We are waiting on a fix for issue 43394, "DOM attributes should be moved to prototype chains and should expose JavaScript getters and setters". Until it fixes, spirits will attach async in Chrome and Safari whenever one of these is involved:

The bug relates to these things being implemented as property setters instead of method calls. Observe the difference:

document.body.textContent = 'hello'; // property setter
document.body.appendChild(textnode); // method call

We can work around this with a MutationObserver. When the DOM changes, the observer will trigger a spiritualization of newly created elements.

The issue is making good progress in Chrome but the status in Safari is unknown, as always.