Re: [w3c/webcomponents] Composite custom element is not defined until the parent element is connected (#765)

Note that if we did bottom-up upgrades like you're suggesting, we'd end up with a bunch of unupgraded custom elements all waiting for all its descendents to be fetched & parsed by the HTML parser. In the worst case scenario, the entire page doesn't get renders / doesn't function until everything is fetched & parsed.

We've also considered upgrading elements as they're accessed but this will impose a significant perf cost on every access DOM API imaginable like firstChild, and deemed unacceptable from implementor's perspective. Having said that, if you're so inclined, you can manually invoke `customElements.upgrade` on all your child nodes manually before accessing them. (I just added the support for it in WebKit/Safari a couple of weeks ago so it's not widely available yet though :()

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