@csswizardry I think another viable use case is defining *some* custom elements as it helps avoid the flash of the element being undefined. To me that frees up the undefined state from needing to serve the defined state (i.e. more equitable progressive enhancement — the experiences don’t need to be exactly the same). Progressive enhancement can be punished with a bad CLS score, but I think this is a more convenient way to avoid it (with its own cost of course).