Polyfilling custom elements right now is ready because only Blink has native shadow DOM, so the polyfill can use a single mutation observer. With native shadow

Polyfilling custom elements right now is ready because only Blink has native shadow DOM, so the polyfill can use a single mutation observer. With native shadow DOM, but no native custom elements the polyfill would need a mutation observer in each shadow root. It would work, but might be slow.