A reftest is a test that compares the visual output of one file (the
test case) with the output of one or more other files (the
references). The test and the reference must be carefully written so
that when the test passes they have identical rendering, but different
rendering when the test fails.

## How to Run Reftests

Reftests can be run manually simply by opening the test and the
reference file in multiple windows or tabs and either placing them
side-by side or flipping between the two. In automation the comparison
is done in an automated fashion, which can lead to differences too
small for the human eye to notice causing tests to fail.

## Components of a Reftest

In the simplest case, a reftest consists of a pair of files called the
*test* and the *reference*.

The *test* file is the one that makes use of the technology being
tested. It also contains a `link` element with `rel="match"` or
`rel="mismatch"` and `href` attribute pointing to the *reference* file
e.g. `<link rel=match href=references/green-box-ref.html>`.

The *reference* file is typically written to be as simple as possible,
and does not use the technology under test. It is desirable that the
reference be rendered correctly even in UAs with relatively poor
support for CSS and no support for the technology under test.

When the `<link>` element in the *test* has `rel="match"`, the test
only passes if the *test* and *reference* have pixel-perfect identical
rendering. `rel="mismatch"` inverts this so the test only passes when
the renderings differ.

In general the files used in a reftest should follow the
[format][format] and [style][style] guidelines. The *test* should also
be [self-describing][selfdesc], to allow a human to determine whether
the the rendering is as expected.

Note that references can be shared between tests; this is strongly
encouraged since it permits optimizations when running tests.

## Controlling When Comparison Occurs

By default reftest screenshots are taken in response to the `load`
event firing. In some cases it is necessary to delay the screenshot
later than this, for example becase some DOM manipulation is
required to set up the desired test conditions. To enable this, the
test may have a `class="reftest-wait"` attribute specified on the root
element. This will cause the screenshot to be delayed until the `load`
event has fired and the `reftest-wait` class has been removed from the
root element (technical note: the implementation in wptrunner uses
mutation observers so the screenshot will be triggered in the
microtask checkpoint after the class is removed. Because the harness
isn't synchronized with the browser event loop it is dangerous to rely
on precise timing here).

## Matching Multiple References

Sometimes it is desirable for a file to match multiple references or,
in rare cases, to allow it to match more than one possible
reference. Note: *this is not currently supported by test runners and
so best avoided if possible until that support improves*.

Multiple references linked from a single file are interpreted as
multiple possible renderings for that file. `<link rel=[mis]match>`
elements in a reference create further conditions that must be met in
order for the test to pass. For example, consider a situation where
`a.html` has `<link rel=match href=b.html>` and `<link rel=match
href=c.html>`, `b.html` has `<link rel=match href=b1.html>` and `c.html`
has `<link rel=mismatch href=c1.html>`. In this case, to pass we must
either have `a.html`, `b.html` and `b1.html` all rendering identically, or
`a.html` and `c.html` rendering identically, but `c.html` rendering
differently from `c1.html`.

## Fuzzy Matching

In some situations a test may have subtle differences in rendering
compared to the reference due to e.g. antialiasing. This may cause the
test to pass on some platforms but fail on others. In this case some
affordance for subtle discrepancies is desirable. However no mechanism
to allow this has yet been standardized.

## Limitations

In some cases, a test cannot be a reftest. For example, there is no
way to create a reference for underlining, since the position and
thickness of the underline depends on the UA, the font, and/or the
platform. However, once it's established that underlining an inline
element works, it's possible to construct a reftest for underlining
a block element, by constructing a reference using underlines on a
```<span>``` that wraps all the content inside the block.

## Example Reftests

These examples are all [self-describing][selfdesc] tests as they
each have a simple statement on the page describing how it should
render to pass the tests.

### HTML example

### Test File

This test verifies that a right-to-left rendering of **SAW** within a
```<bdo>``` element displays as **WAS**.

([view page rendering][html-reftest-example])

```html
<!DOCTYPE html>
<meta charset="utf-8">
<title>BDO element dir=rtl</title>
<link rel="help" href="http://www.whatwg.org/specs/web-apps/current-work/#the-bdo-element">
<meta name="assert" content="BDO element's DIR content attribute renders corrently given value of 'rtl'.">
<link rel="match" href="test-bdo-001.html">
<p>Pass if you see WAS displayed below.</p>
<bdo dir="rtl">SAW</bdo>
```

### Reference File

The reference file must look exactly like the test file,
except that the code behind it is different.

* All metadata is removed.
* The ```title``` need not match.
* The markup that created the actual test data is
  different: here, the same effect is created with
  very mundane, dependable technology.

([view page rendering][html-reffile-example])

```html
<!DOCTYPE html>
<meta charset="utf-8">
<title>HTML Reference File</title>
<p>Pass if you see WAS displayed below.</p>
<p>WAS</p>
```

[testharness]: ./testharness-documentation.html
[format]: ./test-format-guidelines.html
[style]: ./test-style-guidelines.html
[selfdesc]: ./test-style-guidelines.html#self-describing-tests
[reference-links]: ./test-templates.html#reference-links
[html-reftest-example]: ./html-reftest-example.html
[html-reffile-example]: ./html-reffile-example.html
[css-reftest-example]: http://test.csswg.org/source/css21/borders/border-bottom-applies-to-009.xht
[css-reffile-example]: http://test.csswg.org/source/css21/borders/border-bottom-applies-to-001-ref.xht
[svg-reftest-example]: http://test.csswg.org/source/css-transforms-1/translate/svg-translate-001.html
[svg-reffile-example]: http://test.csswg.org/source/css-transforms-1/translate/reference/svg-translate-ref.html
[indicating-failure]: ./test-style-guidelines.html#failure