New documentation on writing LayoutTests.
This CL pulls out the content on writing LayoutTests from
layout_tests.md, and does a significant overhaul of it.
BUG=665494
Review-Url: https://codereview.chromium.org/2492733003
Cr-Commit-Position: refs/heads/master@{#434842}
diff --git a/docs/testing/layout_tests.md b/docs/testing/layout_tests.md
index 1931901..d4be954 100644
--- a/docs/testing/layout_tests.md
+++ b/docs/testing/layout_tests.md
@@ -5,6 +5,10 @@
in a test renderer (`content_shell`) and comparing the rendered output or
JavaScript output against an expected output file.
+This document covers running and debugging existing layout tests. See the
+[Writing Layout Tests documentation](./writing_layout_tests.md) if you find
+yourself writing layout tests.
+
[TOC]
## Running Layout Tests
@@ -62,15 +66,19 @@
[TestExpectations](../../third_party/WebKit/LayoutTests/TestExpectations)
won't be run at all, generally because they cause some intractable tool error.
To force one of them to be run, either rename that file or specify the skipped
-test as the only one on the command line (see below).
+test as the only one on the command line (see below). Read the
+[Layout Test Expectations documentation](./layout_test_expectations.md) to learn
+more about TestExpectations and related files.
-Note that currently only the tests listed in
+*** promo
+Currently only the tests listed in
[SmokeTests](../../third_party/WebKit/LayoutTests/SmokeTests)
are run on the Android bots, since running all layout tests takes too long on
Android (and may still have some infrastructure issues). Most developers focus
their Blink testing on Linux. We rely on the fact that the Linux and Android
behavior is nearly identical for scenarios outside those covered by the smoke
tests.
+***
To run only some of the tests, specify their directories or filenames as
arguments to `run_webkit_tests.py` relative to the layout test directory
@@ -179,9 +187,9 @@
[TestExpectations](../../WebKit/LayoutTests/TestExpectations) file (and related
files, including
[skia_test_expectations.txt](../../skia/skia_test_expectations.txt))
-contains the list of all known layout test failures. See
-[Test Expectations](./layout_test_expectations.md)
-for more on this.
+contains the list of all known layout test failures. See the
+[Layout Test Expectations documentation](./layout_test_expectations.md) for more
+on this.
## Testing Runtime Flags
@@ -275,95 +283,6 @@
You can also use the _Layout Test Failure_ template, which will pre-set these
labels for you.
-## Writing Layout Tests
-
-### Pixel Tests
-
-TODO: Write documentation here.
-
-### Reference Tests
-
-TODO: Write documentation here.
-
-### Script Tests
-
-These tests use a JavaScript test harness and test cases written in script to
-exercise features and make assertions about the behavior. Generally, new tests
-are written using the [testharness.js](https://github.com/w3c/testharness.js/)
-test harness, which is also heavily used in the cross-vendor
-[web-platform-tests](https://github.com/w3c/web-platform-tests) project. Tests
-written with testharness.js generally look something like the following:
-
-```html
-<!DOCTYPE html>
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
-<script>
-test(t => {
- var x = true;
- assert_true(x);
-}, "Truth is true.");
-</script>
-```
-
-Many older tests are written using the **js-test**
-(`LayoutTests/resources/js-test.js`) test harness. This harness is
-**deprecated**, and should not be used for new tests. The tests call
-`testRunner.dumpAsText()` to signal that the page content should be dumped and
-compared against an \*-expected.txt file, and optionally
-`testRunner.waitUntilDone()` and `testRunner.notifyDone()` for asynchronous
-tests.
-
-### Tests that use a HTTP Server
-
-By default, tests are loaded as if via `file:` URLs. Some web platform features
-require tests served via HTTP or HTTPS, for example relative paths (`src=/foo`)
-or features restricted to secure protocols.
-
-HTTP tests are those tests that are under `LayoutTests/http/tests` (or virtual
-variants). Use a locally running HTTP server (Apache) to run. Tests are served
-off of ports 8000, 8080 for HTTP and 8443 for HTTPS. If you run the tests using
-`run-webkit-tests`, the server will be started automatically.To run the server
-manually to reproduce or debug a failure:
-
-```bash
-cd src/third_party/WebKit/Tools/Scripts
-run-blink-httpd start
-```
-
-The layout tests will be served from `http://127.0.0.1:8000`. For example, to
-run the test `http/tests/serviceworker/chromium/service-worker-allowed.html`,
-navigate to
-`http://127.0.0.1:8000/serviceworker/chromium/service-worker-allowed.html`. Some
-tests will behave differently if you go to 127.0.0.1 instead of localhost, so
-use 127.0.0.1.
-
-To kill the server, run `run-blink-httpd --server stop`, or just use `taskkill`
-or the Task Manager on Windows, and `killall` or Activity Monitor on MacOS.
-
-The test server sets up an alias to `LayoutTests/resources` directory. In HTTP
-tests, you can access the testing framework at e.g.
-`src="/js-test-resources/js-test.js"`.
