> How does one specify a declarative shadow root to use a specific custom
> element registry?
Well, I do see this as something that is better designed as part of the
scoped custom element registry proposal. But the basic idea would be to
just to add an attribute that allows the declarative shadow root to opt
out of automatically using the global registry:
<template shadowroot=open scoped-registry>
and that would keep any custom elements contained within the shadow root
from automatically upgrading based on the global registry. Maybe by just
assigning an empty custom registry to that shadow root. We'll need to add
an attribute to ShadowRoot, as part of the SCER proposal, to allow custom
elements to then set the appropriate custom registry later.