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A deep dive into WebKit's Tests.

Correctness Testing in WebKit

WebKit is really big on test driven development, we have many types of tests.

  • JavaScript tests - Resides in top-level JSTests directory. This is the primary method of testing JavaScriptCore. Use Tools/Scripts/run-javascriptcore-tests to run these tests.
  • Layout tests - Resides in top-level LayoutTests directory. This is the primary method of testing WebCore. If you’re making code changes to WebCore, you typically run these tests. Use Tools/Scripts/run-webkit-tests to run these. Pass -1 to run tests using WebKitLegacy (a.k.a. WebKit1). WebKitTestRunner is used to run these tests for WebKit2, and DumpRenderTree is used to these tests for WebKit1. There are a few styles of layout tests but all of them have a test file and expected result (ends with -expected.txt), and the test passes if the test file’s output matches that of the expected result.
  • API tests - Reside in Tools/TestWebKitAPI, these are GTests that test APIs exposed by JavaScriptCore, WebKitLegacy, and WebKit layers as well as unit tests for selected WTF classes. WebKit does not use XCTests. Use Tools/Scripts/run-api-tests to run these tests. Because these API tests are sequentially, it’s preferable to write layout tests when possible.
  • Bindings tests - Reside in Source/WebCore/bindings/scripts/test, these are tests for WebCore’s binding code generator. Use Tools/Scripts/run-bindings-tests to run these tests.
  • webkitpy tests - Tests for WebKit’s various Python scripts in Tools/Scripts/webkitpy. Use Tools/Scripts/test-webkitpy to run these tests.
  • webkitperl tests - Tests for WebKit’s various Perl scripts in Tools/Scripts/webkitperl. Use Tools/Scripts/test-webkitperl to run these tests.

Performance Testing in WebKit

The WebKit project has a "no performance regression" policy. We maintain the performance of the following of the benchmarks and are located under PerformanceTests. If your patch regresses one of these benchmarks even slightly (less than 1%), it will get reverted.

  • JetStream2 - Measures JavaScript and WASM performance.
  • MotionMark - Measures graphics performance.
  • Speedometer 2 - Measures WebKit’s performance for complex web apps.

The following are benchmarks maintained by Apple's WebKit team but not available to other open source contributors since Apple doesn't have the right to redistribute the content. If your WebKit patch regresses one of these tests, your patch may still get reverted.

  • RAMification - Apple's internal JavaScript memory benchmark.
  • ScrollPerf - Apple's internal scrolling performance tests.
  • PLT - Apple's internal page load time tests.
  • Membuster / PLUM - Apple's internal memory tests. Membuster for macOS and PLUM for iOS and iPadOS.

Layout Tests: Tests of the Web for the Web

Layout tests are WebKit tests written using Web technology such as HTML, CSS, and JavaScript, and it’s the primary mechanism by which much of WebCore is tested. Relevant layout test should be ran while you’re making code changes to WebCore and before uploading a patch to While’s Early Warning System will build and run tests on a set of configurations, individual patch authors are ultimately responsible for any test failures that their patches cause.

Test Files and Expected Files

Directory Structure

LayoutTests directory is organized by the category of tests. For example, LayoutTests/accessibility contains accessibility related tests, and LayoutTests/fast/dom/HTMLAnchorElement contains tests for the HTML anchor element.

Any file that ends in .html, .htm, .shtml, .xhtml, .mht, .xht, .xml, .svg, or .php is considered as a test unless it’s preceded with -ref, -notref, -expected, or -expected-mismatch (these are used for ref tests; explained later). It’s accompanied by another file of the same name except it ends in -expected.txt or -expected.png. These are called expected results and constitutes the baseline output of a given test. When layout tests are ran, the test runner generates an output in the form of a plain text file and/or an PNG image, and it is compared against these expected results.

In the case expected results may differ from one platform to another, the expected results for each test is stored in LayoutTests/platform. The expected result of a given test exists in the corresponding directory in each subdirectory of LayoutTests/platform. For example, the expected result of LayoutTests/svg/W3C-SVG-1.1/animate-elem-46-t.svg for macOS Mojave is located at LayoutTests/platform/mac-mojave/svg/W3C-SVG-1.1/animate-elem-46-t-expected.txt.