-
-### Writing tests that need to paint, raster, or draw a frame of intermediate output
-
-A layout test does not actually draw frames of output until the test exits. If
-it is required to generate a painted frame, then use
-`window.testRunner.displayAsyncThen`, which will run the machinery to put up a
-frame, then call the passed callback. There is also a library at
-`fast/repaint/resources/text-based-repaint.js` to help with writing paint
-invalidation and repaint tests.
-
-#### Layout test support for `testRunner`
-
-Some layout tests rely on the testRunner object to expose configuration for
-mocking the platform. This is provided in content_shell, here's a UML diagram of
-testRunner bindings configuring platform implementation:
-
-[](https://docs.google.com/drawings/d/1KNRNjlxK0Q3Tp8rKxuuM5mpWf4OJQZmvm9_kpwu_Wwg/edit)
-
-[Writing reliable layout tests](https://docs.google.com/document/d/1Yl4SnTLBWmY1O99_BTtQvuoffP8YM9HZx2YPkEsaduQ/edit)
-
## Debugging Layout Tests
After the layout tests run, you should get a summary of tests that pass or fail.
diff --git a/docs/testing/writing_layout_tests.md b/docs/testing/writing_layout_tests.md
new file mode 100644
index 0000000..6656762
--- /dev/null
+++ b/docs/testing/writing_layout_tests.md
@@ -0,0 +1,787 @@
+# Writing Layout Tests
+
+_Layout tests_ is a bit of a misnomer. This term is
+[a part of our WebKit heritage](https://webkit.org/blog/1452/layout-tests-theory/),
+and we use it to refer to every test that is written as a Web page (HTML, SVG,
+or XHTML) and lives in
+[third_party/WebKit/LayoutTests/](../../third_party/WebKit/LayoutTests).
+
+[TOC]
+
+## Overview
+
+Layout tests should be used to accomplish one of the following goals:
+
+1. The entire surface of Blink that is exposed to the Web should be covered by
+ tests that we contribute to the
+ [Web Platform Tests Project](https://github.com/w3c/web-platform-tests)
+ (WPT). This helps us avoid regressions, and helps us identify Web Platform
+ areas where the major browsers don't have interoperable implementations.
+ Furthermore, by contributing to projects such as WPT, we share the burden of
+ writing tests with the other browser vendors, and we help all the browsers
+ get better. This is very much in line with our goal to move the Web forward.
+2. When a Blink feature cannot be tested using the tools provided by WPT, and
+ cannot be easily covered by
+ [C++ unit tests](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/web/tests/?q=webframetest&sq=package:chromium&type=cs),
+ the feature must be covered by layout tests, to avoid unexpected regressions.
+ These tests will use Blink-specific testing APIs that are only available in
+ [content_shell](./layout_tests_in_content_shell.md).
+
+*** promo
+If you know that Blink layout tests are upstreamed to other projects, such as
+[test262](https://github.com/tc39/test262), please update this document. Most
+importantly, our guidelines should to make it easy for our tests to be
+upstreamed. The `blink-dev` mailing list will be happy to help you harmonize our
+current guidelines with communal test repositories.
+***
+
+### Test Types
+
+There are four broad types of layout tests, listed in the order of preference.
+
+* *JavaScript Tests* are the layout test implementation of
+ [xUnit tests](https://en.wikipedia.org/wiki/XUnit). These tests contain
+ assertions written in JavaScript, and pass if the assertions evaluate to
+ true.
+* *Reference Tests* render a test page and a reference page, and pass if the two
+ renderings are identical, according to a pixel-by-pixel comparison. These
+ tests are less robust, harder to debug, and significantly slower than
+ JavaScript tests, and are only used when JavaScript tests are insufficient,
+ such as when testing paint code.
+* *Pixel Tests* render a test page and compare the result against a pre-rendered
+ baseline image in the repository. Pixel tests are less robust than all
+ alternatives listed above, because the rendering of a page is influenced by
+ many factors such as the host computer's graphics card and driver, the
+ platform's text rendering system, and various user-configurable operating
+ system settings. For this reason, it is common for a pixel test to have a
+ different reference image for each platform that Blink is tested on. Pixel
+ tests are least preferred, because the reference images are
+ [quite cumbersome to manage](./layout_test_expectations.md).
+* *Dump Render Tree (DRT) Tests* output a textual representation of the render
+ tree, which is the key data structure in Blink's page rendering system. The
+ test passes if the output matches a baseline text file in the repository. In
+ addition to their text result, DRT tests can also produce an image result
+ which is compared to an image baseline, similarly to pixel tests (described
+ above). A DRT test with two results (text and image) passes if _both_ results
+ match the baselines in the repository. DRT tests are less desirable than all
+ the alternatives, because they depend on a browser implementation detail.
+
+## General Principles
+
+The principles below are adapted from
+[Test the Web Forward's Test Format Guidelines](http://testthewebforward.org/docs/test-format-guidelines.html)
+and
+[WebKit's Wiki page on Writing good test cases](https://trac.webkit.org/wiki/Writing%20Layout%20Tests%20for%20DumpRenderTree).
+
+* Tests should be **concise**, without compromising on the principles below.
+ Every element and piece of code on the page should be necessary and relevant
+ to what is being tested. For example, don't build a fully functional signup
+ form if you only need a text field or a button.
