Reftests are one of the primary tools for testing things relating to rendering; they are made up of the test and one or more other pages (”references”) with assertions as to whether they render identically or not. This page describes their aspects exhaustively; the tutorial on writing a reftest offers a more limited but grounded guide to the process.

How to Run Reftests

Reftests can be run manually simply by opening the test and the reference file in multiple windows or tabs and flipping between the two. In automation the comparison is done in an automated fashion, which can lead to differences hard for the human eye to notice to cause the test 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>. A match test only passes if the two files render pixel-for-pixel identically within a 800x600 window including scroll-bars if present; a mismatch test only passes if they don’t render identically.

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.

Writing a Good Reftest

In general the files used in a reftest should follow the general guidelines and the rendering test guidelines. They should also be self-describing, to allow a human to determine whether the the rendering is as expected.

References can be shared between tests; this is strongly encouraged as it makes it easier to tell at a glance whether a test passes (through familiarity) and enables some optimizations in automated test runners. Shared references are typically placed in references directories, either alongside the tests they are expected to be useful for or at the top level if expected to be generally applicable (e.g., many layout tests can be written such that the correct rendering is a 100x100 green square!). For references that are applicable only to a single test, it is recommended to use the test name with a suffix of -ref as their filename; e.g., test.html would have test-ref.html as a reference.

Multiple References

Sometimes, a test’s pass condition cannot be captured in a single reference.

If a test has multiple links, then the test passes if:

  • If there are any match references, at least one must match, and

  • If there are any mismatch references, all must mismatch.

If you need multiple matches to succeed, these can be turned into multiple tests (for example, by just having a reference be a test itself!). If this seems like an unreasonable restriction, please file a bug and let us know!

Controlling When Comparison Occurs

By default, reftest screenshots are taken after the following conditions are met:

  • The load event has fired

  • Web fonts (if any) are loaded

  • Pending paints have completed

In some cases it is necessary to delay the screenshot later than this, for example because 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. In this case the harness will run the following sequence of steps:

  • Wait for the load event to fire and fonts to load.

  • Wait for pending paints to complete.

  • Fire an event named TestRendered at the root element, with the bubbles attribute set to true.

  • Wait for the reftest-wait class to be removed from the root element.

  • Wait for pending paints to complete.

  • Screenshot the viewport.

The TestRendered event provides a hook for tests to make modifications to the test document that are not batched into the initial layout/paint.

Fuzzy Matching

In some situations a test may have subtle differences in rendering compared to the reference due to, e.g., anti-aliasing. To allow for these small differences, we allow tests to specify a fuzziness characterised by two parameters, both of which must be specified:

  • A maximum difference in the per-channel color value for any pixel.

  • A number of total pixels that may be different.

The maximum difference in the per pixel color value is formally defined as follows: let Tx,y,c be the value of colour channel c at pixel coordinates x, y in the test image and Rx,y,c be the corresponding value in the reference image, and let width and height be the dimensions of the image in pixels. Then maxDifference = maxx=[0,width) y=[0,height), c={r,g,b}(|Tx,y,c - Rx,y,c|).

To specify the fuzziness in the test file one may add a <meta name=fuzzy> element (or, in the case of more complex tests, to any page containing the <link rel=[mis]match> elements). In the simplest case this has a content attribute containing the parameters above, separated by a semicolon e.g.

<meta name=fuzzy content="maxDifference=15;totalPixels=300">

would allow for a difference of exactly 15 / 255 on any color channel and 300 exactly pixels total difference. The argument names are optional and may be elided; the above is the same as:

<meta name=fuzzy content="15;300">

The values may also be given as ranges e.g.

<meta name=fuzzy content="maxDifference=10-15;totalPixels=200-300">


<meta name=fuzzy content="10-15;200-300">

In this case the maximum pixel difference must be in the range 10-15 and the total number of different pixels must be in the range 200-300. These range checks are inclusive.

In cases where a single test has multiple possible refs and the fuzziness is not the same for all refs, a ref may be specified by prefixing the content value with the relative url for the ref e.g.

<meta name=fuzzy content="option1-ref.html:10-15;200-300">

One meta element is required per reference requiring a unique fuzziness value, but any unprefixed value will automatically be applied to any ref that doesn’t have a more specific value.

Debugging fuzzy reftests

When debugging a fuzzy reftest via wpt run, it can be useful to know what the allowed and detected differences were. Many of the output logger options will provide this information. For example, by passing --log-mach=- for a run of a hypothetical failing test, one might get:

 0:08.15 TEST_START: /foo/bar.html
 0:09.70 INFO Found 250 pixels different, maximum difference per channel 6 on page 1
 0:09.70 INFO Allowed 0-100 pixels different, maximum difference per channel 0-0
 0:09.70 TEST_END: FAIL, expected PASS - /foo/bar.html ['f83385ed9c9bea168108b8c448366678c7941627']

For other logging flags, see the output of wpt run --help.


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.