There is a large number of flaky tests in this imported directory, many of which are not marked as such. https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#tests=imported%2Fw3c%2Fweb-platform-tests%2Fhtml%2Fbrowsers
The situation was somewhat improved via bug 161083, but there are still many flaky tests on various platforms. Chris, can you look into this?
Also, some of the tests got apparently fixed via bug 161068, and need expectations updated.