+ * Content needed to satisfy the principles below is considered necessary.
+ For example, it is acceptable and desirable to add elements that make
+ the test self-describing (see below), and to add code that makes the
+ test more reliable (see below).
+ * Content that makes test failures easier to debug is considered necessary
+ (to maintaining a good development speed), and is both acceptable and
+ desirable.
+ * Conciseness is particularly important for reference tests and pixel
+ tests, as the test pages are rendered in an 800x600px viewport. Having
+ content outside the viewport is undesirable because the outside content
+ does not get compared, and because the resulting scrollbars are
+ platform-specific UI widgets, making the test results less reliable.
+
+* Tests should be as **fast** as possible, without compromising on the
+ principles below. Blink has several thousand layout tests that are run in
+ parallel, and avoiding unnecessary delays is crucial to keeping our Commit
+ Queue in good shape.
+ * Avoid [window.setTimeout](https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setTimeout),
+ as it wastes time on the testing infrastructure. Instead, use specific
+ event handlers, such as
+ [window.onload](https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers/onload),
+ to decide when to advance to the next step in a test.
+
+* Tests should be **reliable** and yield consistent results for a given
+ implementation. Flaky tests slow down your fellow developers' debugging
+ efforts and the Commit Queue.
+ * `window.setTimeout` is again a primary offender here. Asides from wasting
+ time on a fast system, tests that rely on fixed timeouts can fail when run
+ on systems that are slower than expected.
+ * Follow the guidelines in this
+ [PSA on writing reliable layout tests](https://docs.google.com/document/d/1Yl4SnTLBWmY1O99_BTtQvuoffP8YM9HZx2YPkEsaduQ/edit).
+
+* Tests should be **self-describing**, so that a project member can recognize
+ whether a test passes or fails without having to read the specification of the
+ feature being tested. `testharness.js` makes a test self-describing when used
+ correctly, but tests that degrade to manual tests
+ [must be carefully designed](http://testthewebforward.org/docs/test-style-guidelines.html)
+ to be self-describing.
+
+* Tests should require a **minimal** amount of cognitive effort to read and
+ maintain.
+ * Avoid depending on edge case behavior of features that aren't explicitly
+ covered by the test. For example, except where testing parsing, tests
+ should contain valid markup (no parsing errors).
+ * Tests should provide as much relevant information as possible when
+ failing. `testharness.js` tests should prefer
+ [rich assert_ functions](https://github.com/w3c/testharness.js/blob/master/docs/api.md#list-of-assertions)
+ to combining `assert_true()` with a boolean operator. Using appropriate
+ `assert_` functions results in better diagnostic output when the assertion
+ fails.
+ * Prefer JavaScript's
+ [===](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Operators/Comparison_Operators#Identity_strict_equality_())
+ operator to
+ [==](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Operators/Comparison_Operators#Equality_())
+ so that readers don't have to reason about
+ [type conversion](http://www.ecma-international.org/ecma-262/6.0/#sec-abstract-equality-comparison).
+
+* Tests should be as **cross-platform** as reasonably possible. Avoid
+ assumptions about device type, screen resolution, etc. Unavoidable assumptions
+ should be documented.
+ * When possible, tests should only use Web platform features, as specified
+ in the relevant standards. When the Web platform's APIs are insufficient,
+ tests should prefer to use WPT extended testing APIs, such as
+ `wpt_automation`.
+ * Test pages should use the HTML5 doctype (`<!doctype html>`) unless they
+ specifically cover
+ [quirks mode](https://developer.mozilla.org/docs/Quirks_Mode_and_Standards_Mode)
+ behavior.
+ * Tests should be written under the assumption that they will be upstreamed
+ to the WPT project. For example, tests should follow the
+ [WPT guidelines](http://testthewebforward.org/docs/writing-tests.html).
+ * Tests that use Blink-specific testing APIs should feature-test for the
+ presence of the testing APIs and degrade to
+ [manual tests](http://testthewebforward.org/docs/manual-test.html) when
+ the testing APIs are not present. _This is not currently enforced in code
+ review. However, please keep in mind that a manual test can be debugged in
+ the browser, whereas a test that does not degrade gracefully can only be
+ debugged in the test runner._
+
+* Tests must be **self-contained** and not depend on external network resources.
+ Unless used by multiple test files, CSS and JavaScript should be inlined using
+ `<style>` and `<script>` tags. Content shared by multiple tests should be
+ placed in a `resources/` directory near the tests that share it. See below for
+ using multiple origins in a test.
+
+* Test **file names** should describe what is being tested. File names should
+ use `snake-case`, but preserve the case of any embedded API names. For
+ example, prefer `document-createElement.html` to
+ `document-create-element.html`.
+
+* Tests should prefer **modern features** in JavaScript and in the Web Platform.
+ * Tests should use
+ [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode)
+ for all JavaScript, except when specifically testing sloppy mode behavior.
+ Strict mode flags deprecated features and helps catch some errors, such as
+ forgetting to declare variables.
+ * JavaScript code should prefer
+ [const](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Statements/const)
+ and
+ [let](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Statements/let)
+ over `var`,
+ [classes](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Classes)
+ over other OOP constructs, and
+ [Promises](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Promise)
+ over other mechanisms for structuring asynchronous code.