These platform directories have a fallback order. For example, running tests for WebKit2 on macOS Catalina will use the following fallback path from the most specific to most generic:

  • platform/mac-catalina-wk2 - Results for WebKit2 on macOS Catalina.
  • platform/mac-catalina - Results for WebKit2 and WebKitLegacy on macOS Catalina.
  • platform/mac-wk2 - Results for WebKit2 on all macOS.
  • platform/mac - Results for all macOS.
  • platform/wk2 - Results for WebKit2 on every operating system.
  • generic - Next to the test file.

Imported Tests

Tests under LayoutTests/imported are imported from other repositories. They should not be modified by WebKit patches unless the change is made in respective repositories first.

Most notable is Web Platform Tests, which are imported under LayoutTests/imported/w3c/web-platform-tests. These are cross browser vendor tests developed by W3C. Mozilla, Google, and Apple all contribute many tests to this shared test repository.

HTTP Tests

To open tests under LayoutTests/http or LayoutTests/imported/w3c/web-platform-tests, use Tools/Scripts/open-layout-test with the path to a test.

You can also manually start HTTP servers with Tools/Scripts/run-webkit-httpd. To stop the HTTP servers, exit the script (e.g. Control + C on macOS).

Tests under LayoutTests/http are accessible at except tests in LayoutTests/http/wpt, which are available at http://localhost:8800/WebKit/ instead.

The Web Platform Tests imported under LayoutTests/imported/w3c/web-platform-tests are accessible under HTTP at http://localhost:8800/ and HTTPS at http://localhost:9443/

Note that it's important to use the exact host names such as and localhost above verbatim since some tests rely on or test same-origin or cross-origin behaviors based on those host names.

Test Expectations

FIXME: Explain how test expectations work.

Running Layout Tests

To run layout tests, use Tools/Scripts/run-webkit-tests. It optionally takes file paths to a test file or directory and options on how to run a test. For example, in order to just run LayoutTests/fast/dom/Element/element-traversal.html, do:

Tools/Scripts/run-webkit-tests fast/dom/Element/element-traversal.html

Because there are 50,000+ tests in WebKit, you typically want to run a subset of tests that are relevant to your code change (e.g. LayoutTests/storage/indexeddb/ if you’re working on IndexedDB) while developing the code change, and run all layout tests at the end on your local machine or rely on the Early Warning System on for more thorough testing.

Specify --debug or --release to use either release or debug build. To run tests using iOS simulator, you can specify either --ios-simulator, --iphone-simulator, or --ipad-simulator based on whichever simulator is desired.

By default, run-webkit-tests will run all the tests you specified once in the lexicological order of test paths relative to LayoutTests directory and retry any tests that have failed. If you know the test is going to fail and don’t want retries, specify --no-retry-failures.

Because there are so many tests, run-webkit-tests will runs tests in different directories in parallel (i.e. all tests in a single directory is ran sequentially one after another). You can control the number of parallel test runners using --child-processes option.

run-webkit-tests has many options. Use --help to enumerate all the supported options.

Repeating Layout Tests

When you’re investigating flaky tests or crashes, it might be desirable to adjust this. --iterations X option will specify the number of times the list of tests are ran. For example, if we are running tests A, B, C and --iterations 3 is specified, run-webkit-tests will run: A, B, C, A, B, C, A, B, C. Similarly, --repeat-each option will specify the number of times each test is repeated before moving onto next test. For example, if we’re running tests A, B, C, and --repeat-each 3 is specified, run-webkit-tests will run: A, A, A, B, B, B, C, C, C. --exit-after-n-failures option will specify the total number of test failures before run-webkit-tests will stop. In particular, --exit-after-n-failures=1 is useful when investigating a flaky failure so that run-webkit-tests will stop when the failure actually happens for the first time.

Test Results

Whenever tests do fail, run-webkit-tests will store results in WebKitBuild/Debug/layout-test-results mirroring the same directory structure as LayoutTests. For example, the actual output produced for LayoutTests/editing/inserting/typing-001.html, if failed, will appear in WebKitBuild/Debug/layout-test-results/editing/inserting/typing-001-actual.txt. run-webkit-tests also generates a web page with the summary of results in WebKitBuild/Debug/layout-test-results/results.html and automatically tries to open it in Safari using the local build of WebKit.

If Safari fails to launch, specify --no-show-results and open results.html file manually.

Updating Expected Results

If you’ve updated a test content or test’s output changes with your code change (e.g. more test case passes), then you may have to update -expected.txt file accompanying the test. To do that, first run the test once to make sure the diff and new output makes sense in results.html, and run the test again with --reset-results. This will update the matching -expected.txt file.

