Warning using external.js "Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience"

[jcamp]

I'm using the external.js in a reveal.js presentation with some external sources. I get the follow warnings quite a lot in the debugger:

Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.

They relate to the line of code:

xhr.open( 'GET', url, false );

Line 54 in external.js

Any help always appreciated...

[MichaelNeas]

Are you running your project on a server? Requesting files from a local directory will break it if you're not.