+ * The desire to use modern features must be balanced with the desire for
+ cross-platform tests. Avoid using features that haven't been shipped by
+ other developed major rendering engines (WebKit, Gecko, Edge). When
+ unsure, check [caniuse.com](http://caniuse.com/).
+
+* Tests should use the UTF-8 **character encoding**, which should be declared by
+ `<meta charset=utf-8>`. This does not apply when specifically testing
+ encodings.
+ * At this time, code reviewers may choose to accept layout tests that do
+ not have a `<meta charset>`, as long as the file content is pure ASCII.
+ If going that route, please keep in mind that Firefox currently issues a
+ dev tools warning for pages without a declared charset.
+
+* Tests should aim to have a **coding style** that is consistent with
+ [Google's JavaScript Style Guide](https://google.github.io/styleguide/jsguide.html),
+ and
+ [Google's HTML/CSS Style Guide](https://google.github.io/styleguide/htmlcssguide.xml),
+ with the following exceptions.
+ * Rules related to Google Closure and JSDoc do not apply.
+ * Modern Web Platform and JavaScript features should be preferred to legacy
+ constructs that target old browsers. For example, prefer `const` and `let`
+ to `var`, and prefer `class` over other OOP constructs. This should be
+ balanced with the desire to have cross-platform tests.
+ * Concerns regarding buggy behavior in legacy browsers do not apply. For
+ example, the garbage collection cycle note in the _Closures_ section does
+ not apply.
+ * Per the JavaScript guide, new tests should also follow any per-project
+ style guide, such as the
+ [ServiceWorker Tests Style guide](http://www.chromium.org/blink/serviceworker/testing).
+
+*** note
+This document intentionally uses _should_ a lot more than _must_, as defined in
+[RFC 2119](https://www.ietf.org/rfc/rfc2119.txt). Writing layout tests is a
+careful act of balancing many concerns, and this humble document cannot possibly
+capture the context that rests in the head of an experienced Blink engineer.
+***
+
+## JavaScript Tests
+
+Whenever possible, the testing criteria should be expressed in JavaScript. The
+alternatives, which will be described in future sections, result in slower and
+less reliable tests.
+
+All new JavaScript tests should be written using the
+[testharness.js](https://github.com/w3c/testharness.js/) testing framework. This
+framework is used by the tests in the
+[web-platform-tests](https://github.com/w3c/web-platform-tests) repository,
+which is shared with all the other browser vendors, so `testharness.js` tests
+are more accessible to browser developers.
+
+As a shared framework, `testharness.js` enjoys high-quality documentation, such
+as [a tutorial](http://testthewebforward.org/docs/testharness-tutorial.html) and
+[API documentation](https://github.com/w3c/testharness.js/blob/master/docs/api.md).
+Layout tests should follow the recommendations of the above documents.
+Furthermore, layout tests should include relevant
+[metadata](http://testthewebforward.org/docs/css-metadata.html). The
+specification URL (in `<link rel="help">`) is almost always relevant, and is
+incredibly helpful to a developer who needs to understand the test quickly.
+
+Below is a skeleton for a JavaScript test embedded in an HTML page. Note that,
+in order to follow the minimality guideline, the test omits the tags `<html>`,
+`<head>`, and `<body>`, as they can be inferred by the HTML parser.
+
+```html
+<!doctype html>
+<meta charset="utf-8">
+<title>JavaScript: the true literal</title>
+<link rel="help" href="https://tc39.github.io/ecma262/#sec-boolean-literals">
+<meta name="assert" value="The true literal is equal to itself and immutable">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+'use strict';
+
+// Synchronous test example.
+test(() => {
+ const value = true;
+ assert_true(value, 'true literal');
+ assert_equals(value.toString(), 'true', 'the string representation of true');
+}, 'The literal true in a synchronous test case');
+
+// Asynchronous test example.
+async_test(t => {
+ const originallyTrue = true;
+ setTimeout(t.step_func_done(() => {
+ assert_equals(originallyTrue, true);
+ }), 0);
+}, 'The literal true in a setTimeout callback');
+
+// Promise test example.
+promise_test(() => {
+ return new Promise((resolve, reject) => {
+ resolve(true);
+ }).then(value => {
+ assert_true(value);
+ });
+}, 'The literal true used to resolve a Promise');
+
+</script>
+```
+
+Some points that are not immediately obvious from the example:
+
+* The `<meta name="assert">` describes the purpose of the entire file, and
+ is not redundant to `<title>`. Don't add a `<meta name="assert">` when the
+ information in the `<title>` is sufficient.
+* When calling an `assert_` function that compares two values, the first
+ argument is the actual value (produced by the functionality being tested), and
+ the second argument is the expected value (known good, golden). The order
+ is important, because the testing harness relies on it to generate expressive
+ error messages that are relied upon when debugging test failures.
+* The assertion description (the string argument to `assert_` methods) conveys
+ the way the actual value was obtained.
+ * If the expected value doesn't make it clear, the assertion description
+ should explain the desired behavior.
+ * Test cases with a single assertion should omit the assertion's description
+ when it is sufficiently clear.
