New documentation on writing LayoutTests.

docs/testing/writing_layout_tests.md

+# 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)

[Rick Byers, 2016/11/21 22:37:15]

nit: there's technically more than just WPT (eg. test262 for things defined in
the Ecmascript specification, and csswg-test for CSS stuff - though that's
hopefully going to be merged into web-platform-test before too long).  Maybe
just say "covered by cross-browser tests such as the Web Platform Tests
Project"?

[pwnall, 2016/11/22 20:32:55]

I would rather have specific projects and clear links to them. I think that
while more general wording would be closer to the truth, it would also be less
useful.

csswg-test: I assume that csswg-test is logically a part of WPT. My assumption
is based on the facts that the top of the  WPT README states that the CSS tests
are in a different repository for historic reasons, and that the WPT
documentation covers writing CSS tests. I also assume that folks who follow the
links here will easily arrive to the correct conclusion about the csswg-test /
web-platform-tests split.

test262: Would layout tests fall under test262? I imagined that ECMA262 issues
would be tested in the v8 repository.

I did add a promo early on that begs readers to get in touch and update this
document if we upstream tests to other communal repositories.

+   (WPT). This helps us avoid regressions, and helps us identify Web Platform
+   areas where the major browsers don't have interoperable implementations.

[Rick Byers, 2016/11/21 22:37:14]

also mention that this lowers the cost of testing by sharing work across all the
major engines?

[pwnall, 2016/11/22 20:32:55]

Done.

+2. When a Blink feature cannot be tested using the Web Platform, 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).
+
+### Test Types
+
+There are three 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 layout code.

[Rick Byers, 2016/11/21 22:37:15]

why "layout code"?  Most of layout can be tested in JS IMHO by looking at all
the getClientRects.  Maybe "painting code" would be better?  You could ask eae@
or someone else on layout-dev for their wording.

[pwnall, 2016/11/22 20:32:55]

Done.
I replaced "layout" with paint, and will seek eae's opinion.

I don't really know what I'm doing here :) My goal is to write something that's
close enough to the real thing that knowledgeable people will be willing to fix
what's broken. My rationale is that without this doc, new developers have to
Google around and learn from reviews. Given that we're all time constrained, the
knowledge is going to be less complete than what's summarized here.

+* *Pixel Tests* render a test page and compare the result against a pre-rendered
+  image in the repository. Pixel tests are less robust than JavaScript tests and
+  reference tests, 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 not uncommon 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).
+
+## 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 as **short** as possible. The page should only include
+  elements that are necessary and relevant to what is being tested.
+
+* Tests should be as **fast** as possible. 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 proceed with a test.
+
+* Tests should be **reliable** and yield consistent results for a given
+  implementation. Flaky tests slow down 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 use the **minimal** set of platform features needed to express
+  the test scenario efficiently.
+    * 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).
+    * 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.
+    * Test pages should use the HTML5 doctype (`<!doctype html>`) unless they
+      are testing the quirks mode.
+    * 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.
+
+* 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`, prefer
+      [classes](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Classes)
+      over other OOP constructs, and prefer
+      [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 shipped by other
+      current major rendering engines (WebKit, Gecko, Edge). When unsure, check
+      [caniuse.com](http://caniuse.com/).
+
+* Tests must use the UTF-8 **character encoding**, which should be declared by
+  `<meta charset=utf-8>`. This does not apply when specifically testing
+  encodings.
+
+* Tests must aim to have a **coding style** that is consistent with
+  [Google's JavaScript Style Guide](https://google.github.io/styleguide/javascriptguide.xml),
+  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).
+
+## 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 robust 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_false(!value, 'the logical negtion of true');
+}, 'The literal true in a synchronous test case');
+
+// Asynchronous test example.
+async_test(t => {
+  const originallyTrue = true;
+  setTimeout(t.step_func_done(() => {
+    const value = true;
+    assert_equal(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 must be wrapped in `t.step_func()` calls, so that
+      assertion failures can be connected 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).
+***
+
+
+### Relying on Blink-Specific Testing APIs
+
+Tests that cannot be expressed using the Web Platform APIs rely on
+Blink-specific testing APIs. These APIs are only available in
+[content_shell](./layout_tests_in_content_shell.md).
+
+### Manual Tests
+
+Whenever possible, tests that rely on Blink-specific 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.
+
+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 documents its assertions clearly.
+    * The `<meta name="assert">` describes the purpose of the entire file.
+      However, don't add a `<meta>` when the information in the `<title>` is
+      sufficient.
+    * The `assert_equals` string describes the way the actual value was
+      obtained. If the expected value doesn't make it clear, the assertion
+      description should explain the desired behavior.
+    * Each test case describes the circumstance that it tests.
+* 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 });`

[Rick Byers, 2016/11/21 22:37:14]

What's the implication of this on our bots?  Eg. say we had a bug that caused
eventSender.mouseUp to occasionally be a no-op.  Would the test never time out? 
Presumably our build-bot task timeout would kick in and kill running
webkit_tests, but I think that would be pretty bad.  Perhaps we should only
disable the timeout when eventSender isn't present?

[pwnall, 2016/11/22 20:32:55]

My experiments suggest that we have an external timeout mechanism that
testharness.js can't modify. When my test had a similar bug (I was clicking
outside the button), it still timed out.

+* 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 Blink-specific testing APIs when the
+desired testing conditions cannot be set up using Web Platform APIs.
+
+#### Using Blink-Specific Testing APIs
+
+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.
+
+[![UML of testRunner bindings configuring platform implementation](https://docs.google.com/drawings/u/1/d/1KNRNjlxK0Q3Tp8rKxuuM5mpWf4OJQZmvm9_kpwu_Wwg/export/svg?id=1KNRNjlxK0Q3Tp8rKxuuM5mpWf4OJQZmvm9_kpwu_Wwg&pageid=p)](https://docs.google.com/drawings/d/1KNRNjlxK0Q3Tp8rKxuuM5mpWf4OJQZmvm9_kpwu_Wwg/edit)
+
+### 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 tests that are 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 `LayoutTests/resources` directory. In HTTP
+tests, you can access the testing framework at e.g.
+`src="/js-test-resources/js-test.js"`.
+
+TODO: Document [wptserve](http://wptserve.readthedocs.io/) when we are in a
+position to use it to run layout tests.
+
+## Reference Tests
+
+*** promo
+In the long term, we intend to express reference tests as
+[WPT reftests](http://testthewebforward.org/docs/reftests.html). Currently,
+Chromium's testing infrastructure does not support WPT reftests. In the
+meantime, please use the legacy reference tests format described below.
+***
+
+*** note
+TODO: Summarize best practices for reftests and give examples.
+***
+
+### Legacy Reference Tests
+
+Blink also has inherited a sizable amount of
+[reftests](https://trac.webkit.org/wiki/Writing%20Reftests) from WebKit. In
+these tests, the reference page file name is based on the test page's file name
+and an `-expected.html` suffix.
+
+## 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.
+
+* use a green paragraph / page / square to indicate success
+* use the red color or the word `FAIL` to highlight errors
+* use the [Ahem font](https://www.w3.org/Style/CSS/Test/Fonts/Ahem/README) to
+  minimize the variance introduced by the platform's text rendering system
+
+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. 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.
+
+## 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 a 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.
+***