1463111 - Run the callback from customElements.setElementCreationCallback() immediately if there are already unresolved element

Status: NEW

(Core :: DOM: Core & HTML, enhancement, P3)

Description

[Tim Guan-tin Chien [:timdream] (please needinfo)]

It's possible(*) that by the time a callback is set, there is already unresolved elements in the DOM. 

The element will remain unresolved and my proposed patch in bug 1462703 will still not call the callback if the element is cloned or (re-)appended.

Just to avoid the confusion, I think we should execute the callback and get the definition if there already unresolved elements in the registry.

* this really depends on the timing of how we will use this method. I don't think we will encounter this with usage in bug 1411707 but we still might in the future.