+* Each test case describes the circumstance that it tests, without being
+ redundant.
+ * Do not start test case descriptions with redundant terms like "Testing"
+ or "Test for".
+ * Test files with a single test case should omit the test case description.
+ The file's `<title>` should be sufficient to describe the scenario being
+ tested.
+* Asynchronous tests have a few subtleties.
+ * The `async_test` wrapper calls its function with a test case argument that
+ is used to signal when the test case is done, and to connect assertion
+ failures to the correct test.
+ * `t.done()` must be called after all the test case's assertions have
+ executed.
+ * Test case assertions (actually, any callback code that can throw
+ exceptions) must be wrapped in `t.step_func()` calls, so that
+ assertion failures and exceptions can be traced back to the correct test
+ case.
+ * `t.step_func_done()` is a shortcut that combines `t.step_func()` with a
+ `t.done()` call.
+
+*** promo
+Layout tests that load from `file://` origins must currently use relative paths
+to point to
+[/resources/testharness.js](../../third_party/WebKit/LayoutTests/resources/testharness.js)
+and
+[/resources/testharnessreport.js](../../third_party/WebKit/LayoutTests/resources/testharnessreport.js).
+This is contrary to the WPT guidelines, which call for absolute paths.
+This limitation does not apply to the tests in `LayoutTests/http`, which rely on
+an HTTP server, or to the tests in `LayoutTests/imported/wpt`, which are
+imported from the [WPT repository](https://github.com/w3c/web-platform-tests).
+***
+
+### WPT Supplemental Testing APIs
+
+Some tests simply cannot be expressed using the Web Platform APIs. For example,
+some tests that require a user to perform a gesture, such as a mouse click,
+cannot be implemented using Web APIs. The WPT project covers some of these cases
+via supplemental testing APIs.
+
+*** promo
+In many cases, the user gesture is not actually necessary. For example, many
+event handling tests can use
+[synthetic events](https://developer.mozilla.org/docs/Web/Guide/Events/Creating_and_triggering_events).
+***
+
+*** note
+TODO: document wpt_automation. Manual tests might end up moving here.
+***
+
+### Relying on Blink-Specific Testing APIs
+
+Tests that cannot be expressed using the Web Platform APIs or WPT's testing APIs
+use Blink-specific testing APIs. These APIs are only available in
+[content_shell](./layout_tests_in_content_shell.md), and should only be used as
+a last resort.
+
+A downside of Blink-specific APIs is that they are not as well documented as the
+Web Platform features. Learning to use a Blink-specific feature requires finding
+other tests that use it, or reading its source code.
+
+For example, the most popular Blink-specific API is `testRunner`, which is
+implemented in
+[components/test_runner/test_runner.h](../../components/test_runner/test_runner.h)
+and
+[components/test_runner/test_runner.cpp](../../components/test_runner/test_runner.cpp).
+By skimming the `TestRunnerBindings::Install` method, we learn that the
+testRunner API is presented by the `window.testRunner` and
+`window.layoutTestsController` objects, which are synonyms. Reading the
+`TestRunnerBindings::GetObjectTemplateBuilder` method tells us what properties
+are available on the `window.testRunner` object.
+
+*** aside
+`window.testRunner` is the preferred way to access the `testRunner` APIs.
+`window.layoutTestsController` is still supported because it is used by
+3rd-party tests.
+***
+
+*** note
+`testRunner` is the most popular testing API because it is also used indirectly
+by tests that stick to Web Platform APIs. The `testharnessreport.js` file in
+`testharness.js` is specifically designated to hold glue code that connects
+`testharness.js` to the testing environment. Our implementation is in
+[third_party/WebKit/LayoutTests/resources/testharnessreport.js](../../third_party/WebKit/LayoutTests/resources/testharnessreport.js),
+and uses the `testRunner` API.
+***
+
+See the [components/test_runner/](../../components/test_runner/) directory and
+[WebKit's LayoutTests guide](https://trac.webkit.org/wiki/Writing%20Layout%20Tests%20for%20DumpRenderTree)
+for other useful APIs. For example, `window.eventSender`
+([components/test_runner/event_sender.h](../../components/test_runner/event_sender.h)
+and
+[components/test_runner/event_sender.cpp](../../components/test_runner/event_sender.cpp))
+has methods that simulate events input such as keyboard / mouse input and
+drag-and-drop.
+
+Here is a UML diagram of how the `testRunner` bindings fit into Chromium.
+
+[](https://docs.google.com/drawings/d/1KNRNjlxK0Q3Tp8rKxuuM5mpWf4OJQZmvm9_kpwu_Wwg/edit)
+### Manual Tests
+
+Whenever possible, tests that rely on (WPT's or Blink's) testing APIs should
+also be usable as
+[manual tests](http://testthewebforward.org/docs/manual-test.html). This makes
+it easy to debug the test, and to check whether our behavior matches other
+browsers.
+
+*** note
+The recommendation to have tests that depend on Blink-only testing APIs
+gracefully degrade to manual tests is not currently enforced in code review.
