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The element upgrade algorithm goes as far as saying "For each item ELEMENT in CANDIDATES, set the value of the [[Prototype]] internal property of ELEMENT to PROTOTYPE". This is very different to the element initialization algorithm, which will invoke lifecycle callbacks. If a custom element author wanted to implement the equivalent of <video autoplay>, they would have to do manual tree walking to effect this for elements in the tree before document.register is called.
https://dvcs.w3.org/hg/webcomponents/
https://dvcs.w3.org/hg/webcomponents/rev/b6affe831db3