Test Format Guidelines

This page describes the available test types and the requirements for authoring that apply to all test types. There is also a supplementary guide to writing good testcases.

Test Locations

Each top level directory in the repository corresponds to tests for a single specification. For W3C specs, these directories are named after the shortname of the spec (i.e. the name used for snapshot publications under /TR/).

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. This layout 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.

When adding new tests to existing specifications, try to follow the structure of existing tests.

Because of path length limitations on Windows, test paths must be less that 150 characters relative to the test root directory (this gives vendors just over 100 characters for their own paths when running in automation).

Choosing the Test Type

Tests should be written using the mechanism that is most conducive to running in automation. In general the following order of preference holds:

  • idlharness.js tests - for testing anything in a WebIDL block.

  • testharness.js tests - for any test that can be written using script alone.

  • Reftests - for most tests of rendering.

  • WebDriver tests - for testing the webdriver protocol itself or (in the future) for certain tests that require access to privileged APIs.

  • Manual tests - as a last resort for anything that can't be tested using one of the above techniques.

Some scenarios demand certain test types. For example:

  • Tests for layout will generally be reftests. In some cases it will not be possible to construct a reference and a test that will always render the same, in which case a manual test, accompanied by testharness tests that inspect the layout via the DOM must be written.

  • Features that require human interaction for security reasons (e.g. to pick a file from the local filesystem) typically have to be manual tests.

General Test Design Requirements


Tests should be as short as possible. For reftests in particular scrollbars at 800×600px window size must be avoided unless scrolling behaviour is specifically being tested. For all tests extraneous elements on the page should be avoided so it is clear what is part of the test (for a typical testharness test, the only content on the page will be rendered by the harness itself).


Tests should generally avoid depending on edge case behaviour of features that they don't explicitly intend to test. For example, except where testing parsing, tests should contain no parse errors. Of course tests which intentionally address the interactions between multiple platform features are not only acceptable but encouraged.


Tests should be as cross-platform as reasonably possible, working across different devices, screen resolutions, paper sizes, etc. Exceptions should document their assumptions.


Tests must not depend on external network resources, including w3c-test.org. When these tests are run on CI systems they are typically configured with access to external resources disabled, so tests that try to access them will fail. Where tests want to use multiple hosts this is possible thorough a known set of subdomains and features of wptserve (see "Tests Involving Multiple Origins").

File Names

Generally file names should be somewhat descriptive of what is being tested; very generic names like 001.html are discouraged. A common format, required by CSS tests, is described in CSS Naming Conventions.

File Formats

Tests must be HTML, XHTML or SVG files.

Note: For CSS tests, the test source will be parsed and re-serialized. This re-serialization will cause minor changes to the test file, notably: attribute values will always be quoted, whitespace between attributes will be collapsed to a single space, duplicate attributes will be removed, optional closing tags will be inserted, and invalid markup will be normalized. If these changes should make the test inoperable, for example if the test is testing markup error recovery, add the flag asis to prevent re-serialization. This flag will also prevent format conversions so it may be necessary to provide alternate versions of the test in other formats (XHTML, HTML, etc.)

Character Encoding

Except when specifically testing encoding, tests must be encoded in UTF-8, marked through the use of e.g. <meta charset=utf-8>, or in pure ASCII.

Support files

Various support files are available in in the /common/ and /media/ directories (web-platform-tests) and /support/ (CSS). Reusing existing resources is encouraged where possible, as is adding generally useful files to these common areas rather than to specific testsuites.

For CSS tests the following standard images are available in the support directory:

  • 1x1 color swatches
  • 15x15 color swatches
  • 15x15 bordered color swatches
  • assorted rulers and red/green grids
  • a cat
  • a 4-part picture


Sometimes you may want to add a script to the repository that's meant to be used from the command line, not from a browser (e.g., a script for generating test files). If you want to ensure (e.g., or security reasons) that such scripts won't be handled by the HTTP server, but will instead only be usable from the command line, then place them in either:

  • the tools subdir at the root of the repository, or
  • the tools subdir at the root of any top-level directory in the repo which contains the tests the script is meant to be used with

Any files in those tools directories won't be handled by the HTTP server; instead the server will return a 404 if a user navigates to the URL for a file within them.

