[webcomponents] Changes to element creation during parsing per F2F (#366)

During parsing, the moment the open tag is seen:

1. Increment the [ignore destructive writes counter](https://html.spec.whatwg.org/multipage/webappapis.html#ignore-destructive-writes-counter)
1. Drop out of the parser, like you do for script elements
1. Queue the constructor.
1. Queue the attributeChangedCallbacks, in order, as you see new attributes.
1. Run the custom element callbacks queue, synchronously
1. If the return value of the constructor is not of the correct registered name, just create a HTMLUnknownElement instead and stop processing the queue
1. If any of the callbacks run throw, just create a HTMLUnknownElement instead and stop processing the queue
1. Decrement the counter
1. Resume parsing

Still TBD: does innerHTML do step "run the custom element callbacks queue, synchronously" or not?


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/366