The following test has a ImageOnlyFailure flake: compositing/video/poster.html https://build.webkit.org/results/GTK%20Linux%2064-bit%20Release%20(Tests)/r229090%20(5649)/compositing/video/poster-diffs.html It contains this code: <video controls id="v" poster="../resources/simple_image.png"></video> <script> var v = document.getElementById("v"); if (window.testRunner) { testRunner.waitUntilDone(); v.addEventListener("canplaythrough", function() { testRunner.notifyDone(); }); } v.setAttribute("src", findMediaFile("video", "../resources/video")); </script> The video poster looks perfectly fine, the problem is with the controls; particularly the bar. notifyDone() is called after the "canplaythrough" event. At that point, enough video should be buffered for smooth playback, but most of the time when the screenshot is taken the bar is still empty.
The test or tests filed under this bug are not failing anymore. Test expectations updated in https://commits.webkit.org/258724@main.