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).
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.
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.
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
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.
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.,
B.htmlmust 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.
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 OWNERS
files in those same directories and their parents (i.e., they work
a/OWNERS will get notified for
If you want to be notified about changes to tests in a directory, feel free to add yourself to the OWNERS file: there’s no requirement to own anything as a result!
The tests are designed to be run from your local computer. The test environment requires Python 2.7+ (but not Python 3.x).
To get the tests running, you need to set up the test domains in your
The necessary content can be generated with
./wpt make-hosts-file; on
Windows, you will need to preceed the prior command with
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 (note this requires an Administrator privileged shell):
python wpt make-hosts-file >> %SystemRoot%\System32\drivers\etc\hosts
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
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, copy the
config.default.json file to
config.json and edit the new file,
replacing the part that reads:
"http": [8000, "auto"], "https":
to some ports of your choice e.g.
"http": [1234, "auto"], "https":
hosts file is configured, the servers will be locally accessible at:
*See Trusting Root CA
Running tests automatically
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
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
python wpt serve