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tests/wpt/web-platform-tests/docs/writing-tests/visual.md

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+# Visual Tests
+
+Visual tests are typically used when testing rendering of things that
+cannot be tested with [reftests](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.
+
+## Writing a Visual Test
+
+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.