The scoped custom elements proposal needs to define the upgrade order of custom elements within shadow roots that have a scoped registry.

We've discussed the question of scoped registry upgrade ordering in other issues, but it deserves its own issue so we can more easily track the discussion and consensus.

The discussion on ordering starts here: #716 (comment) and is also brought up here: #865 (comment) with the second link having some more detailed discussion.

To reiterate, the two main options seem to be:

1. Keep document order upgrading.
2. Order upgrades by the order that shadow roots were associated with the registry, then tree order within those roots.

It seemed from the discussion in the last face-to-face and in #865 that if disconnected trees are not upgraded and registries are restricted to one document that keeping a list of shadow roots associated with a registry - in the order they were created, and upgrading only those roots was preferable. I did want open this to double check with @rniwa @annevk and @emilio especially.