If there's invalid dom nesting, there will be mismatches following but
the nesting is the most important cause.

We could also silence the mismatch warning (but not thrown error) too.

This really relies on us throwing to abort hydrating siblings so we don't
get extra mismatches after.

That this is according to the rules of HTML, not React, and that this
will cause a hydration error if not addressed.

"appear as" is a bit academic.

Because otherwise we'll act as if we're already inside the child so
the child is already inside itself.

I don't think we use the context for anything at runtime.