Web platform tests can be marked as slow in the test files, with <meta> tag (html) or // META comment (js). We do support this metadata when running the tests: controllers.Manager._is_test_slow also takes into account the result from Port.is_slow_wpt_test, which reads the info from Manifest. In other words, they are equivalent to [ Slow ] expectations.

However, at the end of the whole test run, we may still list these tests under "%s slowest tests that are not marked as SLOW and did not timeout/crash:", which is confusing. Marking these slow WPT as [ Slow ] would not have any effect.