A slot has `<slot><span part="placeholder">none</span></slot>`. Light DOM assigns a real node so fallback is hidden. A consumer writes `my-el::part(placeholder)`. Does the fallback `<span>`'s part remain stylable by the consumer while it is NOT rendered, and what governs `::part` matching here?
<!-- shadow: <slot><span part="placeholder">none</span></slot> -->
<my-el><b>real</b></my-el>
<!-- consumer: my-el::part(placeholder){color:red} -->