I ran this command: MOZ_WEBRENDER=1 ./mach reftest layout/reftests/transform-3d/animate-preserve3d-child.html --verify 2>&1 | tee log with some local patches (that affect the reftest, but not mach or the mochitest harness). As the output was going by I saw what looked like a failure. But at the end of the run, the test verification summary showed everything passed. Good thing I captured all the output into a logfile, because I double-checked, and sure enough, there were some failures during the runs that the harness decided to ignore for no apparent reason: $ grep "UNEXPECTED" log REFTEST TEST-UNEXPECTED-FAIL | file:///c:/Users/kats/zspace/mozilla-w2/layout/reftests/transform-3d/animate-preserve3d-child.html == file:///c:/Users/kats/zspace/mozilla-w2/layout/reftests/transform-3d/animate-preserve3d-ref.html | image comparison, max difference: 255, number of differing pixels: 18228 REFTEST TEST-UNEXPECTED-FAIL | file:///c:/Users/kats/zspace/mozilla-w2/layout/reftests/transform-3d/animate-preserve3d-child.html == file:///c:/Users/kats/zspace/mozilla-w2/layout/reftests/transform-3d/animate-preserve3d-ref.html | image comparison, max difference: 255, number of differing pixels: 18228 The full log is attached.

Not sure why I put this in mochitest, since this was a reftest run.