Visual tests are typically used when testing rendering of things that cannot be tested with [reftests][].
Their main advantage of over manual tests is they can be verified using browser-specific and platform-specific screenshots; note, however, that many browser vendors treat them identically to manual tests hence they are similarly discouraged as they very infrequently, if ever, get run by them.
Visuals tests are test files which have -visual at the end of their filename, before the extension. There is nothing needed in them to make them work.
They should follow the [general test guidelines][general guidelines], especially noting the requirement to be self-describing (i.e., they must give a clear pass condition in their rendering).
Similarly, they should consider the [rendering test guidelines][rendering], especially those about color, to ensure those running the test don't incorrectly judge its result.
[general guidelines]: {{ site.baseurl }}{% link _writing-tests/general-guidelines.md %} [reftests]: {{ site.baseurl }}{% link _writing-tests/reftests.md %} [rendering]: {{ site.baseurl }}{% link _writing-tests/rendering.md %}