You may need to manually copy the new result to other -expected.txt files that exist under LayoutTests for other platforms and configurations. Find other -expected.txt files when you’re doing this.

When a new test is added, run-webkit-tests will automatically generate new -expected.txt file for your test. You can disable this feature by specifying --no-new-test-results e.g. when the test is still under development.

Different Styles of Layout Tests

There are multiple styles of layout tests in WebKit.

Render tree dumps

This is the oldest style of layout tests, and the default mode of layout tests. It’s a text serialization of WebKit’s render tree and its output looks like this:

layer at (0,0) size 800x600
  RenderView at (0,0) size 800x600
layer at (0,0) size 800x600
  RenderBlock {HTML} at (0,0) size 800x600
    RenderBody {BODY} at (8,8) size 784x584
      RenderInline {A} at (0,0) size 238x18 [color=#0000EE]
        RenderInline {B} at (0,0) size 238x18
          RenderText {#text} at (0,0) size 238x18
            text run at (0,0) width 238: "the second copy should not be bold"
      RenderText {#text} at (237,0) size 5x18
        text run at (237,0) width 5: " "
      RenderText {#text} at (241,0) size 227x18
        text run at (241,0) width 227: "the second copy should not be bold"

This style of layout tests is discouraged today because its outputs are highly dependent on each platform, and end up requiring a specific expected result in each platform. But they’re still useful when testing new rendering and layout feature or bugs thereof.

These tests also have accompanying -expected.png files but run-webkit-tests doesn't check the PNG output against the expected result by default. To do this check, pass --pixel. Unfortunately, many pixel tests will fail because we have not been updating the expected PNG results a good chunk of the last decade. However, these pixel results might be useful when diagnosing a new test failure. For this reason, run-webkit-tests will automatically generate PNG results when retrying the test, effectively enabling --pixel option for retries.

dumpAsText test

These are tests that uses the plain text serialization of the test page as the output (as if the entire page’s content is copied as plain text). All these tests call testRunner.dumpAsText to trigger this behavior. The output typically contains a log of text or other informative output scripts in the page produced. For example, LayoutTests/fast/dom/anchor-toString.html is written as follows:

<a href="http://localhost/sometestfile.html" id="anchor">
A link!
        if (window.testRunner)

        var anchor = document.getElementById("anchor");
        document.write("Writing just the anchor object - " + anchor);

        var anchorString = String(anchor);
        document.write("<br><br>Writing the result of the String(anchor) - " + anchorString);

        var anchorToString = anchor.toString();
        document.write("<br><br>Writing the result of the anchor's toString() method - " + anchorToString);

and generates the following output:

A link! 

Writing just the anchor object - http://localhost/sometestfile.html

Writing the result of the String(anchor) - http://localhost/sometestfile.html

Writing the result of the anchor's toString() method - http://localhost/sometestfile.html

js-test.js and js-test-pre.js tests

These are variants of dumpAsText test which uses WebKit’s assertion library: LayoutTests/resources/js-test.js and LayoutTests/resources/js-test-pre.js. It consists of shouldX function calls which takes two JavaScript code snippet which are then executed and outputs of which are compared. js-test.js is simply a new variant of js-test-pre.js that doesn’t require the inclusion of LayoutTests/resources/js-test-post.js at the end. Use js-test.js in new tests, not js-test-pre.js.

For example, LayoutTests/fast/dom/Comment/remove.html which tests remove() method on Comment node is written as:

<!DOCTYPE html>
<script src="../../../resources/js-test-pre.js"></script>
<div id="test"></div>

description('This tests the DOM 4 remove method on a Comment.');

var testDiv = document.getElementById('test');
var comment = document.createComment('Comment');
shouldBe('testDiv.childNodes.length', '1');
shouldBe('testDiv.childNodes.length', '0');
shouldBe('testDiv.childNodes.length', '0');

<script src="../../../resources/js-test-post.js"></script>

with the following expected result (output):

This tests the DOM 4 remove method on a Comment.

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

PASS testDiv.childNodes.length is 1
PASS testDiv.childNodes.length is 0
PASS testDiv.childNodes.length is 0
PASS successfullyParsed is true


description function specifies the description of this test, and subsequent shouldBe calls takes two strings, both of which are evaluated as JavaScript and then compared.

