[whatwg/dom] connectedCallback + parser gotcha (#611)

Right now it's easy to assume that `connectedCallback` will call once the element has been fully populated, as this is how it'll work over a fast/local connection.

This means failures are likely to be seen intermittently in real-world conditions.

I'm not sure what the best answer is. Maybe `connectedCallback` could be called sync by the parser if the JS stack is empty. That means it'd consistently call before children have been added, regardless of timing. This makes it more likely to show up in dev.

If you're setting `innerHTML` or building the DOM with JS, `connectedCallback` could continue to use a microtask.

This means we'd only be changing the behaviour of cases that are already potentially broken. We'd just be breaking them more consistently.

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