See: WebIDL issue: https://github.com/heycam/webidl/issues/496 wpt tests: https://github.com/w3c/web-platform-tests/pull/9490 PR: https://github.com/heycam/webidl/pull/494
Turns out that WebKit was already doing the right thing: > Object.getPrototypeOf(Element.prototype[Symbol.unscopables]) < null