palemoon27/testing/web-platform/tests/docs/reftests.md

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

A reftest has three parts:

1. The Reftest Test File

The test file uses the technology to be tested. This file should follow the format and style guidelines.

It is preferable that a reftest is self-describing, since it allows for both machine comparison and manual verification. Having the expected rendering described on the page lets the tester check that the test and the reference are not both being rendered incorrectly and triggering a false pass. Designing it for an obvious fail makes it easier to find what went wrong when the reftest does fail.

2. The Reftest Reference File

This is a different, usually simpler, file that results in the same rendering as the test. The reference file must not use the same features that are being tested, but uses a different method to produce the same rendering as a test file. Sometimes more than one reference file is required. Multiple tests can (and often do) share the same reference file. If it is possible to reuse an existing reference for a test this should always be preferred as it enables substantial speedups when running a large number of tests.

In some cases when creating the reference file, it is necessary to use features that, although different from the tested features, may themselves fail in such a manner as to cause the reference to render identically to a failed test. When this is the case, in order to reduce the possibility of false positive testing outcomes, multiple reference files should be used, each using a different technique to render the reference. One possibility is to create one or more references that must not match the test file, i.e.: a file that renders in the same manner as a failed test.

3. The Reftest Comparison

In a reftest the test is required to either match or mismatch the rendering of the reference file. In order to specify this relationship, use 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.html>.

If possible, share the same reference file amongst multiple 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 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)

<!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)

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