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The last iteration of the algorithm mutates the DOM tree (see bug 20490) and has terminology issues (see bug 20489). Given that in bug 20913 we're converging on the decision that guarantees that the platform object of a custom element is always instantiated properly, we can avoid DOM mutation and instead only update prototype chains when upgrading.
*** Bug 20489 has been marked as a duplicate of this bug. ***
*** Bug 20490 has been marked as a duplicate of this bug. ***
https://dvcs.w3.org/hg/webcomponents/rev/a6bb69c7284b
https://dvcs.w3.org/hg/webcomponents/rev/1e48e5c483c1