+When considering skipping this recommendation, please keep in mind that a manual
+test can be debugged in the browser, whereas a test that does not degrade
+gracefully can only be debugged in the test runner. Fellow project members and
+future you will thank you for having your test work as a manual test.
+***
+
+Manual tests should minimize the chance of user error. This implies keeping the
+manual steps to a minimum, and having simple and clear instructions that
+describe all the configuration changes and user gestures that match the effect
+of the Blink-specific APIs used by the test.
+
+Below is an example of a fairly minimal test that uses a Blink-Specific API
+(`window.eventSender`), and gracefully degrades to a manual test.
+
+```html
+<!doctype html>
+<meta charset="utf-8">
+<title>DOM: Event.isTrusted for UI events</title>
+<link rel="help" href="https://dom.spec.whatwg.org/#dom-event-istrusted">
+<link rel="help" href="https://dom.spec.whatwg.org/#constructing-events">
+<meta name="assert"
+ content="Event.isTrusted is true for events generated by user interaction">
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+
+<p>Please click on the button below.</p>
+<button>Click Me!</button>
+
+<script>
+'use strict';
+
+setup({ explicit_timeout: true });
+
+promise_test(() => {
+ const button = document.querySelector('button');
+ return new Promise((resolve, reject) => {
+ const button = document.querySelector('button');
+ button.addEventListener('click', (event) => {
+ resolve(event);
+ });
+
+ if (window.eventSender) {
+ eventSender.mouseMoveTo(button.offsetLeft, button.offsetTop);
+ eventSender.mouseDown();
+ eventSender.mouseUp();
+ }
+ }).then((clickEvent) => {
+ assert_true(clickEvent.isTrusted);
+ });
+
+}, 'Click generated by user interaction');
+
+</script>
+```
+
+The test exhibits the following desirable features:
+
+* It has a second specification URL (`<link rel="help">`), because the paragraph
+ that documents the tested feature (referenced by the primary URL) is not very
+ informative on its own.
+* It links to the
+ [WHATWG Living Standard](https://wiki.whatwg.org/wiki/FAQ#What_does_.22Living_Standard.22_mean.3F),
+ rather than to a frozen version of the specification.
+* It contains clear instructions for manually triggering the test conditions.
+ The test starts with a paragraph (`<p>`) that tells the tester exactly what to
+ do, and the `<button>` that needs to be clicked is clearly labeled.
+* It disables the timeout mechanism built into `testharness.js` by calling
+ `setup({ explicit_timeout: true });`
+* It checks for the presence of the Blink-specific testing APIs
+ (`window.eventSender`) before invoking them. The test does not automatically
+ fail when the APIs are not present.
+* It uses [Promises](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Promise)
+ to separate the test setup from the assertions. This is particularly helpful
+ for manual tests that depend on a sequence of events to occur, as Promises
+ offer a composable way to express waiting for asynchronous events that avoids
+ [callback hell](http://stackabuse.com/avoiding-callback-hell-in-node-js/).
+
+Notice that the test is pretty heavy compared to a minimal JavaScript test that
+does not rely on testing APIs. Only use testing APIs when the desired testing
+conditions cannot be set up using Web Platform APIs.
+
+### Text Test Baselines
+
+By default, all the test cases in a file that uses `testharness.js` are expected
+to pass. However, in some cases, we prefer to add failing test cases to the
+repository, so that we can be notified when the failure modes change (e.g., we
+want to know if a test starts crashing rather than returning incorrect output).
+In these situations, a test file will be accompanied by a baseline, which is an
+`-expected.txt` file that contains the test's expected output.
+
+The baselines are generated automatically when appropriate by
+`run-webkit-tests`, which is described [here](./layout_tests.md), and by the
+[rebaselining tools](./layout_test_expectations.md).
+
+Text baselines for `testharness.js` should be avoided, as having a text baseline
+associated with a `testharness.js` indicates the presence of a bug. For this
+reason, CLs that add text baselines must include a
+[crbug.com](https://crbug.com) link for an issue tracking the removal of the
+text expectations.
+
+* When creating tests that will be upstreamed to WPT, and Blink's current
+ behavior does not match the specification that is being tested, a text
+ baseline is necessary. Remember to create an issue tracking the expectation's
+ removal, and to link the issue in the CL description.
+* Layout tests that cannot be upstreamed to WPT should use JavaScript to
+ document Blink's current behavior, rather than using JavaScript to document
+ desired behavior and a text file to document current behavior.
+
+### The js-test.js Legacy Harness
+
+*** promo
+For historical reasons, older tests are written using the `js-test` harness.
+This harness is **deprecated**, and should not be used for new tests.
+***
+
+If you need to understand old tests, the best `js-test` documentation is its
+implementation at
+[third_party/WebKit/LayoutTests/resources/js-test.js](../../third_party/WebKit/LayoutTests/resources/js-test.js).
+
+`js-test` tests lean heavily on the Blink-specific `testRunner` testing API.
+In a nutshell, the tests call `testRunner.dumpAsText()` to signal that the page
+content should be dumped and compared against a text baseline (an
+`-expected.txt` file). As a consequence, `js-test` tests are always accompanied
+by text baselines. Asynchronous tests also use `testRunner.waitUntilDone()` and
+`testRunner.notifyDone()` to tell the testing tools when they are complete.
