"Whenever an unresolved element is created, it must be added to the upgrade candidates map."

It would be good to clarify what order is implied here. Does "added" mean "added to the end of a list"?

Consider three hypothetical fragments of markup A B C that are each markup for a custom element, and a script S which registers the custom elements in ABC.

If the document is structured SABC then the ready callbacks are delivered in order CBA per Section 7 ("Place ... at the front of the ... queue.")

However if the document is structured ABCS then the order in which the ready callbacks are delivered is unclear, but it is presumably ABC.

Is this interpretation correct? Is this seeming inconsistency desirable? (I guess it will encourage authors to be robust to initialization order, or if they desire a stable order, put their registration script last which has the side-effect of doing depth-first initialization which is pretty nice.)