What's the issue?

Chrome 65+ allows synchronous XHR to be disabled. Blocking the UI thread on the network is a perf anti-pattern and so should probably not be allowed in AMP documents or embeds.

How do we reproduce the issue?

1. Create an amp page with an amp-iframe to a site doing a sync-xhr
2. View in the top stories carousel
3. Observe jank if you try to swipe when the sync-XHR occurs (especially on a slow network - use throttling in devtools to emulate).

What browsers are affected?

Chrome for now, but at TPAC all other browsers indicated a desire to add this feature as a first step of trying to phase-out sync-xhr.

Which AMP version is affected?

All

/cc @clelland @cramforce with whom this has been discussed offline.