Please don't make XHR run in synchronous mode

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The Windows Error Reporting team reports that 8.4% of all hangs in IE9 in the past month are caused by XMLHttpRequest objects blocking the UI thread with a synchronous request.

https://blogs.msdn.com/b/wer/archive/2011/08/03/why-you-should-use-xmlhttprequest-asynchronously.aspx

If your page uses XHR in synchronous mode, your first thought should be "I'm doing something wrong."

For a live example of why you shouldn't do this, see my Synchronous XHR test page.