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"Any time a script calls a method, reads or sets a property that is implemented by the user agent, the following actions must occur: " This needs to become part of the actual algorithms such as innerHTML, cloneNode(), etc. The easiest way to do that is tying it to IDL I think just as is done in the Chrome implementation. I recommend working with heycam on doing that per my outline in http://lists.w3.org/Archives/Public/public-webapps/2014JanMar/0142.html as that makes it much more explicit when everything happens relative to each other.
Note that we also need this mechanism for callbacks implemented by the user agent. E.g. when adoption happens we want to queue a callback that runs before the method that caused the adoption to return and mangles prototypes.
I was just reading up on extended attributes: http://heycam.github.io/webidl/#idl-extended-attributes. A simple ExtendedAttributeNoArgs thingy should suffice in this case -- something like [InvokesCustomElementCallbacks] or [ProcessesCustomElementCallbacks] or [ElementCallbackBarrier]? Heycam, should I just write this down in custom elements spec, or should it be part of Web IDL spec, like ES-specific attributes: http://heycam.github.io/webidl/#es-extended-attributes?
*** Bug 27179 has been marked as a duplicate of this bug. ***
(Currently Web IDL offers no way for other specifications to define extended attributes.)
Moved to https://github.com/w3c/webcomponents/issues/186