+
+### Tests that use an HTTP Server
+
+By default, tests are loaded as if via `file:` URLs. Some web platform features
+require tests served via HTTP or HTTPS, for example absolute paths (`src=/foo`)
+or features restricted to secure protocols.
+
+HTTP tests are those under `LayoutTests/http/tests` (or virtual variants). Use a
+locally running HTTP server (Apache) to run them. Tests are served off of ports
+8000 and 8080 for HTTP, and 8443 for HTTPS. If you run the tests using
+`run-webkit-tests`, the server will be started automatically. To run the server
+manually to reproduce or debug a failure:
+
+```bash
+cd src/third_party/WebKit/Tools/Scripts
+run-blink-httpd start
+```
+
+The layout tests will be served from `http://127.0.0.1:8000`. For example, to
+run the test `http/tests/serviceworker/chromium/service-worker-allowed.html`,
+navigate to
+`http://127.0.0.1:8000/serviceworker/chromium/service-worker-allowed.html`. Some
+tests will behave differently if you go to 127.0.0.1 instead of localhost, so
+use 127.0.0.1.
+
+To kill the server, run `run-blink-httpd --server stop`, or just use `taskkill`
+or the Task Manager on Windows, and `killall` or Activity Monitor on MacOS.
+
+The test server sets up an alias to the `LayoutTests/resources` directory. In
+HTTP tests, you can access the testing framework at e.g.
+`src="/js-test-resources/testharness.js"`.
+
+TODO: Document [wptserve](http://wptserve.readthedocs.io/) when we are in a
+position to use it to run layout tests.
+
+## Reference Tests (Reftests)
+
+Reference tests, also known as reftests, perform a pixel-by-pixel comparison
+between the rendered image of a test page and the rendered image of a reference
+page. Most reference tests pass if the two images match, but there are cases
+where it is useful to have a test pass when the two images do _not_ match.
+
+Reference tests are more difficult to debug than JavaScript tests, and tend to
+be slower as well. Therefore, they should only be used for functionality that
+cannot be covered by JavaScript tests.
+
+New reference tests should follow the
+[WPT reftests guidelines](http://testthewebforward.org/docs/reftests.html). The
+most important points are summarized below.
+
+* The test page declares the reference page using a `<link rel="match">` or
+ `<link rel="mismatch">`, depending on whether the test passes when the test
+ image matches or does not match the reference image.
+* The reference page must not use the feature being tested. Otherwise, the test
+ is meaningless.
+* The reference page should be as simple as possible, and should not depend on
+ advanced features. Ideally, the reference page should render as intended even
+ on browsers with poor CSS support.
+* Reference tests should be self-describing.
+* Reference tests do _not_ include `testharness.js`.
+
+Our testing infrastructure was designed for the
+[WebKit reftests](https://trac.webkit.org/wiki/Writing%20Reftests) that Blink
+has inherited. The consequences are summarized below.
+
+* Each reference page must be in the same directory as its associated test.
+ Given a test page named `foo` (e.g. `foo.html` or `foo.svg`),
+ * The reference page must be named `foo-expected` (e.g.,
+ `foo-expected.html`) if the test passes when the two images match.
+ * The reference page must be named `foo-expected-mismatch` (e.g.,
+ `foo-expected-mismatch.svg`) if the test passes when the two images do
+ _not_ match.
+* Multiple references and chained references are not supported.
+
+The following example demonstrates a reference test for
+[`<ol>`'s reversed attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/ol).
+The example assumes that the test page is named `ol-reversed.html`.
+
+```html
+<!doctype html>
+<meta charset="utf-8">
+<link rel="match" href="ol-reversed-expected.html">
+
+<ol reversed>
+ <li>A</li>
+ <li>B</li>
+ <li>C</li>
+</ol>
+```
+
+The reference page, which must be named `ol-reversed-expected.html`, is below.
+
+```html
+<!doctype html>
+<meta charset="utf-8">
+
+<ol>
+ <li value="3">A</li>
+ <li value="2">B</li>
+ <li value="1">C</li>
+</ol>
+```
+
+## Pixel Tests
+
+`testRunner` APIs such as `window.testRunner.dumpAsTextWithPixelResults()` and
+`window.testRunner.dumpDragImage()` create an image result that is associated
+with the test. The image result is compared against an image baseline, which is
+an `-expected.png` file associated with the test, and the test passes if the
+image result is identical to the baseline, according to a pixel-by-pixel
+comparison. Tests that have image results (and baselines) are called **pixel
+tests**.
+
+Pixel tests should still follow the principles laid out above. Pixel tests pose
+unique challenges to the desire to have *self-describing* and *cross-platform*
+tests. The
+[WPT test style guidelines](http://testthewebforward.org/docs/test-style-guidelines.html)
+contain useful guidance. The most relevant pieces of advice are below.
+
+* Whenever possible, use a green paragraph / page / square to indicate success.
+ If that is not possible, make the test self-describing by including a textual
+ description of the desired (passing) outcome.
+* Only use the red color or the word `FAIL` to highlight errors. This does not
+ apply when testing the color red.
