http://www.w3.org/TR/custom-elements/#enqueuing-and-invoking-callbacks has the following normative text for when custom elements' callbacks are called:

```
Any time a script calls a method, reads or sets a property that is implemented by the user agent, the following actions must occur:

When transitioning from script to the user agent code, push a new element queue to the processing stack
When transitioning back from the user agent code to script, pop the element queue from the processing stack and invoke callbacks in that queue.
```

This needs to be more precisely defined in Web IDL, and the custom elements spec should refer to that.