Given custom elements are pretty widely supported by browsers now, I assume you are referring to js being turned off.

In terms of designing for that situation - you can follow a pattern where your custom element wraps ( <custom-ele><stdelement></></> ) the element you want to enhance. If js is turned off, then the custom element defaults to rendering it's contents....

https://simonwillison.net/2022/Apr/21/web-components-as-prog...