Some old js-test-pre.js tests may put its test code in a separate JS file but we don’t do that anymore to keep all the test code in one place.

js-test.js and js-test-pre.js provide all kinds of other assertion and helper functions. Here are some examples:

  • debug(msg) - Inserts a debug / log string in the output.
  • evalAndLog(code) - Similar to debug() but evaluates code as JavaScript.
  • shouldNotBe(a, b) - Generates PASS if the results of evaluating a and b differ.
  • shouldBeTrue(code) - Shorthand for shouldBe(code, 'true').
  • shouldBeFalse(code) - Shorthand for shouldBe(code, 'false').
  • shouldBeNaN(code) - Shorthand for shouldBe(code, 'NaN').
  • shouldBeNull(code) - Shorthand for shouldBe(code, 'null').
  • shouldBeZero(code) - Shorthand for shouldBe(code, '0').
  • shouldBeEqualToString(code, string) - Similar to shouldBe but the second argument is not evaluated as string.
  • finishJSTest() - When js-test.js style test needs to do some async work, define the global variable named jsTestIsAsync and set it to true. When the test is done, call this function to notify the test runner (don’t call testRunner.notifyDone mentioned later directly). See an example.

It’s important to note that these shouldX functions only add output strings that say PASS or FAIL. If the expected result also contains the same FAIL strings, then run-webkit-tests will consider the whole test file to have passed.

Another way to think about this is that -expected.txt files are baseline outputs, and baseline outputs can contain known failures.

There is a helper script to create a template for a new js-test.js test. The following will create new test named new-test.html in LayoutTests/fast/dom:

Tools/Scripts/make-new-script-test fast/dom/new-test.html

dump-as-markup.js Tests

A dump-as-markup.js test is yet another variant of dumpAsText test, which uses LayoutTests/resources/dump-as-markup.js. This style of test is used when it’s desirable to compare the state of the DOM tree before and after some operations. For example, many tests under LayoutTests/editing use this style of testing to test complex DOM mutation operations such as pasting HTML from the users’ clipboard. dump-as-markup.js adds Markup on the global object and exposes a few helper functions. Like js-test.js tests, a test description can be specified via Markup.description. The test then involves Markup.dump(node, description) to serialize the state of DOM tree as plain text where element is either a DOM node under which the state should be serialized or its id.

For example, LayoutTests/editing/inserting/insert-list-in-table-cell-01.html is written as follows:

<!DOCTYPE html>
<div id="container" contenteditable="true"><table border="1"><tr><td id="element">fsdf</td><td>fsdf</td></tr><tr><td>gghfg</td><td>fsfg</td></tr></table></div>
<script src="../editing.js"></script>
<script src="../../resources/dump-as-markup.js"></script>
    Markup.description('Insert list items in a single table cell:');

    var e = document.getElementById("element");
    setSelectionCommand(e, 0, e, 1);
    Markup.dump('container', 'Before');

    Markup.dump('container', 'After');

with the following expected result:

Insert list items in a single table cell:

| <table>
|   border="1"
|   <tbody>
|     <tr>
|       <td>
|         id="element"
|         "<#selection-anchor>fsdf<#selection-focus>"
|       <td>
|         "fsdf"
|     <tr>
|       <td>
|         "gghfg"
|       <td>
|         "fsfg"

| <table>
|   border="1"
|   <tbody>
|     <tr>
|       <td>
|         id="element"
|         <ol>
|           <li>
|             "<#selection-anchor>fsdf<#selection-focus>"
|             <br>
|       <td>
|         "fsdf"
|     <tr>
|       <td>
|         "gghfg"
|       <td>
|         "fsfg"

testharness.js Tests

This is yet another variant of dumpAsText test which uses the test harness of Web Platform Tests, which is W3C’s official tests for the Web. There is an extensive documentation on how this harness works.

As mentioned above, do not modify tests in LayoutTests/imported/w3c/web-platform-tests unless the same test changes are made in Web Platform Tests’ primary repository.

Reference Tests

Reference tests are special in that they don’t have accompanying -expected.txt files. Instead, they have a matching or mismatching expected result file. Both the test file and the accompanying matching or mismatching expected result generate PNG outputs. The test passes if the PNG outputs of the test and the matching expected result are the same; the test fails otherwise. For a test with a mismatching expected result, the test passes if the PNG outputs of the test and the mismatching expected result are not the same, and fails if they are the same.

