This is a testharness.js-based test.
Found 2 FAIL, 0 TIMEOUT, 0 NOTRUN.
[FAIL] "::part(mypart):is(:first-child)" should be a valid selector
assert_equals: serialization should be canonical expected "::part(mypart):is(:first-child)" but got "::part(mypart):is()"
[FAIL] "::part(mypart):where(:first-child)" should be a valid selector
assert_equals: serialization should be canonical expected "::part(mypart):where(:first-child)" but got "::part(mypart):where()"
Harness: the test ran to completion.