If you want to add a script for use with a particular set of tests but there isn't yet any tools subdir at the root of a top-level directory in the repository containing those tests, you can create a tools subdir at the root of that top-level directory and place your scripts there.

For example, if you wanted to add a script for use with tests in the notifications directory, create the notifications/tools subdir and put your script there.

Style Rules

A number of style rules should be applied to the test file. These are not uniformly enforced throughout the existing tests, but will be for new tests. Any of these rules may be broken if the test demands it:

  • No trailing whitespace

  • Use spaces rather than tabs for indentation

  • Use UNIX-style line endings (i.e. no CR characters at EOL).

Advanced Testing Features

Certain test scenarios require more than just static HTML generation. This is supported through the wptserve server. Several scenarios in particular are common:

Standalone workers tests

Tests that only require assertions in a dedicated worker scope can use standalone workers tests. In this case, the test is a JavaScript file with extension .worker.js that imports testharness.js. The test can then use all the usual APIs, and can be run from the path to the JavaScript file with the .js removed.

For example, one could write a test for the Blob constructor by creating a FileAPI/Blob-constructor.worker.js as follows:

test(function () {
  var blob = new Blob();
  assert_equals(blob.size, 0);
  assert_equals(blob.type, "");
}, "The Blob constructor.");

This test could then be run from FileAPI/Blob-constructor.worker.

Tests Involving Multiple Origins

In the test environment, five subdomains are available; www, www1, www2, 天気の良い日 and élève. These must be used for cross-origin tests. In addition two ports are available for http and one for websockets. Tests must not hardcode the hostname of the server that they expect to be running on or the port numbers, as these are not guaranteed by the test environment. Instead tests can get this information in one of two ways:

  • From script, using the location API.

  • By using a textual substitution feature of the server.

In order for the latter to work, a file must either have a name of the form {name}.sub.{ext} e.g. example-test.sub.html or be referenced through a URL containing pipe=sub in the query string e.g. example-test.html?pipe=sub. The substitution syntax uses `` to delimit items for substitution. For example to substitute in the host name on which the tests are running, one would write:

As well as the host, one can get full domains, including subdomains using the domains dictionary. For example:

would be replaced by the fully qualified domain name of the www subdomain. Ports are also available on a per-protocol basis e.g.

is replaced with the first (and only) websockets port, whilst

is replaced with the second HTTP port.

The request URL itself can be used as part of the substitution using the location dictionary, which has entries matching the window.location API. For example

is replaced by hostname:port for the current request.

Tests Requiring Special Headers

For tests requiring that a certain HTTP header is set to some static value, a file with the same path as the test file except for an an additional .headers suffix may be created. For example for /example/test.html, the headers file would be /example/test.html.headers. This file consists of lines of the form

header-name: header-value

For example

Content-Type: text/html; charset=big5

To apply the same headers to all files in a directory use a __dir__.headers file. This will only apply to the immediate directory and not subdirectories.

Headers files may be used in combination with substitutions by naming the file e.g. test.html.sub.headers.

Tests Requiring Full Control Over The HTTP Response

For full control over the request and response the server provides the ability to write .asis files; these are served as literal HTTP responses. It also provides the ability to write python scripts that have access to request data and can manipulate the content and timing of the response. For details see the wptserve documentation.

CSS-Specific Requirements

Tests for CSS specs have some additional requirements that have to be met in order to be included in an official specification testsuite.

Lint tool

We have a lint tool for catching common mistakes in test files. You can run it manually by starting the lint executable from the root of your local web-platform-tests working directory like this:


The lint tool is also run automatically for every submitted pull request, and reviewers will not merge branches with tests that have lint errors, so you must fix any errors the lint tool reports. For details on doing that, see the lint-tool documentation.

But in the unusual case of error reports for things essential to a certain test or that for other exceptional reasons shouldn't prevent a merge of a test, update and commit the lint.whitelist file in the web-platform-tests root directory to suppress the error reports. For details on doing that, see the lint-tool documentation.