A matching expected result or a mismatching expected result can be specified in a few ways:

  • The file with the same name as the test name except it ends with -expected.* or -ref.* is a matching expected result for the test.
  • The file with the same name as the test name except it ends with -expected-mismatch.* or -notref.* is a matching expected result for the test.
  • The file specified by a HTML link element in the test file with match relation: <link rel=match href=X> where X is the relative file path is a matching expected result.
  • The file specified by a HTML link element in the test file with mismatch relation: <link rel=mismatch href=X> where X is the relative file path is a mismatching expected result.

For example, LayoutTests/imported/w3c/web-platform-tests/html/rendering/replaced-elements/images/space.html specifies space-ref.html in the same directory as the matching expected result as follows:

<!doctype html>
<meta charset=utf-8>
<title>img hspace/vspace</title>
<link rel=match href=space-ref.html>
span { background: blue; }
<div style=width:400px;>
<p><span><img src=/images/green.png></span>
<p><span><img src=/images/green.png hspace=10></span>
<p><span><img src=/images/green.png vspace=10></span>
<p><span><img src=/images/green.png hspace=10%></span>
<p><span><img src=/images/green.png vspace=10%></span>

Test Runners

Most layout tests are designed to be runnable inside a browser but run-webkit-tests uses a special program to run them. Our continuous integration system as well as the Early Warning System uses run-webkit-tests to run layout tests. In WebKit2, this is appropriately named WebKitTestRunner. In WebKit1 or WebKitLegacy, it’s DumpRenderTree, which is named after the very first type of layout tests, which generated the text representation of the render tree.

Extra Interfaces Available in Test Runners

Both WebKitTestRunner and DumpRenderTree expose a few extra interfaces to JavaScript on window (i.e. global object) in order to emulate user inputs, enable or disable a feature, or to improve the reliability of testing.

  • GCController
    • GCController.collect() triggers a synchronous full garbage collection. This function is useful for testing crashes or erroneous premature collection of JS wrappers and leaks.
  • testRunner
    • TestRunner interface exposes many methods to control the behaviors of WebKitTestRunner and DumpRenderTree. Some the most commonly used methods are as follows:
    • waitUntilDone() / notifyDone() - These functions are useful when writing tests that involve asynchronous tasks which may require the test to continue running beyond when it finished loading. testRunner.waitUntilDone() makes WebKitTestRunner and DumpRenderTree not end the test when a layout test has finished loading. The test continues until testRunner.notifyDone() is called.
    • dumpAsText(boolean dumpPixels) - Makes WebKitTestRunner and DumpRenderTree output the plain text of the loaded page instead of the state of the render tree.
    • overridePreference(DOMString preference, DOMString value) - Overrides WebKit’s preferences. For WebKitLegacy, these are defined in Source/WebKitLegacy/mac/WebView/WebPreferences.h for macOS and Source/WebKitLegacy/win/WebPreferences.h for Windows.
  • eventSender
    • Exposes methods to emulate mouse, keyboard, and touch actions. Use ui-helpers.js script instead of directly calling methods on this function. This will ensure the test will be most compatible with all the test configurations we have.
  • UIScriptController
    • Exposes methods to emulate user inputs like eventSender mostly on iOS WebKit2. Use ui-helpers.js script instead of directly calling methods on this function. This will ensure the test will be most compatible with all the test configurations we have.
  • textInputController

Additionally, WebCore/testing exposes a few testing hooks to test its internals:

  • internals
    • Exposes various hooks into WebCore that shouldn’t be part of WebKit or WebKitLegacy API.
  • internals.settings
    • Exposes various WebCore settings and let tests override them. Note that WebKit layer code may depend on preferences in UI process and the aforementioned testRunner.overridePreference may need to be used instead. It’s in fact preferable to override the equivalent preference via testRunner.overridePreference unless you know for sure WebKit or WebKitLegacy layer of code isn’t affected by the setting you’re overriding.

Enabling or Disabling a Feature in Test Runners

FIXME: Mention test-runner-options

Test Harness Scripts

FIXME: Write about dump-as-markup.js, and ui-helper.js

Investigating Test Failures Observed on Bots

There are multiple tools to investigate test failures happening on our continuous integration system ( The most notable is flakiness dashboard:

FIXME: Write how to investigate a test failure.

Dive into API tests

FIXME: Talk about how to debug API tests.