Chrome shows this on the console for every MMV load:
> Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check http://xhr.spec.whatwg.org/.
[[ https://xhr.spec.whatwg.org/#sync-warning | According to ]] the WHATWG spec:
> Synchronous XMLHttpRequest outside of workers is in the process of being removed from the web platform as it has detrimental effects to the end user's experience. (This is a long process that takes many years.) Developers must not pass false for the async argument when the JavaScript global environment is a document environment. User agents are strongly encouraged to warn about such usage in developer tools and may experiment with throwing an InvalidAccessError exception when it occurs.
Using sync XHR queries was always a debatable idea and now that EventLogging supports sendBeacon, it should be no longer necessary.