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

Watch a Talk

If you prefer watching a video, here is a talk introducing web-platform-tests:


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, with the exception of css/ which contains testsuites for CSS WG specifications. 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 META.yml files in those same directories and their parents (i.e., they work recursively: a/META.yml 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 META.yml file!

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).

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 preceed 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 %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.

The test environment can then be started using

./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

The wpt run command provides a frontend for running tests automatically in various browsers. The general syntax is:

wpt run [options] <product> [test paths]

e.g. to run dom/historical.html in Firefox, the required command is:

wpt run firefox dom/historical.html

Windows Notes

Generally Windows Subsystem for Linux will provide the smoothest user experience for running web-platform-tests on Windows.

The standard Windows shell requires that all wpt commands are prefixed by the Python binary i.e. assuming python is on your path the server is started using:

python wpt serve