick up
any local modifications.

Most tests are written using testharness.js, see
[Writing Web Tests](./writing_web_tests.md) and
[Web Tests Tips](./web_tests_tips.md) for general guidelines.

### Write tests against specifications

Tests in web-platform-tests are expected to match behavior defined by the
relevant specification. In other words, all assertions that a test makes
should be derived from a specification's normative requirements, and not go
beyond them. It is often necessary to change the specification to clarify what
is and isn't required.

When implementation experience is needed to inform the specification work,
[tentative tests](https://web-platform-tests.org/writing-tests/file-names.html)
can be appropriate. It should be apparent in context why the test is tentative
and what needs to be resolved to make it non-tentative.

### Tests that require testing APIs

[testdriver.js](https://web-platform-tests.org/writing-tests/testdriver.html)
provides a means to automate tests that cannot be written purely using web
platform APIs, similar to `internals.*` and `eventSender.*` in regular Blink
web tests.

If no testdriver.js API exists, check if it's a
[known issue](https://github.com/web-platform-tests/wpt/labels/testdriver.js)
and otherwise consider filing a new issue. For instructions on how to add a new
testing API, see [WPT Test Automation for
Chromium](https://docs.google.com/document/d/18BpD41vyX1cFZ77CE0a_DJYlGpdvyLlx3pwXVRxUzvI/preview#)

An alternative is to write manual tests that are automated with scripts from
[wpt_automation](../../third_party/blink/web_tests/external/wpt_automation).
Injection of JS in manual tests is determined by `loadAutomationScript` in
[testharnessreport.js](../../third_party/blink/web_tests/resources/testharnessreport.js).

Such tests still require case-by-case automation to run for other browser
engines, but are more valuable than purely manual tests.

Manual tests that have no automation are still imported, but skipped in
[NeverFixTests](../../third_party/blink/web_tests/NeverFixTests); see
[issue 738489](https://crbug.com/738489).

### Adding new top-level directories

Entirely new top-level directories should generally be added upstream, since
that's the only way to add an OWNERS file upstream. After adding a new top-level
directory upstream, you should add a line for it in `W3CImportExpectations`.

Adding the new directory (and `W3CImportExpectations` entry) in Chromium and
later adding an OWNERS file upstream also works.

### Will the exported commits be linked to my GitHub profile?

The email you commit with in Chromium will be the author of the commit on
GitHub. You can [add it as a secondary address on your GitHub
account](https://help.github.com/articles/adding-an-email-address-to-your-github-account/)
to link your exported commits to your GitHub profile.

If you are a Googler, you can also register your GitHub account at go/github,
making it easier for other Googlers to find you.

### What if there are conflicts?

This cannot be avoided entirely as the two repositories are independent, but
should be rare with frequent imports and exports. When it does happen, manual
intervention will be needed and in non-trivial cases you may be asked to help
resolve the conflict.

### Direct pull requests

It's still possible to make direct pull requests to web-platform-tests, see
https://web-platform-tests.org/appendix/github-intro.html.

## Running tests

Same as Blink web tests, you can use
[`run_web_tests.py`](web_tests.md#running-the-tests) to run any WPT test.

One thing to note is that glob patterns for WPT tests are not yet supported.

See [Running WPT tests in Content Shell](web_tests_in_content_shell.md#Running-WPT-Tests-in-Content-Shell)
for debugging etc.

## Reviewing tests

Anyone who can review code and tests in Chromium can also review changes in
[external/wpt](../../third_party/blink/web_tests/external/wpt)
that will be automatically upstreamed. There will be no additional review in
web-platform-tests as part of the export process.

If upstream reviewers have feedback on the changes, discuss on the pull request
created during export, and if necessary work on a new pull request to iterate
until everyone is satisfied.

When reviewing tests, check that they match the relevant specification, which
may not fully match the implementation. See also
[Write tests against specifications](#Write-tests-against-specifications).