This is a deliberate change from preexisting behavior, see https://www.w3.org/2015/10/28-htmlcue-minutes.html https://github.com/whatwg/html/issues/293 Test https://github.com/w3c/web-platform-tests/pull/2941
Safari passes following test: https://wpt.fyi/results/html/semantics/embedded-content/media-elements/track/track-element?label=master&label=experimental&aligned&view=subtest&q=track%20kind rniwa - Anything else required here?