Synchronous XMLHttpRequest is being deprecated, meaning support for it will be removed eventually, here is the message I get in Chrome:
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/.
So, my question is if, and, if yes, when, major browsers (esp. Chrome) will drop support for Synchronous XMLHttpRequest?
Update: Chrome's announcement on removal of synchronous XHR during microtask execution at version 54 is unrelated.
I have no insider information, but here is some research.
The decision to deprecate was made in Feb 2014. Chrome declared it deprecated the same month but did not show a warning until Nov 2014. Firefox deprecated it in June 2014 (Firefox 30).
In general, important features for the web take time to migrate. For example, Chrome deprecated NPAPI support in Sep 2013. It is disabled by default in April 2015 (Chrome 42), and is scheduled for complete removal in Sep 2015.
In this case, ExtJS still uses this feature for module loading. Other modules use it inside an unload event to ping the server, but beacon replacement technology is far from ready for prime time. There are some more compelling use cases.
Roughly 2.5% of websites (Update: dropped to 0.8% in Oct 2017) are still using synchronous XMLHttpRequest. It does not look like it will go away any time soon.
来源：https://stackoverflow.com/questions/30876093/will-chrome-and-other-browsers-drop-support-for-synchronous-xmlhttprequest