How do we employ progressive enhancement using custom elements? Given their reliance on JavaScript, as an old-school front-end developer, this feels like an unfortunate oversight.
The idea is much the same as server-rendering anything else: you give the components a first run to generate HTML server-side, send that first, and then let JS progressively enhance interactivity on top in the client if it can, or just stick with the initially rendered version if it can't.
(Server components is currently still on the V0 version of the spec, and this article is talking about V1, but they're very similar and we're migrating at the moment).
Fortunately, we can reliably depend on javascript now. What we cannot rely on is internet/processing speed.
I've always found progressive enhancement to work best as an additive process. You want to define the functionality of your page and then add enhancements and verify that the base functionality still holds. If you start from the most complex behavior it is a lot harder to reason about how removing functionality might hurt the page.