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.
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.
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 and style guidelines. The test should also be self-describing, 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.
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).
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.
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.
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.
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.
This test verifies that a right-to-left rendering of SAW within a <bdo> element displays as WAS.
<!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>
The reference file must look exactly like the test file, except that the code behind it is different.
title need not match.<!DOCTYPE html> <meta charset="utf-8"> <title>HTML Reference File</title> <p>Pass if you see WAS displayed below.</p> <p>WAS</p>