# Test Suite Design
The vast majority of the test suite is formed of HTML pages, which can
be loaded in a browser and either programmatically provide a result or
provide a set of steps to run the test and obtain the result.
The tests are, in general, short, cross-platform, and self-contained,
and should be easy to run in any browser.
## Test Layout
Most of the repository's top-level directories hold tests for specific web
standards. For [W3C specs](https://www.w3.org/standards/), these directories
are typically named after the shortname of the spec (i.e. the name used for
snapshot publications under `/TR/`); for [WHATWG
specs](https://spec.whatwg.org/), they are typically named after the subdomain
of the spec (i.e. trimming `.spec.whatwg.org` from the URL); for other specs,
something deemed sensible is used. The `css/` directory contains test suites
for [the CSS Working Group
specifications](https://www.w3.org/Style/CSS/current-work).
Within the specification-specific directory there are two common ways
of laying out tests: the first is a flat structure which is sometimes
adopted for very short specifications; the alternative is a nested
structure with each subdirectory corresponding to the id of a heading
in the specification. The latter provides some implicit metadata about
the part of a specification being tested according to its location in
the filesystem, and is preferred for larger specifications.
For example, tests in HTML for ["The History
interface"](https://html.spec.whatwg.org/multipage/history.html#the-history-interface)
are located in `html/browsers/history/the-history-interface/`.
Many directories also include a file named `META.yml`. This file may define any
of the following properties:
- `spec` - a link to the specification covered by the tests in the directory
- `suggested_reviewers` - a list of GitHub account username belonging to
people who are notified when pull requests modify files in the directory
Various resources that tests depend on are in `common`, `images`, `fonts`,
`media`, and `resources`.
## Test Types
Tests in this project use a few different approaches to verify expected
behavior. The tests can be classified based on the way they express
expectations:
* Rendering tests ensure that the browser graphically displays pages as
expected. There are a few different ways this is done:
* [Reftests][] render two (or more) web pages and combine them with equality
assertions about their rendering (e.g., `A.html` and `B.html` must render
identically), run either by the user switching between tabs/windows and
trying to observe differences or through [automated
scripts][running-from-local-system].
* [Visual tests][visual] display a page where the result is determined either
by a human looking at it or by comparing it with a saved screenshot for
that user agent on that platform.
* [testharness.js][] tests verify that JavaScript interfaces behave as
expected. They get their name from the JavaScript harness that's used to
execute them.
* [wdspec][] tests are written in Python and test [the WebDriver browser
automation protocol](https://w3c.github.io/webdriver/)
* [Manual tests][manual] rely on a human to run them and determine their
result.
[reftests]: writing-tests/reftests
[testharness.js]: writing-tests/testharness
[visual]: writing-tests/visual
[manual]: writing-tests/manual
[running-from-local-system]: running-tests/from-local-system
[wdspec]: writing-tests/wdspec
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