It seems odd that the custom elements API and spec refers to upgrading custom elements and yet the CSS pseudo class that corresponds to a custom element not being upgraded is called :unresolved. unresolved doesn't tell us what hasn't been resolved and inconsistent with the rest of the spec / API terminology we're using.

Perhaps we should consider negating it to :upgraded and use :not(:upgraded) for this use case for consistency. I do admit the the latter form is a bit more verbose but I think consistency in the terminology is very valuable here.