+* Use the [Ahem font](https://www.w3.org/Style/CSS/Test/Fonts/Ahem/README) to
+ reduce the variance introduced by the platform's text rendering system. This
+ does not apply when testing text, text flow, font selection, font fallback,
+ font features, or other typographic information.
+
+*** promo
+When using `window.testRunner.dumpAsTextWithPixelResults()`, the image result
+will always be 800x600px, because test pages are rendered in an 800x600px
+viewport. Pixel tests that do not specifically cover scrolling should fit in an
+800x600px viewport without creating scrollbars.
+***
+
+The following snippet includes the Ahem font in a layout test.
+
+```html
+<style>
+body {
+ font: 10px Ahem;
+}
+</style>
+<script src="/resources/ahem.js"></script>
+```
+
+*** promo
+Tests outside `LayoutTests/http` and `LayoutTests/imported/wpt` currently need
+to use a relative path to
+[/third_party/WebKit/LayoutTests/resources/ahem.js](../../third_party/WebKit/LayoutTests/resources/ahem.js)
+***
+
+### Tests that need to paint, raster, or draw a frame of intermediate output
+
+A layout test does not actually draw frames of output until the test exits.
+Tests that need to generate a painted frame can use
+`window.testRunner.displayAsyncThen`, which will run the machinery to put up a
+frame, then call the passed callback. There is also a library at
+`fast/repaint/resources/text-based-repaint.js` to help with writing paint
+invalidation and repaint tests.
+
+## Dump Render Tree (DRT) Tests
+
+A Dump Render Tree test renders a web page and produces up to two results, which
+are compared against baseline files:
+
+* All tests output a textual representation of Blink's
+ [render tree](https://developers.google.com/web/fundamentals/performance/critical-rendering-path/render-tree-construction),
+ which is compared against an `-expected.txt` text baseline.
+* Some tests also output the image of the rendered page, which is compared
+ against an `-expected.png` image baseline, using the same method as pixel
+ tests.
+
+TODO: Document the API used by DRT tests to opt out of producing image results.
+
+A DRT test passes if _all_ of its results match their baselines. Like pixel
+tests, the output of DRT tests depends on platform-specific mechanisms, so DRT
+tests often require per-platform baselines. Furthermore, DRT tests depend on the
+render tree data structure, which means that if we replace the render tree data
+structure, we will need to look at each DRT test and consider whether it is
+still meaningful.
+
+For these reasons, DRT tests should **only** be used to cover aspects of the
+layout code that can only be tested by looking at the render tree. Any
+combination of the other test types is preferable to a DRT test. DRT tests are
+[inherited from WebKit](https://webkit.org/blog/1456/layout-tests-practice/), so
+the repository may have some unfortunate examples of DRT tests.
+
+The following page is an example of a DRT test.
+
+```html
+<!doctype html>
+<meta charset="utf-8">
+<style>
+body { font: 10px Ahem; }
+span::after {
+ content: "pass";
+ color: green;
+}
+</style>
+<script src="/resources/ahem.js"></script>
+
+<p><span>Pass if a green PASS appears to the right: </span></p>
+```
+
+The most important aspects of the example are that the test page does not
+include a testing framework, and that it follows the guidelines for pixel tests.
+The test page produces the text result below.
+
+```
+layer at (0,0) size 800x600
+ LayoutView at (0,0) size 800x600
+layer at (0,0) size 800x30
+ LayoutBlockFlow {HTML} at (0,0) size 800x30
+ LayoutBlockFlow {BODY} at (8,10) size 784x10
+ LayoutBlockFlow {P} at (0,0) size 784x10
+ LayoutInline {SPAN} at (0,0) size 470x10
+ LayoutText {#text} at (0,0) size 430x10
+ text run at (0,0) width 430: "Pass if a green PASS appears to the right: "
+ LayoutInline {<pseudo:after>} at (0,0) size 40x10 [color=#008000]
+ LayoutTextFragment (anonymous) at (430,0) size 40x10
+ text run at (430,0) width 40: "pass"
+```
+
+Notice that the test result above depends on the size of the `<p>` text. The
+test page uses the Ahem font (introduced above), whose main design goal is
+consistent cross-platform rendering. Had the test used another font, its text
+baseline would have depended on the fonts installed on the testing computer, and
+on the platform's font rendering system. Please follow the pixel tests
+guidelines and write reliable DRT tests!
+
+WebKit's render tree is described in
+[a series of posts](https://webkit.org/blog/114/webcore-rendering-i-the-basics/)
+on WebKit's blog. Some of the concepts there still apply to Blink's render tree.
+
+## Directory Structure
+
+The [LayoutTests directory](../../third_party/WebKit/LayoutTests) currently
+lacks a strict, formal structure. The following directories have special
+meaning:
+
+* The `http/` directory hosts tests that require an HTTP server (see above).
+* The `resources/` subdirectory in every directory contains binary files, such
+ as media files, and code that is shared by multiple test files.
+
+*** note
+Some layout tests consist of a minimal HTML page that references a JavaScript
+file in `resources/`. Please do not use this pattern for new tests, as it goes
+against the minimality principle. JavaScript and CSS files should only live in
+`resources/` if they are shared by at least two test files.
+***
