This is a testharness.js-based test.
FAIL Focus should cover assigned elements of an assigned slot, as well as elements that are directly assigned to a slot. assert_equals: Focus should move forward from i0 to assigned/x-foo/a0 expected Element node <input id="a0" tabindex="1" value="a0"></input> but got Element node <input id="i0" tabindex="0" value="i0"></input>
Harness: the test ran to completion.

